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+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
+ "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
+
+<!-- ****************************************************** -->
+<!-- Header -->
+<!-- ****************************************************** -->
+<book id="Writing-an-ALSA-Driver">
+ <bookinfo>
+ <title>Writing an ALSA Driver</title>
+ <author>
+ <firstname>Takashi</firstname>
+ <surname>Iwai</surname>
+ <affiliation>
+ <address>
+ <email>tiwai@suse.de</email>
+ </address>
+ </affiliation>
+ </author>
+
+ <date>Oct 15, 2007</date>
+ <edition>0.3.7</edition>
+
+ <abstract>
+ <para>
+ This document describes how to write an ALSA (Advanced Linux
+ Sound Architecture) driver.
+ </para>
+ </abstract>
+
+ <legalnotice>
+ <para>
+ Copyright (c) 2002-2005 Takashi Iwai <email>tiwai@suse.de</email>
+ </para>
+
+ <para>
+ This document is free; you can redistribute it and/or modify it
+ under the terms of the GNU General Public License as published by
+ the Free Software Foundation; either version 2 of the License, or
+ (at your option) any later version.
+ </para>
+
+ <para>
+ This document is distributed in the hope that it will be useful,
+ but <emphasis>WITHOUT ANY WARRANTY</emphasis>; without even the
+ implied warranty of <emphasis>MERCHANTABILITY or FITNESS FOR A
+ PARTICULAR PURPOSE</emphasis>. See the GNU General Public License
+ for more details.
+ </para>
+
+ <para>
+ You should have received a copy of the GNU General Public
+ License along with this program; if not, write to the Free
+ Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
+ MA 02111-1307 USA
+ </para>
+ </legalnotice>
+
+ </bookinfo>
+
+<!-- ****************************************************** -->
+<!-- Preface -->
+<!-- ****************************************************** -->
+ <preface id="preface">
+ <title>Preface</title>
+ <para>
+ This document describes how to write an
+ <ulink url="http://www.alsa-project.org/"><citetitle>
+ ALSA (Advanced Linux Sound Architecture)</citetitle></ulink>
+ driver. The document focuses mainly on PCI soundcards.
+ In the case of other device types, the API might
+ be different, too. However, at least the ALSA kernel API is
+ consistent, and therefore it would be still a bit help for
+ writing them.
+ </para>
+
+ <para>
+ This document targets people who already have enough
+ C language skills and have basic linux kernel programming
+ knowledge. This document doesn't explain the general
+ topic of linux kernel coding and doesn't cover low-level
+ driver implementation details. It only describes
+ the standard way to write a PCI sound driver on ALSA.
+ </para>
+
+ <para>
+ If you are already familiar with the older ALSA ver.0.5.x API, you
+ can check the drivers such as <filename>sound/pci/es1938.c</filename> or
+ <filename>sound/pci/maestro3.c</filename> which have also almost the same
+ code-base in the ALSA 0.5.x tree, so you can compare the differences.
+ </para>
+
+ <para>
+ This document is still a draft version. Any feedback and
+ corrections, please!!
+ </para>
+ </preface>
+
+
+<!-- ****************************************************** -->
+<!-- File Tree Structure -->
+<!-- ****************************************************** -->
+ <chapter id="file-tree">
+ <title>File Tree Structure</title>
+
+ <section id="file-tree-general">
+ <title>General</title>
+ <para>
+ The ALSA drivers are provided in two ways.
+ </para>
+
+ <para>
+ One is the trees provided as a tarball or via cvs from the
+ ALSA's ftp site, and another is the 2.6 (or later) Linux kernel
+ tree. To synchronize both, the ALSA driver tree is split into
+ two different trees: alsa-kernel and alsa-driver. The former
+ contains purely the source code for the Linux 2.6 (or later)
+ tree. This tree is designed only for compilation on 2.6 or
+ later environment. The latter, alsa-driver, contains many subtle
+ files for compiling ALSA drivers outside of the Linux kernel tree,
+ wrapper functions for older 2.2 and 2.4 kernels, to adapt the latest kernel API,
+ and additional drivers which are still in development or in
+ tests. The drivers in alsa-driver tree will be moved to
+ alsa-kernel (and eventually to the 2.6 kernel tree) when they are
+ finished and confirmed to work fine.
+ </para>
+
+ <para>
+ The file tree structure of ALSA driver is depicted below. Both
+ alsa-kernel and alsa-driver have almost the same file
+ structure, except for <quote>core</quote> directory. It's
+ named as <quote>acore</quote> in alsa-driver tree.
+
+ <example>
+ <title>ALSA File Tree Structure</title>
+ <literallayout>
+ sound
+ /core
+ /oss
+ /seq
+ /oss
+ /instr
+ /ioctl32
+ /include
+ /drivers
+ /mpu401
+ /opl3
+ /i2c
+ /l3
+ /synth
+ /emux
+ /pci
+ /(cards)
+ /isa
+ /(cards)
+ /arm
+ /ppc
+ /sparc
+ /usb
+ /pcmcia /(cards)
+ /oss
+ </literallayout>
+ </example>
+ </para>
+ </section>
+
+ <section id="file-tree-core-directory">
+ <title>core directory</title>
+ <para>
+ This directory contains the middle layer which is the heart
+ of ALSA drivers. In this directory, the native ALSA modules are
+ stored. The sub-directories contain different modules and are
+ dependent upon the kernel config.
+ </para>
+
+ <section id="file-tree-core-directory-oss">
+ <title>core/oss</title>
+
+ <para>
+ The codes for PCM and mixer OSS emulation modules are stored
+ in this directory. The rawmidi OSS emulation is included in
+ the ALSA rawmidi code since it's quite small. The sequencer
+ code is stored in <filename>core/seq/oss</filename> directory (see
+ <link linkend="file-tree-core-directory-seq-oss"><citetitle>
+ below</citetitle></link>).
+ </para>
+ </section>
+
+ <section id="file-tree-core-directory-ioctl32">
+ <title>core/ioctl32</title>
+
+ <para>
+ This directory contains the 32bit-ioctl wrappers for 64bit
+ architectures such like x86-64, ppc64 and sparc64. For 32bit
+ and alpha architectures, these are not compiled.
+ </para>
+ </section>
+
+ <section id="file-tree-core-directory-seq">
+ <title>core/seq</title>
+ <para>
+ This directory and its sub-directories are for the ALSA
+ sequencer. This directory contains the sequencer core and
+ primary sequencer modules such like snd-seq-midi,
+ snd-seq-virmidi, etc. They are compiled only when
+ <constant>CONFIG_SND_SEQUENCER</constant> is set in the kernel
+ config.
+ </para>
+ </section>
+
+ <section id="file-tree-core-directory-seq-oss">
+ <title>core/seq/oss</title>
+ <para>
+ This contains the OSS sequencer emulation codes.
+ </para>
+ </section>
+
+ <section id="file-tree-core-directory-deq-instr">
+ <title>core/seq/instr</title>
+ <para>
+ This directory contains the modules for the sequencer
+ instrument layer.
+ </para>
+ </section>
+ </section>
+
+ <section id="file-tree-include-directory">
+ <title>include directory</title>
+ <para>
+ This is the place for the public header files of ALSA drivers,
+ which are to be exported to user-space, or included by
+ several files at different directories. Basically, the private
+ header files should not be placed in this directory, but you may
+ still find files there, due to historical reasons :)
+ </para>
+ </section>
+
+ <section id="file-tree-drivers-directory">
+ <title>drivers directory</title>
+ <para>
+ This directory contains code shared among different drivers
+ on different architectures. They are hence supposed not to be
+ architecture-specific.
+ For example, the dummy pcm driver and the serial MIDI
+ driver are found in this directory. In the sub-directories,
+ there is code for components which are independent from
+ bus and cpu architectures.
+ </para>
+
+ <section id="file-tree-drivers-directory-mpu401">
+ <title>drivers/mpu401</title>
+ <para>
+ The MPU401 and MPU401-UART modules are stored here.
+ </para>
+ </section>
+
+ <section id="file-tree-drivers-directory-opl3">
+ <title>drivers/opl3 and opl4</title>
+ <para>
+ The OPL3 and OPL4 FM-synth stuff is found here.
+ </para>
+ </section>
+ </section>
+
+ <section id="file-tree-i2c-directory">
+ <title>i2c directory</title>
+ <para>
+ This contains the ALSA i2c components.
+ </para>
+
+ <para>
+ Although there is a standard i2c layer on Linux, ALSA has its
+ own i2c code for some cards, because the soundcard needs only a
+ simple operation and the standard i2c API is too complicated for
+ such a purpose.
+ </para>
+
+ <section id="file-tree-i2c-directory-l3">
+ <title>i2c/l3</title>
+ <para>
+ This is a sub-directory for ARM L3 i2c.
+ </para>
+ </section>
+ </section>
+
+ <section id="file-tree-synth-directory">
+ <title>synth directory</title>
+ <para>
+ This contains the synth middle-level modules.
+ </para>
+
+ <para>
+ So far, there is only Emu8000/Emu10k1 synth driver under
+ the <filename>synth/emux</filename> sub-directory.
+ </para>
+ </section>
+
+ <section id="file-tree-pci-directory">
+ <title>pci directory</title>
+ <para>
+ This directory and its sub-directories hold the top-level card modules
+ for PCI soundcards and the code specific to the PCI BUS.
+ </para>
+
+ <para>
+ The drivers compiled from a single file are stored directly
+ in the pci directory, while the drivers with several source files are
+ stored on their own sub-directory (e.g. emu10k1, ice1712).
+ </para>
+ </section>
+
+ <section id="file-tree-isa-directory">
+ <title>isa directory</title>
+ <para>
+ This directory and its sub-directories hold the top-level card modules
+ for ISA soundcards.
+ </para>
+ </section>
+
+ <section id="file-tree-arm-ppc-sparc-directories">
+ <title>arm, ppc, and sparc directories</title>
+ <para>
+ They are used for top-level card modules which are
+ specific to one of these architectures.
+ </para>
+ </section>
+
+ <section id="file-tree-usb-directory">
+ <title>usb directory</title>
+ <para>
+ This directory contains the USB-audio driver. In the latest version, the
+ USB MIDI driver is integrated in the usb-audio driver.
+ </para>
+ </section>
+
+ <section id="file-tree-pcmcia-directory">
+ <title>pcmcia directory</title>
+ <para>
+ The PCMCIA, especially PCCard drivers will go here. CardBus
+ drivers will be in the pci directory, because their API is identical
+ to that of standard PCI cards.
+ </para>
+ </section>
+
+ <section id="file-tree-oss-directory">
+ <title>oss directory</title>
+ <para>
+ The OSS/Lite source files are stored here in Linux 2.6 (or
+ later) tree. In the ALSA driver tarball, this directory is empty,
+ of course :)
+ </para>
+ </section>
+ </chapter>
+
+
+<!-- ****************************************************** -->
+<!-- Basic Flow for PCI Drivers -->
+<!-- ****************************************************** -->
+ <chapter id="basic-flow">
+ <title>Basic Flow for PCI Drivers</title>
+
+ <section id="basic-flow-outline">
+ <title>Outline</title>
+ <para>
+ The minimum flow for PCI soundcards is as follows:
+
+ <itemizedlist>
+ <listitem><para>define the PCI ID table (see the section
+ <link linkend="pci-resource-entries"><citetitle>PCI Entries
+ </citetitle></link>).</para></listitem>
+ <listitem><para>create <function>probe()</function> callback.</para></listitem>
+ <listitem><para>create <function>remove()</function> callback.</para></listitem>
+ <listitem><para>create a <structname>pci_driver</structname> structure
+ containing the three pointers above.</para></listitem>
+ <listitem><para>create an <function>init()</function> function just calling
+ the <function>pci_register_driver()</function> to register the pci_driver table
+ defined above.</para></listitem>
+ <listitem><para>create an <function>exit()</function> function to call
+ the <function>pci_unregister_driver()</function> function.</para></listitem>
+ </itemizedlist>
+ </para>
+ </section>
+
+ <section id="basic-flow-example">
+ <title>Full Code Example</title>
+ <para>
+ The code example is shown below. Some parts are kept
+ unimplemented at this moment but will be filled in the
+ next sections. The numbers in the comment lines of the
+ <function>snd_mychip_probe()</function> function
+ refer to details explained in the following section.
+
+ <example>
+ <title>Basic Flow for PCI Drivers - Example</title>
+ <programlisting>
+<![CDATA[
+ #include <linux/init.h>
+ #include <linux/pci.h>
+ #include <linux/slab.h>
+ #include <sound/core.h>
+ #include <sound/initval.h>
+
+ /* module parameters (see "Module Parameters") */
+ /* SNDRV_CARDS: maximum number of cards supported by this module */
+ static int index[SNDRV_CARDS] = SNDRV_DEFAULT_IDX;
+ static char *id[SNDRV_CARDS] = SNDRV_DEFAULT_STR;
+ static bool enable[SNDRV_CARDS] = SNDRV_DEFAULT_ENABLE_PNP;
+
+ /* definition of the chip-specific record */
+ struct mychip {
+ struct snd_card *card;
+ /* the rest of the implementation will be in section
+ * "PCI Resource Management"
+ */
+ };
+
+ /* chip-specific destructor
+ * (see "PCI Resource Management")
+ */
+ static int snd_mychip_free(struct mychip *chip)
+ {
+ .... /* will be implemented later... */
+ }
+
+ /* component-destructor
+ * (see "Management of Cards and Components")
+ */
+ static int snd_mychip_dev_free(struct snd_device *device)
+ {
+ return snd_mychip_free(device->device_data);
+ }
+
+ /* chip-specific constructor
+ * (see "Management of Cards and Components")
+ */
+ static int snd_mychip_create(struct snd_card *card,
+ struct pci_dev *pci,
+ struct mychip **rchip)
+ {
+ struct mychip *chip;
+ int err;
+ static struct snd_device_ops ops = {
+ .dev_free = snd_mychip_dev_free,
+ };
+
+ *rchip = NULL;
+
+ /* check PCI availability here
+ * (see "PCI Resource Management")
+ */
+ ....
+
+ /* allocate a chip-specific data with zero filled */
+ chip = kzalloc(sizeof(*chip), GFP_KERNEL);
+ if (chip == NULL)
+ return -ENOMEM;
+
+ chip->card = card;
+
+ /* rest of initialization here; will be implemented
+ * later, see "PCI Resource Management"
+ */
+ ....
+
+ err = snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops);
+ if (err < 0) {
+ snd_mychip_free(chip);
+ return err;
+ }
+
+ *rchip = chip;
+ return 0;
+ }
+
+ /* constructor -- see "Constructor" sub-section */
+ static int snd_mychip_probe(struct pci_dev *pci,
+ const struct pci_device_id *pci_id)
+ {
+ static int dev;
+ struct snd_card *card;
+ struct mychip *chip;
+ int err;
+
+ /* (1) */
+ if (dev >= SNDRV_CARDS)
+ return -ENODEV;
+ if (!enable[dev]) {
+ dev++;
+ return -ENOENT;
+ }
+
+ /* (2) */
+ err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
+ 0, &card);
+ if (err < 0)
+ return err;
+
+ /* (3) */
+ err = snd_mychip_create(card, pci, &chip);
+ if (err < 0) {
+ snd_card_free(card);
+ return err;
+ }
+
+ /* (4) */
+ strcpy(card->driver, "My Chip");
+ strcpy(card->shortname, "My Own Chip 123");
+ sprintf(card->longname, "%s at 0x%lx irq %i",
+ card->shortname, chip->ioport, chip->irq);
+
+ /* (5) */
+ .... /* implemented later */
+
+ /* (6) */
+ err = snd_card_register(card);
+ if (err < 0) {
+ snd_card_free(card);
+ return err;
+ }
+
+ /* (7) */
+ pci_set_drvdata(pci, card);
+ dev++;
+ return 0;
+ }
+
+ /* destructor -- see the "Destructor" sub-section */
+ static void snd_mychip_remove(struct pci_dev *pci)
+ {
+ snd_card_free(pci_get_drvdata(pci));
+ pci_set_drvdata(pci, NULL);
+ }
+]]>
+ </programlisting>
+ </example>
+ </para>
+ </section>
+
+ <section id="basic-flow-constructor">
+ <title>Constructor</title>
+ <para>
+ The real constructor of PCI drivers is the <function>probe</function> callback.
+ The <function>probe</function> callback and other component-constructors which are called
+ from the <function>probe</function> callback cannot be used with
+ the <parameter>__init</parameter> prefix
+ because any PCI device could be a hotplug device.
+ </para>
+
+ <para>
+ In the <function>probe</function> callback, the following scheme is often used.
+ </para>
+
+ <section id="basic-flow-constructor-device-index">
+ <title>1) Check and increment the device index.</title>
+ <para>
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static int dev;
+ ....
+ if (dev >= SNDRV_CARDS)
+ return -ENODEV;
+ if (!enable[dev]) {
+ dev++;
+ return -ENOENT;
+ }
+]]>
+ </programlisting>
+ </informalexample>
+
+ where enable[dev] is the module option.
+ </para>
+
+ <para>
+ Each time the <function>probe</function> callback is called, check the
+ availability of the device. If not available, simply increment
+ the device index and returns. dev will be incremented also
+ later (<link
+ linkend="basic-flow-constructor-set-pci"><citetitle>step
+ 7</citetitle></link>).
+ </para>
+ </section>
+
+ <section id="basic-flow-constructor-create-card">
+ <title>2) Create a card instance</title>
+ <para>
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ struct snd_card *card;
+ int err;
+ ....
+ err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
+ 0, &card);
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ The details will be explained in the section
+ <link linkend="card-management-card-instance"><citetitle>
+ Management of Cards and Components</citetitle></link>.
+ </para>
+ </section>
+
+ <section id="basic-flow-constructor-create-main">
+ <title>3) Create a main component</title>
+ <para>
+ In this part, the PCI resources are allocated.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ struct mychip *chip;
+ ....
+ err = snd_mychip_create(card, pci, &chip);
+ if (err < 0) {
+ snd_card_free(card);
+ return err;
+ }
+]]>
+ </programlisting>
+ </informalexample>
+
+ The details will be explained in the section <link
+ linkend="pci-resource"><citetitle>PCI Resource
+ Management</citetitle></link>.
+ </para>
+ </section>
+
+ <section id="basic-flow-constructor-main-component">
+ <title>4) Set the driver ID and name strings.</title>
+ <para>
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ strcpy(card->driver, "My Chip");
+ strcpy(card->shortname, "My Own Chip 123");
+ sprintf(card->longname, "%s at 0x%lx irq %i",
+ card->shortname, chip->ioport, chip->irq);
+]]>
+ </programlisting>
+ </informalexample>
+
+ The driver field holds the minimal ID string of the
+ chip. This is used by alsa-lib's configurator, so keep it
+ simple but unique.
+ Even the same driver can have different driver IDs to
+ distinguish the functionality of each chip type.
+ </para>
+
+ <para>
+ The shortname field is a string shown as more verbose
+ name. The longname field contains the information
+ shown in <filename>/proc/asound/cards</filename>.
+ </para>
+ </section>
+
+ <section id="basic-flow-constructor-create-other">
+ <title>5) Create other components, such as mixer, MIDI, etc.</title>
+ <para>
+ Here you define the basic components such as
+ <link linkend="pcm-interface"><citetitle>PCM</citetitle></link>,
+ mixer (e.g. <link linkend="api-ac97"><citetitle>AC97</citetitle></link>),
+ MIDI (e.g. <link linkend="midi-interface"><citetitle>MPU-401</citetitle></link>),
+ and other interfaces.
+ Also, if you want a <link linkend="proc-interface"><citetitle>proc
+ file</citetitle></link>, define it here, too.
+ </para>
+ </section>
+
+ <section id="basic-flow-constructor-register-card">
+ <title>6) Register the card instance.</title>
+ <para>
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ err = snd_card_register(card);
+ if (err < 0) {
+ snd_card_free(card);
+ return err;
+ }
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ Will be explained in the section <link
+ linkend="card-management-registration"><citetitle>Management
+ of Cards and Components</citetitle></link>, too.
+ </para>
+ </section>
+
+ <section id="basic-flow-constructor-set-pci">
+ <title>7) Set the PCI driver data and return zero.</title>
+ <para>
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ pci_set_drvdata(pci, card);
+ dev++;
+ return 0;
+]]>
+ </programlisting>
+ </informalexample>
+
+ In the above, the card record is stored. This pointer is
+ used in the remove callback and power-management
+ callbacks, too.
+ </para>
+ </section>
+ </section>
+
+ <section id="basic-flow-destructor">
+ <title>Destructor</title>
+ <para>
+ The destructor, remove callback, simply releases the card
+ instance. Then the ALSA middle layer will release all the
+ attached components automatically.
+ </para>
+
+ <para>
+ It would be typically like the following:
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static void snd_mychip_remove(struct pci_dev *pci)
+ {
+ snd_card_free(pci_get_drvdata(pci));
+ pci_set_drvdata(pci, NULL);
+ }
+]]>
+ </programlisting>
+ </informalexample>
+
+ The above code assumes that the card pointer is set to the PCI
+ driver data.
+ </para>
+ </section>
+
+ <section id="basic-flow-header-files">
+ <title>Header Files</title>
+ <para>
+ For the above example, at least the following include files
+ are necessary.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ #include <linux/init.h>
+ #include <linux/pci.h>
+ #include <linux/slab.h>
+ #include <sound/core.h>
+ #include <sound/initval.h>
+]]>
+ </programlisting>
+ </informalexample>
+
+ where the last one is necessary only when module options are
+ defined in the source file. If the code is split into several
+ files, the files without module options don't need them.
+ </para>
+
+ <para>
+ In addition to these headers, you'll need
+ <filename><linux/interrupt.h></filename> for interrupt
+ handling, and <filename><asm/io.h></filename> for I/O
+ access. If you use the <function>mdelay()</function> or
+ <function>udelay()</function> functions, you'll need to include
+ <filename><linux/delay.h></filename> too.
+ </para>
+
+ <para>
+ The ALSA interfaces like the PCM and control APIs are defined in other
+ <filename><sound/xxx.h></filename> header files.
+ They have to be included after
+ <filename><sound/core.h></filename>.
+ </para>
+
+ </section>
+ </chapter>
+
+
+<!-- ****************************************************** -->
+<!-- Management of Cards and Components -->
+<!-- ****************************************************** -->
+ <chapter id="card-management">
+ <title>Management of Cards and Components</title>
+
+ <section id="card-management-card-instance">
+ <title>Card Instance</title>
+ <para>
+ For each soundcard, a <quote>card</quote> record must be allocated.
+ </para>
+
+ <para>
+ A card record is the headquarters of the soundcard. It manages
+ the whole list of devices (components) on the soundcard, such as
+ PCM, mixers, MIDI, synthesizer, and so on. Also, the card
+ record holds the ID and the name strings of the card, manages
+ the root of proc files, and controls the power-management states
+ and hotplug disconnections. The component list on the card
+ record is used to manage the correct release of resources at
+ destruction.
+ </para>
+
+ <para>
+ As mentioned above, to create a card instance, call
+ <function>snd_card_new()</function>.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ struct snd_card *card;
+ int err;
+ err = snd_card_new(&pci->dev, index, id, module, extra_size, &card);
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ The function takes six arguments: the parent device pointer,
+ the card-index number, the id string, the module pointer (usually
+ <constant>THIS_MODULE</constant>),
+ the size of extra-data space, and the pointer to return the
+ card instance. The extra_size argument is used to
+ allocate card->private_data for the
+ chip-specific data. Note that these data
+ are allocated by <function>snd_card_new()</function>.
+ </para>
+
+ <para>
+ The first argument, the pointer of struct
+ <structname>device</structname>, specifies the parent device.
+ For PCI devices, typically &pci-> is passed there.
+ </para>
+ </section>
+
+ <section id="card-management-component">
+ <title>Components</title>
+ <para>
+ After the card is created, you can attach the components
+ (devices) to the card instance. In an ALSA driver, a component is
+ represented as a struct <structname>snd_device</structname> object.
+ A component can be a PCM instance, a control interface, a raw
+ MIDI interface, etc. Each such instance has one component
+ entry.
+ </para>
+
+ <para>
+ A component can be created via
+ <function>snd_device_new()</function> function.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ snd_device_new(card, SNDRV_DEV_XXX, chip, &ops);
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ This takes the card pointer, the device-level
+ (<constant>SNDRV_DEV_XXX</constant>), the data pointer, and the
+ callback pointers (<parameter>&ops</parameter>). The
+ device-level defines the type of components and the order of
+ registration and de-registration. For most components, the
+ device-level is already defined. For a user-defined component,
+ you can use <constant>SNDRV_DEV_LOWLEVEL</constant>.
+ </para>
+
+ <para>
+ This function itself doesn't allocate the data space. The data
+ must be allocated manually beforehand, and its pointer is passed
+ as the argument. This pointer (<parameter>chip</parameter> in the
+ above example) is used as the identifier for the instance.
+ </para>
+
+ <para>
+ Each pre-defined ALSA component such as ac97 and pcm calls
+ <function>snd_device_new()</function> inside its
+ constructor. The destructor for each component is defined in the
+ callback pointers. Hence, you don't need to take care of
+ calling a destructor for such a component.
+ </para>
+
+ <para>
+ If you wish to create your own component, you need to
+ set the destructor function to the dev_free callback in
+ the <parameter>ops</parameter>, so that it can be released
+ automatically via <function>snd_card_free()</function>.
+ The next example will show an implementation of chip-specific
+ data.
+ </para>
+ </section>
+
+ <section id="card-management-chip-specific">
+ <title>Chip-Specific Data</title>
+ <para>
+ Chip-specific information, e.g. the I/O port address, its
+ resource pointer, or the irq number, is stored in the
+ chip-specific record.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ struct mychip {
+ ....
+ };
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ In general, there are two ways of allocating the chip record.
+ </para>
+
+ <section id="card-management-chip-specific-snd-card-new">
+ <title>1. Allocating via <function>snd_card_new()</function>.</title>
+ <para>
+ As mentioned above, you can pass the extra-data-length
+ to the 5th argument of <function>snd_card_new()</function>, i.e.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
+ sizeof(struct mychip), &card);
+]]>
+ </programlisting>
+ </informalexample>
+
+ struct <structname>mychip</structname> is the type of the chip record.
+ </para>
+
+ <para>
+ In return, the allocated record can be accessed as
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ struct mychip *chip = card->private_data;
+]]>
+ </programlisting>
+ </informalexample>
+
+ With this method, you don't have to allocate twice.
+ The record is released together with the card instance.
+ </para>
+ </section>
+
+ <section id="card-management-chip-specific-allocate-extra">
+ <title>2. Allocating an extra device.</title>
+
+ <para>
+ After allocating a card instance via
+ <function>snd_card_new()</function> (with
+ <constant>0</constant> on the 4th arg), call
+ <function>kzalloc()</function>.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ struct snd_card *card;
+ struct mychip *chip;
+ err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
+ 0, &card);
+ .....
+ chip = kzalloc(sizeof(*chip), GFP_KERNEL);
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ The chip record should have the field to hold the card
+ pointer at least,
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ struct mychip {
+ struct snd_card *card;
+ ....
+ };
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ Then, set the card pointer in the returned chip instance.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ chip->card = card;
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ Next, initialize the fields, and register this chip
+ record as a low-level device with a specified
+ <parameter>ops</parameter>,
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static struct snd_device_ops ops = {
+ .dev_free = snd_mychip_dev_free,
+ };
+ ....
+ snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops);
+]]>
+ </programlisting>
+ </informalexample>
+
+ <function>snd_mychip_dev_free()</function> is the
+ device-destructor function, which will call the real
+ destructor.
+ </para>
+
+ <para>
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static int snd_mychip_dev_free(struct snd_device *device)
+ {
+ return snd_mychip_free(device->device_data);
+ }
+]]>
+ </programlisting>
+ </informalexample>
+
+ where <function>snd_mychip_free()</function> is the real destructor.
+ </para>
+ </section>
+ </section>
+
+ <section id="card-management-registration">
+ <title>Registration and Release</title>
+ <para>
+ After all components are assigned, register the card instance
+ by calling <function>snd_card_register()</function>. Access
+ to the device files is enabled at this point. That is, before
+ <function>snd_card_register()</function> is called, the
+ components are safely inaccessible from external side. If this
+ call fails, exit the probe function after releasing the card via
+ <function>snd_card_free()</function>.
+ </para>
+
+ <para>
+ For releasing the card instance, you can call simply
+ <function>snd_card_free()</function>. As mentioned earlier, all
+ components are released automatically by this call.
+ </para>
+
+ <para>
+ For a device which allows hotplugging, you can use
+ <function>snd_card_free_when_closed</function>. This one will
+ postpone the destruction until all devices are closed.
+ </para>
+
+ </section>
+
+ </chapter>
+
+
+<!-- ****************************************************** -->
+<!-- PCI Resource Management -->
+<!-- ****************************************************** -->
+ <chapter id="pci-resource">
+ <title>PCI Resource Management</title>
+
+ <section id="pci-resource-example">
+ <title>Full Code Example</title>
+ <para>
+ In this section, we'll complete the chip-specific constructor,
+ destructor and PCI entries. Example code is shown first,
+ below.
+
+ <example>
+ <title>PCI Resource Management Example</title>
+ <programlisting>
+<![CDATA[
+ struct mychip {
+ struct snd_card *card;
+ struct pci_dev *pci;
+
+ unsigned long port;
+ int irq;
+ };
+
+ static int snd_mychip_free(struct mychip *chip)
+ {
+ /* disable hardware here if any */
+ .... /* (not implemented in this document) */
+
+ /* release the irq */
+ if (chip->irq >= 0)
+ free_irq(chip->irq, chip);
+ /* release the I/O ports & memory */
+ pci_release_regions(chip->pci);
+ /* disable the PCI entry */
+ pci_disable_device(chip->pci);
+ /* release the data */
+ kfree(chip);
+ return 0;
+ }
+
+ /* chip-specific constructor */
+ static int snd_mychip_create(struct snd_card *card,
+ struct pci_dev *pci,
+ struct mychip **rchip)
+ {
+ struct mychip *chip;
+ int err;
+ static struct snd_device_ops ops = {
+ .dev_free = snd_mychip_dev_free,
+ };
+
+ *rchip = NULL;
+
+ /* initialize the PCI entry */
+ err = pci_enable_device(pci);
+ if (err < 0)
+ return err;
+ /* check PCI availability (28bit DMA) */
+ if (pci_set_dma_mask(pci, DMA_BIT_MASK(28)) < 0 ||
+ pci_set_consistent_dma_mask(pci, DMA_BIT_MASK(28)) < 0) {
+ printk(KERN_ERR "error to set 28bit mask DMA\n");
+ pci_disable_device(pci);
+ return -ENXIO;
+ }
+
+ chip = kzalloc(sizeof(*chip), GFP_KERNEL);
+ if (chip == NULL) {
+ pci_disable_device(pci);
+ return -ENOMEM;
+ }
+
+ /* initialize the stuff */
+ chip->card = card;
+ chip->pci = pci;
+ chip->irq = -1;
+
+ /* (1) PCI resource allocation */
+ err = pci_request_regions(pci, "My Chip");
+ if (err < 0) {
+ kfree(chip);
+ pci_disable_device(pci);
+ return err;
+ }
+ chip->port = pci_resource_start(pci, 0);
+ if (request_irq(pci->irq, snd_mychip_interrupt,
+ IRQF_SHARED, KBUILD_MODNAME, chip)) {
+ printk(KERN_ERR "cannot grab irq %d\n", pci->irq);
+ snd_mychip_free(chip);
+ return -EBUSY;
+ }
+ chip->irq = pci->irq;
+
+ /* (2) initialization of the chip hardware */
+ .... /* (not implemented in this document) */
+
+ err = snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops);
+ if (err < 0) {
+ snd_mychip_free(chip);
+ return err;
+ }
+
+ *rchip = chip;
+ return 0;
+ }
+
+ /* PCI IDs */
+ static struct pci_device_id snd_mychip_ids[] = {
+ { PCI_VENDOR_ID_FOO, PCI_DEVICE_ID_BAR,
+ PCI_ANY_ID, PCI_ANY_ID, 0, 0, 0, },
+ ....
+ { 0, }
+ };
+ MODULE_DEVICE_TABLE(pci, snd_mychip_ids);
+
+ /* pci_driver definition */
+ static struct pci_driver driver = {
+ .name = KBUILD_MODNAME,
+ .id_table = snd_mychip_ids,
+ .probe = snd_mychip_probe,
+ .remove = snd_mychip_remove,
+ };
+
+ /* module initialization */
+ static int __init alsa_card_mychip_init(void)
+ {
+ return pci_register_driver(&driver);
+ }
+
+ /* module clean up */
+ static void __exit alsa_card_mychip_exit(void)
+ {
+ pci_unregister_driver(&driver);
+ }
+
+ module_init(alsa_card_mychip_init)
+ module_exit(alsa_card_mychip_exit)
+
+ EXPORT_NO_SYMBOLS; /* for old kernels only */
+]]>
+ </programlisting>
+ </example>
+ </para>
+ </section>
+
+ <section id="pci-resource-some-haftas">
+ <title>Some Hafta's</title>
+ <para>
+ The allocation of PCI resources is done in the
+ <function>probe()</function> function, and usually an extra
+ <function>xxx_create()</function> function is written for this
+ purpose.
+ </para>
+
+ <para>
+ In the case of PCI devices, you first have to call
+ the <function>pci_enable_device()</function> function before
+ allocating resources. Also, you need to set the proper PCI DMA
+ mask to limit the accessed I/O range. In some cases, you might
+ need to call <function>pci_set_master()</function> function,
+ too.
+ </para>
+
+ <para>
+ Suppose the 28bit mask, and the code to be added would be like:
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ err = pci_enable_device(pci);
+ if (err < 0)
+ return err;
+ if (pci_set_dma_mask(pci, DMA_BIT_MASK(28)) < 0 ||
+ pci_set_consistent_dma_mask(pci, DMA_BIT_MASK(28)) < 0) {
+ printk(KERN_ERR "error to set 28bit mask DMA\n");
+ pci_disable_device(pci);
+ return -ENXIO;
+ }
+
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+ </section>
+
+ <section id="pci-resource-resource-allocation">
+ <title>Resource Allocation</title>
+ <para>
+ The allocation of I/O ports and irqs is done via standard kernel
+ functions. Unlike ALSA ver.0.5.x., there are no helpers for
+ that. And these resources must be released in the destructor
+ function (see below). Also, on ALSA 0.9.x, you don't need to
+ allocate (pseudo-)DMA for PCI like in ALSA 0.5.x.
+ </para>
+
+ <para>
+ Now assume that the PCI device has an I/O port with 8 bytes
+ and an interrupt. Then struct <structname>mychip</structname> will have the
+ following fields:
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ struct mychip {
+ struct snd_card *card;
+
+ unsigned long port;
+ int irq;
+ };
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ For an I/O port (and also a memory region), you need to have
+ the resource pointer for the standard resource management. For
+ an irq, you have to keep only the irq number (integer). But you
+ need to initialize this number as -1 before actual allocation,
+ since irq 0 is valid. The port address and its resource pointer
+ can be initialized as null by
+ <function>kzalloc()</function> automatically, so you
+ don't have to take care of resetting them.
+ </para>
+
+ <para>
+ The allocation of an I/O port is done like this:
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ err = pci_request_regions(pci, "My Chip");
+ if (err < 0) {
+ kfree(chip);
+ pci_disable_device(pci);
+ return err;
+ }
+ chip->port = pci_resource_start(pci, 0);
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ <!-- obsolete -->
+ It will reserve the I/O port region of 8 bytes of the given
+ PCI device. The returned value, chip->res_port, is allocated
+ via <function>kmalloc()</function> by
+ <function>request_region()</function>. The pointer must be
+ released via <function>kfree()</function>, but there is a
+ problem with this. This issue will be explained later.
+ </para>
+
+ <para>
+ The allocation of an interrupt source is done like this:
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ if (request_irq(pci->irq, snd_mychip_interrupt,
+ IRQF_SHARED, KBUILD_MODNAME, chip)) {
+ printk(KERN_ERR "cannot grab irq %d\n", pci->irq);
+ snd_mychip_free(chip);
+ return -EBUSY;
+ }
+ chip->irq = pci->irq;
+]]>
+ </programlisting>
+ </informalexample>
+
+ where <function>snd_mychip_interrupt()</function> is the
+ interrupt handler defined <link
+ linkend="pcm-interface-interrupt-handler"><citetitle>later</citetitle></link>.
+ Note that chip->irq should be defined
+ only when <function>request_irq()</function> succeeded.
+ </para>
+
+ <para>
+ On the PCI bus, interrupts can be shared. Thus,
+ <constant>IRQF_SHARED</constant> is used as the interrupt flag of
+ <function>request_irq()</function>.
+ </para>
+
+ <para>
+ The last argument of <function>request_irq()</function> is the
+ data pointer passed to the interrupt handler. Usually, the
+ chip-specific record is used for that, but you can use what you
+ like, too.
+ </para>
+
+ <para>
+ I won't give details about the interrupt handler at this
+ point, but at least its appearance can be explained now. The
+ interrupt handler looks usually like the following:
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id)
+ {
+ struct mychip *chip = dev_id;
+ ....
+ return IRQ_HANDLED;
+ }
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ Now let's write the corresponding destructor for the resources
+ above. The role of destructor is simple: disable the hardware
+ (if already activated) and release the resources. So far, we
+ have no hardware part, so the disabling code is not written here.
+ </para>
+
+ <para>
+ To release the resources, the <quote>check-and-release</quote>
+ method is a safer way. For the interrupt, do like this:
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ if (chip->irq >= 0)
+ free_irq(chip->irq, chip);
+]]>
+ </programlisting>
+ </informalexample>
+
+ Since the irq number can start from 0, you should initialize
+ chip->irq with a negative value (e.g. -1), so that you can
+ check the validity of the irq number as above.
+ </para>
+
+ <para>
+ When you requested I/O ports or memory regions via
+ <function>pci_request_region()</function> or
+ <function>pci_request_regions()</function> like in this example,
+ release the resource(s) using the corresponding function,
+ <function>pci_release_region()</function> or
+ <function>pci_release_regions()</function>.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ pci_release_regions(chip->pci);
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ When you requested manually via <function>request_region()</function>
+ or <function>request_mem_region</function>, you can release it via
+ <function>release_resource()</function>. Suppose that you keep
+ the resource pointer returned from <function>request_region()</function>
+ in chip->res_port, the release procedure looks like:
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ release_and_free_resource(chip->res_port);
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ Don't forget to call <function>pci_disable_device()</function>
+ before the end.
+ </para>
+
+ <para>
+ And finally, release the chip-specific record.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ kfree(chip);
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ We didn't implement the hardware disabling part in the above.
+ If you need to do this, please note that the destructor may be
+ called even before the initialization of the chip is completed.
+ It would be better to have a flag to skip hardware disabling
+ if the hardware was not initialized yet.
+ </para>
+
+ <para>
+ When the chip-data is assigned to the card using
+ <function>snd_device_new()</function> with
+ <constant>SNDRV_DEV_LOWLELVEL</constant> , its destructor is
+ called at the last. That is, it is assured that all other
+ components like PCMs and controls have already been released.
+ You don't have to stop PCMs, etc. explicitly, but just
+ call low-level hardware stopping.
+ </para>
+
+ <para>
+ The management of a memory-mapped region is almost as same as
+ the management of an I/O port. You'll need three fields like
+ the following:
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ struct mychip {
+ ....
+ unsigned long iobase_phys;
+ void __iomem *iobase_virt;
+ };
+]]>
+ </programlisting>
+ </informalexample>
+
+ and the allocation would be like below:
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ if ((err = pci_request_regions(pci, "My Chip")) < 0) {
+ kfree(chip);
+ return err;
+ }
+ chip->iobase_phys = pci_resource_start(pci, 0);
+ chip->iobase_virt = ioremap_nocache(chip->iobase_phys,
+ pci_resource_len(pci, 0));
+]]>
+ </programlisting>
+ </informalexample>
+
+ and the corresponding destructor would be:
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static int snd_mychip_free(struct mychip *chip)
+ {
+ ....
+ if (chip->iobase_virt)
+ iounmap(chip->iobase_virt);
+ ....
+ pci_release_regions(chip->pci);
+ ....
+ }
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ </section>
+
+ <section id="pci-resource-entries">
+ <title>PCI Entries</title>
+ <para>
+ So far, so good. Let's finish the missing PCI
+ stuff. At first, we need a
+ <structname>pci_device_id</structname> table for this
+ chipset. It's a table of PCI vendor/device ID number, and some
+ masks.
+ </para>
+
+ <para>
+ For example,
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static struct pci_device_id snd_mychip_ids[] = {
+ { PCI_VENDOR_ID_FOO, PCI_DEVICE_ID_BAR,
+ PCI_ANY_ID, PCI_ANY_ID, 0, 0, 0, },
+ ....
+ { 0, }
+ };
+ MODULE_DEVICE_TABLE(pci, snd_mychip_ids);
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ The first and second fields of
+ the <structname>pci_device_id</structname> structure are the vendor and
+ device IDs. If you have no reason to filter the matching
+ devices, you can leave the remaining fields as above. The last
+ field of the <structname>pci_device_id</structname> struct contains
+ private data for this entry. You can specify any value here, for
+ example, to define specific operations for supported device IDs.
+ Such an example is found in the intel8x0 driver.
+ </para>
+
+ <para>
+ The last entry of this list is the terminator. You must
+ specify this all-zero entry.
+ </para>
+
+ <para>
+ Then, prepare the <structname>pci_driver</structname> record:
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static struct pci_driver driver = {
+ .name = KBUILD_MODNAME,
+ .id_table = snd_mychip_ids,
+ .probe = snd_mychip_probe,
+ .remove = snd_mychip_remove,
+ };
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ The <structfield>probe</structfield> and
+ <structfield>remove</structfield> functions have already
+ been defined in the previous sections.
+ The <structfield>name</structfield>
+ field is the name string of this device. Note that you must not
+ use a slash <quote>/</quote> in this string.
+ </para>
+
+ <para>
+ And at last, the module entries:
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static int __init alsa_card_mychip_init(void)
+ {
+ return pci_register_driver(&driver);
+ }
+
+ static void __exit alsa_card_mychip_exit(void)
+ {
+ pci_unregister_driver(&driver);
+ }
+
+ module_init(alsa_card_mychip_init)
+ module_exit(alsa_card_mychip_exit)
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ Note that these module entries are tagged with
+ <parameter>__init</parameter> and
+ <parameter>__exit</parameter> prefixes.
+ </para>
+
+ <para>
+ Oh, one thing was forgotten. If you have no exported symbols,
+ you need to declare it in 2.2 or 2.4 kernels (it's not necessary in 2.6 kernels).
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ EXPORT_NO_SYMBOLS;
+]]>
+ </programlisting>
+ </informalexample>
+
+ That's all!
+ </para>
+ </section>
+ </chapter>
+
+
+<!-- ****************************************************** -->
+<!-- PCM Interface -->
+<!-- ****************************************************** -->
+ <chapter id="pcm-interface">
+ <title>PCM Interface</title>
+
+ <section id="pcm-interface-general">
+ <title>General</title>
+ <para>
+ The PCM middle layer of ALSA is quite powerful and it is only
+ necessary for each driver to implement the low-level functions
+ to access its hardware.
+ </para>
+
+ <para>
+ For accessing to the PCM layer, you need to include
+ <filename><sound/pcm.h></filename> first. In addition,
+ <filename><sound/pcm_params.h></filename> might be needed
+ if you access to some functions related with hw_param.
+ </para>
+
+ <para>
+ Each card device can have up to four pcm instances. A pcm
+ instance corresponds to a pcm device file. The limitation of
+ number of instances comes only from the available bit size of
+ the Linux's device numbers. Once when 64bit device number is
+ used, we'll have more pcm instances available.
+ </para>
+
+ <para>
+ A pcm instance consists of pcm playback and capture streams,
+ and each pcm stream consists of one or more pcm substreams. Some
+ soundcards support multiple playback functions. For example,
+ emu10k1 has a PCM playback of 32 stereo substreams. In this case, at
+ each open, a free substream is (usually) automatically chosen
+ and opened. Meanwhile, when only one substream exists and it was
+ already opened, the successful open will either block
+ or error with <constant>EAGAIN</constant> according to the
+ file open mode. But you don't have to care about such details in your
+ driver. The PCM middle layer will take care of such work.
+ </para>
+ </section>
+
+ <section id="pcm-interface-example">
+ <title>Full Code Example</title>
+ <para>
+ The example code below does not include any hardware access
+ routines but shows only the skeleton, how to build up the PCM
+ interfaces.
+
+ <example>
+ <title>PCM Example Code</title>
+ <programlisting>
+<![CDATA[
+ #include <sound/pcm.h>
+ ....
+
+ /* hardware definition */
+ static struct snd_pcm_hardware snd_mychip_playback_hw = {
+ .info = (SNDRV_PCM_INFO_MMAP |
+ SNDRV_PCM_INFO_INTERLEAVED |
+ SNDRV_PCM_INFO_BLOCK_TRANSFER |
+ SNDRV_PCM_INFO_MMAP_VALID),
+ .formats = SNDRV_PCM_FMTBIT_S16_LE,
+ .rates = SNDRV_PCM_RATE_8000_48000,
+ .rate_min = 8000,
+ .rate_max = 48000,
+ .channels_min = 2,
+ .channels_max = 2,
+ .buffer_bytes_max = 32768,
+ .period_bytes_min = 4096,
+ .period_bytes_max = 32768,
+ .periods_min = 1,
+ .periods_max = 1024,
+ };
+
+ /* hardware definition */
+ static struct snd_pcm_hardware snd_mychip_capture_hw = {
+ .info = (SNDRV_PCM_INFO_MMAP |
+ SNDRV_PCM_INFO_INTERLEAVED |
+ SNDRV_PCM_INFO_BLOCK_TRANSFER |
+ SNDRV_PCM_INFO_MMAP_VALID),
+ .formats = SNDRV_PCM_FMTBIT_S16_LE,
+ .rates = SNDRV_PCM_RATE_8000_48000,
+ .rate_min = 8000,
+ .rate_max = 48000,
+ .channels_min = 2,
+ .channels_max = 2,
+ .buffer_bytes_max = 32768,
+ .period_bytes_min = 4096,
+ .period_bytes_max = 32768,
+ .periods_min = 1,
+ .periods_max = 1024,
+ };
+
+ /* open callback */
+ static int snd_mychip_playback_open(struct snd_pcm_substream *substream)
+ {
+ struct mychip *chip = snd_pcm_substream_chip(substream);
+ struct snd_pcm_runtime *runtime = substream->runtime;
+
+ runtime->hw = snd_mychip_playback_hw;
+ /* more hardware-initialization will be done here */
+ ....
+ return 0;
+ }
+
+ /* close callback */
+ static int snd_mychip_playback_close(struct snd_pcm_substream *substream)
+ {
+ struct mychip *chip = snd_pcm_substream_chip(substream);
+ /* the hardware-specific codes will be here */
+ ....
+ return 0;
+
+ }
+
+ /* open callback */
+ static int snd_mychip_capture_open(struct snd_pcm_substream *substream)
+ {
+ struct mychip *chip = snd_pcm_substream_chip(substream);
+ struct snd_pcm_runtime *runtime = substream->runtime;
+
+ runtime->hw = snd_mychip_capture_hw;
+ /* more hardware-initialization will be done here */
+ ....
+ return 0;
+ }
+
+ /* close callback */
+ static int snd_mychip_capture_close(struct snd_pcm_substream *substream)
+ {
+ struct mychip *chip = snd_pcm_substream_chip(substream);
+ /* the hardware-specific codes will be here */
+ ....
+ return 0;
+
+ }
+
+ /* hw_params callback */
+ static int snd_mychip_pcm_hw_params(struct snd_pcm_substream *substream,
+ struct snd_pcm_hw_params *hw_params)
+ {
+ return snd_pcm_lib_malloc_pages(substream,
+ params_buffer_bytes(hw_params));
+ }
+
+ /* hw_free callback */
+ static int snd_mychip_pcm_hw_free(struct snd_pcm_substream *substream)
+ {
+ return snd_pcm_lib_free_pages(substream);
+ }
+
+ /* prepare callback */
+ static int snd_mychip_pcm_prepare(struct snd_pcm_substream *substream)
+ {
+ struct mychip *chip = snd_pcm_substream_chip(substream);
+ struct snd_pcm_runtime *runtime = substream->runtime;
+
+ /* set up the hardware with the current configuration
+ * for example...
+ */
+ mychip_set_sample_format(chip, runtime->format);
+ mychip_set_sample_rate(chip, runtime->rate);
+ mychip_set_channels(chip, runtime->channels);
+ mychip_set_dma_setup(chip, runtime->dma_addr,
+ chip->buffer_size,
+ chip->period_size);
+ return 0;
+ }
+
+ /* trigger callback */
+ static int snd_mychip_pcm_trigger(struct snd_pcm_substream *substream,
+ int cmd)
+ {
+ switch (cmd) {
+ case SNDRV_PCM_TRIGGER_START:
+ /* do something to start the PCM engine */
+ ....
+ break;
+ case SNDRV_PCM_TRIGGER_STOP:
+ /* do something to stop the PCM engine */
+ ....
+ break;
+ default:
+ return -EINVAL;
+ }
+ }
+
+ /* pointer callback */
+ static snd_pcm_uframes_t
+ snd_mychip_pcm_pointer(struct snd_pcm_substream *substream)
+ {
+ struct mychip *chip = snd_pcm_substream_chip(substream);
+ unsigned int current_ptr;
+
+ /* get the current hardware pointer */
+ current_ptr = mychip_get_hw_pointer(chip);
+ return current_ptr;
+ }
+
+ /* operators */
+ static struct snd_pcm_ops snd_mychip_playback_ops = {
+ .open = snd_mychip_playback_open,
+ .close = snd_mychip_playback_close,
+ .ioctl = snd_pcm_lib_ioctl,
+ .hw_params = snd_mychip_pcm_hw_params,
+ .hw_free = snd_mychip_pcm_hw_free,
+ .prepare = snd_mychip_pcm_prepare,
+ .trigger = snd_mychip_pcm_trigger,
+ .pointer = snd_mychip_pcm_pointer,
+ };
+
+ /* operators */
+ static struct snd_pcm_ops snd_mychip_capture_ops = {
+ .open = snd_mychip_capture_open,
+ .close = snd_mychip_capture_close,
+ .ioctl = snd_pcm_lib_ioctl,
+ .hw_params = snd_mychip_pcm_hw_params,
+ .hw_free = snd_mychip_pcm_hw_free,
+ .prepare = snd_mychip_pcm_prepare,
+ .trigger = snd_mychip_pcm_trigger,
+ .pointer = snd_mychip_pcm_pointer,
+ };
+
+ /*
+ * definitions of capture are omitted here...
+ */
+
+ /* create a pcm device */
+ static int snd_mychip_new_pcm(struct mychip *chip)
+ {
+ struct snd_pcm *pcm;
+ int err;
+
+ err = snd_pcm_new(chip->card, "My Chip", 0, 1, 1, &pcm);
+ if (err < 0)
+ return err;
+ pcm->private_data = chip;
+ strcpy(pcm->name, "My Chip");
+ chip->pcm = pcm;
+ /* set operators */
+ snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK,
+ &snd_mychip_playback_ops);
+ snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_CAPTURE,
+ &snd_mychip_capture_ops);
+ /* pre-allocation of buffers */
+ /* NOTE: this may fail */
+ snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_DEV,
+ snd_dma_pci_data(chip->pci),
+ 64*1024, 64*1024);
+ return 0;
+ }
+]]>
+ </programlisting>
+ </example>
+ </para>
+ </section>
+
+ <section id="pcm-interface-constructor">
+ <title>Constructor</title>
+ <para>
+ A pcm instance is allocated by the <function>snd_pcm_new()</function>
+ function. It would be better to create a constructor for pcm,
+ namely,
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static int snd_mychip_new_pcm(struct mychip *chip)
+ {
+ struct snd_pcm *pcm;
+ int err;
+
+ err = snd_pcm_new(chip->card, "My Chip", 0, 1, 1, &pcm);
+ if (err < 0)
+ return err;
+ pcm->private_data = chip;
+ strcpy(pcm->name, "My Chip");
+ chip->pcm = pcm;
+ ....
+ return 0;
+ }
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ The <function>snd_pcm_new()</function> function takes four
+ arguments. The first argument is the card pointer to which this
+ pcm is assigned, and the second is the ID string.
+ </para>
+
+ <para>
+ The third argument (<parameter>index</parameter>, 0 in the
+ above) is the index of this new pcm. It begins from zero. If
+ you create more than one pcm instances, specify the
+ different numbers in this argument. For example,
+ <parameter>index</parameter> = 1 for the second PCM device.
+ </para>
+
+ <para>
+ The fourth and fifth arguments are the number of substreams
+ for playback and capture, respectively. Here 1 is used for
+ both arguments. When no playback or capture substreams are available,
+ pass 0 to the corresponding argument.
+ </para>
+
+ <para>
+ If a chip supports multiple playbacks or captures, you can
+ specify more numbers, but they must be handled properly in
+ open/close, etc. callbacks. When you need to know which
+ substream you are referring to, then it can be obtained from
+ struct <structname>snd_pcm_substream</structname> data passed to each callback
+ as follows:
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ struct snd_pcm_substream *substream;
+ int index = substream->number;
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ After the pcm is created, you need to set operators for each
+ pcm stream.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK,
+ &snd_mychip_playback_ops);
+ snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_CAPTURE,
+ &snd_mychip_capture_ops);
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ The operators are defined typically like this:
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static struct snd_pcm_ops snd_mychip_playback_ops = {
+ .open = snd_mychip_pcm_open,
+ .close = snd_mychip_pcm_close,
+ .ioctl = snd_pcm_lib_ioctl,
+ .hw_params = snd_mychip_pcm_hw_params,
+ .hw_free = snd_mychip_pcm_hw_free,
+ .prepare = snd_mychip_pcm_prepare,
+ .trigger = snd_mychip_pcm_trigger,
+ .pointer = snd_mychip_pcm_pointer,
+ };
+]]>
+ </programlisting>
+ </informalexample>
+
+ All the callbacks are described in the
+ <link linkend="pcm-interface-operators"><citetitle>
+ Operators</citetitle></link> subsection.
+ </para>
+
+ <para>
+ After setting the operators, you probably will want to
+ pre-allocate the buffer. For the pre-allocation, simply call
+ the following:
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_DEV,
+ snd_dma_pci_data(chip->pci),
+ 64*1024, 64*1024);
+]]>
+ </programlisting>
+ </informalexample>
+
+ It will allocate a buffer up to 64kB as default.
+ Buffer management details will be described in the later section <link
+ linkend="buffer-and-memory"><citetitle>Buffer and Memory
+ Management</citetitle></link>.
+ </para>
+
+ <para>
+ Additionally, you can set some extra information for this pcm
+ in pcm->info_flags.
+ The available values are defined as
+ <constant>SNDRV_PCM_INFO_XXX</constant> in
+ <filename><sound/asound.h></filename>, which is used for
+ the hardware definition (described later). When your soundchip
+ supports only half-duplex, specify like this:
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ pcm->info_flags = SNDRV_PCM_INFO_HALF_DUPLEX;
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+ </section>
+
+ <section id="pcm-interface-destructor">
+ <title>... And the Destructor?</title>
+ <para>
+ The destructor for a pcm instance is not always
+ necessary. Since the pcm device will be released by the middle
+ layer code automatically, you don't have to call the destructor
+ explicitly.
+ </para>
+
+ <para>
+ The destructor would be necessary if you created
+ special records internally and needed to release them. In such a
+ case, set the destructor function to
+ pcm->private_free:
+
+ <example>
+ <title>PCM Instance with a Destructor</title>
+ <programlisting>
+<![CDATA[
+ static void mychip_pcm_free(struct snd_pcm *pcm)
+ {
+ struct mychip *chip = snd_pcm_chip(pcm);
+ /* free your own data */
+ kfree(chip->my_private_pcm_data);
+ /* do what you like else */
+ ....
+ }
+
+ static int snd_mychip_new_pcm(struct mychip *chip)
+ {
+ struct snd_pcm *pcm;
+ ....
+ /* allocate your own data */
+ chip->my_private_pcm_data = kmalloc(...);
+ /* set the destructor */
+ pcm->private_data = chip;
+ pcm->private_free = mychip_pcm_free;
+ ....
+ }
+]]>
+ </programlisting>
+ </example>
+ </para>
+ </section>
+
+ <section id="pcm-interface-runtime">
+ <title>Runtime Pointer - The Chest of PCM Information</title>
+ <para>
+ When the PCM substream is opened, a PCM runtime instance is
+ allocated and assigned to the substream. This pointer is
+ accessible via <constant>substream->runtime</constant>.
+ This runtime pointer holds most information you need
+ to control the PCM: the copy of hw_params and sw_params configurations, the buffer
+ pointers, mmap records, spinlocks, etc.
+ </para>
+
+ <para>
+ The definition of runtime instance is found in
+ <filename><sound/pcm.h></filename>. Here are
+ the contents of this file:
+ <informalexample>
+ <programlisting>
+<![CDATA[
+struct _snd_pcm_runtime {
+ /* -- Status -- */
+ struct snd_pcm_substream *trigger_master;
+ snd_timestamp_t trigger_tstamp; /* trigger timestamp */
+ int overrange;
+ snd_pcm_uframes_t avail_max;
+ snd_pcm_uframes_t hw_ptr_base; /* Position at buffer restart */
+ snd_pcm_uframes_t hw_ptr_interrupt; /* Position at interrupt time*/
+
+ /* -- HW params -- */
+ snd_pcm_access_t access; /* access mode */
+ snd_pcm_format_t format; /* SNDRV_PCM_FORMAT_* */
+ snd_pcm_subformat_t subformat; /* subformat */
+ unsigned int rate; /* rate in Hz */
+ unsigned int channels; /* channels */
+ snd_pcm_uframes_t period_size; /* period size */
+ unsigned int periods; /* periods */
+ snd_pcm_uframes_t buffer_size; /* buffer size */
+ unsigned int tick_time; /* tick time */
+ snd_pcm_uframes_t min_align; /* Min alignment for the format */
+ size_t byte_align;
+ unsigned int frame_bits;
+ unsigned int sample_bits;
+ unsigned int info;
+ unsigned int rate_num;
+ unsigned int rate_den;
+
+ /* -- SW params -- */
+ struct timespec tstamp_mode; /* mmap timestamp is updated */
+ unsigned int period_step;
+ unsigned int sleep_min; /* min ticks to sleep */
+ snd_pcm_uframes_t start_threshold;
+ snd_pcm_uframes_t stop_threshold;
+ snd_pcm_uframes_t silence_threshold; /* Silence filling happens when
+ noise is nearest than this */
+ snd_pcm_uframes_t silence_size; /* Silence filling size */
+ snd_pcm_uframes_t boundary; /* pointers wrap point */
+
+ snd_pcm_uframes_t silenced_start;
+ snd_pcm_uframes_t silenced_size;
+
+ snd_pcm_sync_id_t sync; /* hardware synchronization ID */
+
+ /* -- mmap -- */
+ volatile struct snd_pcm_mmap_status *status;
+ volatile struct snd_pcm_mmap_control *control;
+ atomic_t mmap_count;
+
+ /* -- locking / scheduling -- */
+ spinlock_t lock;
+ wait_queue_head_t sleep;
+ struct timer_list tick_timer;
+ struct fasync_struct *fasync;
+
+ /* -- private section -- */
+ void *private_data;
+ void (*private_free)(struct snd_pcm_runtime *runtime);
+
+ /* -- hardware description -- */
+ struct snd_pcm_hardware hw;
+ struct snd_pcm_hw_constraints hw_constraints;
+
+ /* -- interrupt callbacks -- */
+ void (*transfer_ack_begin)(struct snd_pcm_substream *substream);
+ void (*transfer_ack_end)(struct snd_pcm_substream *substream);
+
+ /* -- timer -- */
+ unsigned int timer_resolution; /* timer resolution */
+
+ /* -- DMA -- */
+ unsigned char *dma_area; /* DMA area */
+ dma_addr_t dma_addr; /* physical bus address (not accessible from main CPU) */
+ size_t dma_bytes; /* size of DMA area */
+
+ struct snd_dma_buffer *dma_buffer_p; /* allocated buffer */
+
+#if defined(CONFIG_SND_PCM_OSS) || defined(CONFIG_SND_PCM_OSS_MODULE)
+ /* -- OSS things -- */
+ struct snd_pcm_oss_runtime oss;
+#endif
+};
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ For the operators (callbacks) of each sound driver, most of
+ these records are supposed to be read-only. Only the PCM
+ middle-layer changes / updates them. The exceptions are
+ the hardware description (hw), interrupt callbacks
+ (transfer_ack_xxx), DMA buffer information, and the private
+ data. Besides, if you use the standard buffer allocation
+ method via <function>snd_pcm_lib_malloc_pages()</function>,
+ you don't need to set the DMA buffer information by yourself.
+ </para>
+
+ <para>
+ In the sections below, important records are explained.
+ </para>
+
+ <section id="pcm-interface-runtime-hw">
+ <title>Hardware Description</title>
+ <para>
+ The hardware descriptor (struct <structname>snd_pcm_hardware</structname>)
+ contains the definitions of the fundamental hardware
+ configuration. Above all, you'll need to define this in
+ <link linkend="pcm-interface-operators-open-callback"><citetitle>
+ the open callback</citetitle></link>.
+ Note that the runtime instance holds the copy of the
+ descriptor, not the pointer to the existing descriptor. That
+ is, in the open callback, you can modify the copied descriptor
+ (<constant>runtime->hw</constant>) as you need. For example, if the maximum
+ number of channels is 1 only on some chip models, you can
+ still use the same hardware descriptor and change the
+ channels_max later:
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ struct snd_pcm_runtime *runtime = substream->runtime;
+ ...
+ runtime->hw = snd_mychip_playback_hw; /* common definition */
+ if (chip->model == VERY_OLD_ONE)
+ runtime->hw.channels_max = 1;
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ Typically, you'll have a hardware descriptor as below:
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static struct snd_pcm_hardware snd_mychip_playback_hw = {
+ .info = (SNDRV_PCM_INFO_MMAP |
+ SNDRV_PCM_INFO_INTERLEAVED |
+ SNDRV_PCM_INFO_BLOCK_TRANSFER |
+ SNDRV_PCM_INFO_MMAP_VALID),
+ .formats = SNDRV_PCM_FMTBIT_S16_LE,
+ .rates = SNDRV_PCM_RATE_8000_48000,
+ .rate_min = 8000,
+ .rate_max = 48000,
+ .channels_min = 2,
+ .channels_max = 2,
+ .buffer_bytes_max = 32768,
+ .period_bytes_min = 4096,
+ .period_bytes_max = 32768,
+ .periods_min = 1,
+ .periods_max = 1024,
+ };
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ <itemizedlist>
+ <listitem><para>
+ The <structfield>info</structfield> field contains the type and
+ capabilities of this pcm. The bit flags are defined in
+ <filename><sound/asound.h></filename> as
+ <constant>SNDRV_PCM_INFO_XXX</constant>. Here, at least, you
+ have to specify whether the mmap is supported and which
+ interleaved format is supported.
+ When the hardware supports mmap, add the
+ <constant>SNDRV_PCM_INFO_MMAP</constant> flag here. When the
+ hardware supports the interleaved or the non-interleaved
+ formats, <constant>SNDRV_PCM_INFO_INTERLEAVED</constant> or
+ <constant>SNDRV_PCM_INFO_NONINTERLEAVED</constant> flag must
+ be set, respectively. If both are supported, you can set both,
+ too.
+ </para>
+
+ <para>
+ In the above example, <constant>MMAP_VALID</constant> and
+ <constant>BLOCK_TRANSFER</constant> are specified for the OSS mmap
+ mode. Usually both are set. Of course,
+ <constant>MMAP_VALID</constant> is set only if the mmap is
+ really supported.
+ </para>
+
+ <para>
+ The other possible flags are
+ <constant>SNDRV_PCM_INFO_PAUSE</constant> and
+ <constant>SNDRV_PCM_INFO_RESUME</constant>. The
+ <constant>PAUSE</constant> bit means that the pcm supports the
+ <quote>pause</quote> operation, while the
+ <constant>RESUME</constant> bit means that the pcm supports
+ the full <quote>suspend/resume</quote> operation.
+ If the <constant>PAUSE</constant> flag is set,
+ the <structfield>trigger</structfield> callback below
+ must handle the corresponding (pause push/release) commands.
+ The suspend/resume trigger commands can be defined even without
+ the <constant>RESUME</constant> flag. See <link
+ linkend="power-management"><citetitle>
+ Power Management</citetitle></link> section for details.
+ </para>
+
+ <para>
+ When the PCM substreams can be synchronized (typically,
+ synchronized start/stop of a playback and a capture streams),
+ you can give <constant>SNDRV_PCM_INFO_SYNC_START</constant>,
+ too. In this case, you'll need to check the linked-list of
+ PCM substreams in the trigger callback. This will be
+ described in the later section.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <structfield>formats</structfield> field contains the bit-flags
+ of supported formats (<constant>SNDRV_PCM_FMTBIT_XXX</constant>).
+ If the hardware supports more than one format, give all or'ed
+ bits. In the example above, the signed 16bit little-endian
+ format is specified.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <structfield>rates</structfield> field contains the bit-flags of
+ supported rates (<constant>SNDRV_PCM_RATE_XXX</constant>).
+ When the chip supports continuous rates, pass
+ <constant>CONTINUOUS</constant> bit additionally.
+ The pre-defined rate bits are provided only for typical
+ rates. If your chip supports unconventional rates, you need to add
+ the <constant>KNOT</constant> bit and set up the hardware
+ constraint manually (explained later).
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <structfield>rate_min</structfield> and
+ <structfield>rate_max</structfield> define the minimum and
+ maximum sample rate. This should correspond somehow to
+ <structfield>rates</structfield> bits.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <structfield>channel_min</structfield> and
+ <structfield>channel_max</structfield>
+ define, as you might already expected, the minimum and maximum
+ number of channels.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ <structfield>buffer_bytes_max</structfield> defines the
+ maximum buffer size in bytes. There is no
+ <structfield>buffer_bytes_min</structfield> field, since
+ it can be calculated from the minimum period size and the
+ minimum number of periods.
+ Meanwhile, <structfield>period_bytes_min</structfield> and
+ define the minimum and maximum size of the period in bytes.
+ <structfield>periods_max</structfield> and
+ <structfield>periods_min</structfield> define the maximum and
+ minimum number of periods in the buffer.
+ </para>
+
+ <para>
+ The <quote>period</quote> is a term that corresponds to
+ a fragment in the OSS world. The period defines the size at
+ which a PCM interrupt is generated. This size strongly
+ depends on the hardware.
+ Generally, the smaller period size will give you more
+ interrupts, that is, more controls.
+ In the case of capture, this size defines the input latency.
+ On the other hand, the whole buffer size defines the
+ output latency for the playback direction.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ There is also a field <structfield>fifo_size</structfield>.
+ This specifies the size of the hardware FIFO, but currently it
+ is neither used in the driver nor in the alsa-lib. So, you
+ can ignore this field.
+ </para>
+ </listitem>
+ </itemizedlist>
+ </para>
+ </section>
+
+ <section id="pcm-interface-runtime-config">
+ <title>PCM Configurations</title>
+ <para>
+ Ok, let's go back again to the PCM runtime records.
+ The most frequently referred records in the runtime instance are
+ the PCM configurations.
+ The PCM configurations are stored in the runtime instance
+ after the application sends <type>hw_params</type> data via
+ alsa-lib. There are many fields copied from hw_params and
+ sw_params structs. For example,
+ <structfield>format</structfield> holds the format type
+ chosen by the application. This field contains the enum value
+ <constant>SNDRV_PCM_FORMAT_XXX</constant>.
+ </para>
+
+ <para>
+ One thing to be noted is that the configured buffer and period
+ sizes are stored in <quote>frames</quote> in the runtime.
+ In the ALSA world, 1 frame = channels * samples-size.
+ For conversion between frames and bytes, you can use the
+ <function>frames_to_bytes()</function> and
+ <function>bytes_to_frames()</function> helper functions.
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ period_bytes = frames_to_bytes(runtime, runtime->period_size);
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ Also, many software parameters (sw_params) are
+ stored in frames, too. Please check the type of the field.
+ <type>snd_pcm_uframes_t</type> is for the frames as unsigned
+ integer while <type>snd_pcm_sframes_t</type> is for the frames
+ as signed integer.
+ </para>
+ </section>
+
+ <section id="pcm-interface-runtime-dma">
+ <title>DMA Buffer Information</title>
+ <para>
+ The DMA buffer is defined by the following four fields,
+ <structfield>dma_area</structfield>,
+ <structfield>dma_addr</structfield>,
+ <structfield>dma_bytes</structfield> and
+ <structfield>dma_private</structfield>.
+ The <structfield>dma_area</structfield> holds the buffer
+ pointer (the logical address). You can call
+ <function>memcpy</function> from/to
+ this pointer. Meanwhile, <structfield>dma_addr</structfield>
+ holds the physical address of the buffer. This field is
+ specified only when the buffer is a linear buffer.
+ <structfield>dma_bytes</structfield> holds the size of buffer
+ in bytes. <structfield>dma_private</structfield> is used for
+ the ALSA DMA allocator.
+ </para>
+
+ <para>
+ If you use a standard ALSA function,
+ <function>snd_pcm_lib_malloc_pages()</function>, for
+ allocating the buffer, these fields are set by the ALSA middle
+ layer, and you should <emphasis>not</emphasis> change them by
+ yourself. You can read them but not write them.
+ On the other hand, if you want to allocate the buffer by
+ yourself, you'll need to manage it in hw_params callback.
+ At least, <structfield>dma_bytes</structfield> is mandatory.
+ <structfield>dma_area</structfield> is necessary when the
+ buffer is mmapped. If your driver doesn't support mmap, this
+ field is not necessary. <structfield>dma_addr</structfield>
+ is also optional. You can use
+ <structfield>dma_private</structfield> as you like, too.
+ </para>
+ </section>
+
+ <section id="pcm-interface-runtime-status">
+ <title>Running Status</title>
+ <para>
+ The running status can be referred via <constant>runtime->status</constant>.
+ This is the pointer to the struct <structname>snd_pcm_mmap_status</structname>
+ record. For example, you can get the current DMA hardware
+ pointer via <constant>runtime->status->hw_ptr</constant>.
+ </para>
+
+ <para>
+ The DMA application pointer can be referred via
+ <constant>runtime->control</constant>, which points to the
+ struct <structname>snd_pcm_mmap_control</structname> record.
+ However, accessing directly to this value is not recommended.
+ </para>
+ </section>
+
+ <section id="pcm-interface-runtime-private">
+ <title>Private Data</title>
+ <para>
+ You can allocate a record for the substream and store it in
+ <constant>runtime->private_data</constant>. Usually, this
+ is done in
+ <link linkend="pcm-interface-operators-open-callback"><citetitle>
+ the open callback</citetitle></link>.
+ Don't mix this with <constant>pcm->private_data</constant>.
+ The <constant>pcm->private_data</constant> usually points to the
+ chip instance assigned statically at the creation of PCM, while the
+ <constant>runtime->private_data</constant> points to a dynamic
+ data structure created at the PCM open callback.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static int snd_xxx_open(struct snd_pcm_substream *substream)
+ {
+ struct my_pcm_data *data;
+ ....
+ data = kmalloc(sizeof(*data), GFP_KERNEL);
+ substream->runtime->private_data = data;
+ ....
+ }
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ The allocated object must be released in
+ <link linkend="pcm-interface-operators-open-callback"><citetitle>
+ the close callback</citetitle></link>.
+ </para>
+ </section>
+
+ <section id="pcm-interface-runtime-intr">
+ <title>Interrupt Callbacks</title>
+ <para>
+ The field <structfield>transfer_ack_begin</structfield> and
+ <structfield>transfer_ack_end</structfield> are called at
+ the beginning and at the end of
+ <function>snd_pcm_period_elapsed()</function>, respectively.
+ </para>
+ </section>
+
+ </section>
+
+ <section id="pcm-interface-operators">
+ <title>Operators</title>
+ <para>
+ OK, now let me give details about each pcm callback
+ (<parameter>ops</parameter>). In general, every callback must
+ return 0 if successful, or a negative error number
+ such as <constant>-EINVAL</constant>. To choose an appropriate
+ error number, it is advised to check what value other parts of
+ the kernel return when the same kind of request fails.
+ </para>
+
+ <para>
+ The callback function takes at least the argument with
+ <structname>snd_pcm_substream</structname> pointer. To retrieve
+ the chip record from the given substream instance, you can use the
+ following macro.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ int xxx() {
+ struct mychip *chip = snd_pcm_substream_chip(substream);
+ ....
+ }
+]]>
+ </programlisting>
+ </informalexample>
+
+ The macro reads <constant>substream->private_data</constant>,
+ which is a copy of <constant>pcm->private_data</constant>.
+ You can override the former if you need to assign different data
+ records per PCM substream. For example, the cmi8330 driver assigns
+ different private_data for playback and capture directions,
+ because it uses two different codecs (SB- and AD-compatible) for
+ different directions.
+ </para>
+
+ <section id="pcm-interface-operators-open-callback">
+ <title>open callback</title>
+ <para>
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static int snd_xxx_open(struct snd_pcm_substream *substream);
+]]>
+ </programlisting>
+ </informalexample>
+
+ This is called when a pcm substream is opened.
+ </para>
+
+ <para>
+ At least, here you have to initialize the runtime->hw
+ record. Typically, this is done by like this:
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static int snd_xxx_open(struct snd_pcm_substream *substream)
+ {
+ struct mychip *chip = snd_pcm_substream_chip(substream);
+ struct snd_pcm_runtime *runtime = substream->runtime;
+
+ runtime->hw = snd_mychip_playback_hw;
+ return 0;
+ }
+]]>
+ </programlisting>
+ </informalexample>
+
+ where <parameter>snd_mychip_playback_hw</parameter> is the
+ pre-defined hardware description.
+ </para>
+
+ <para>
+ You can allocate a private data in this callback, as described
+ in <link linkend="pcm-interface-runtime-private"><citetitle>
+ Private Data</citetitle></link> section.
+ </para>
+
+ <para>
+ If the hardware configuration needs more constraints, set the
+ hardware constraints here, too.
+ See <link linkend="pcm-interface-constraints"><citetitle>
+ Constraints</citetitle></link> for more details.
+ </para>
+ </section>
+
+ <section id="pcm-interface-operators-close-callback">
+ <title>close callback</title>
+ <para>
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static int snd_xxx_close(struct snd_pcm_substream *substream);
+]]>
+ </programlisting>
+ </informalexample>
+
+ Obviously, this is called when a pcm substream is closed.
+ </para>
+
+ <para>
+ Any private instance for a pcm substream allocated in the
+ open callback will be released here.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static int snd_xxx_close(struct snd_pcm_substream *substream)
+ {
+ ....
+ kfree(substream->runtime->private_data);
+ ....
+ }
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+ </section>
+
+ <section id="pcm-interface-operators-ioctl-callback">
+ <title>ioctl callback</title>
+ <para>
+ This is used for any special call to pcm ioctls. But
+ usually you can pass a generic ioctl callback,
+ <function>snd_pcm_lib_ioctl</function>.
+ </para>
+ </section>
+
+ <section id="pcm-interface-operators-hw-params-callback">
+ <title>hw_params callback</title>
+ <para>
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static int snd_xxx_hw_params(struct snd_pcm_substream *substream,
+ struct snd_pcm_hw_params *hw_params);
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ This is called when the hardware parameter
+ (<structfield>hw_params</structfield>) is set
+ up by the application,
+ that is, once when the buffer size, the period size, the
+ format, etc. are defined for the pcm substream.
+ </para>
+
+ <para>
+ Many hardware setups should be done in this callback,
+ including the allocation of buffers.
+ </para>
+
+ <para>
+ Parameters to be initialized are retrieved by
+ <function>params_xxx()</function> macros. To allocate
+ buffer, you can call a helper function,
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ snd_pcm_lib_malloc_pages(substream, params_buffer_bytes(hw_params));
+]]>
+ </programlisting>
+ </informalexample>
+
+ <function>snd_pcm_lib_malloc_pages()</function> is available
+ only when the DMA buffers have been pre-allocated.
+ See the section <link
+ linkend="buffer-and-memory-buffer-types"><citetitle>
+ Buffer Types</citetitle></link> for more details.
+ </para>
+
+ <para>
+ Note that this and <structfield>prepare</structfield> callbacks
+ may be called multiple times per initialization.
+ For example, the OSS emulation may
+ call these callbacks at each change via its ioctl.
+ </para>
+
+ <para>
+ Thus, you need to be careful not to allocate the same buffers
+ many times, which will lead to memory leaks! Calling the
+ helper function above many times is OK. It will release the
+ previous buffer automatically when it was already allocated.
+ </para>
+
+ <para>
+ Another note is that this callback is non-atomic
+ (schedulable) as default, i.e. when no
+ <structfield>nonatomic</structfield> flag set.
+ This is important, because the
+ <structfield>trigger</structfield> callback
+ is atomic (non-schedulable). That is, mutexes or any
+ schedule-related functions are not available in
+ <structfield>trigger</structfield> callback.
+ Please see the subsection
+ <link linkend="pcm-interface-atomicity"><citetitle>
+ Atomicity</citetitle></link> for details.
+ </para>
+ </section>
+
+ <section id="pcm-interface-operators-hw-free-callback">
+ <title>hw_free callback</title>
+ <para>
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static int snd_xxx_hw_free(struct snd_pcm_substream *substream);
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ This is called to release the resources allocated via
+ <structfield>hw_params</structfield>. For example, releasing the
+ buffer via
+ <function>snd_pcm_lib_malloc_pages()</function> is done by
+ calling the following:
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ snd_pcm_lib_free_pages(substream);
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ This function is always called before the close callback is called.
+ Also, the callback may be called multiple times, too.
+ Keep track whether the resource was already released.
+ </para>
+ </section>
+
+ <section id="pcm-interface-operators-prepare-callback">
+ <title>prepare callback</title>
+ <para>
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static int snd_xxx_prepare(struct snd_pcm_substream *substream);
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ This callback is called when the pcm is
+ <quote>prepared</quote>. You can set the format type, sample
+ rate, etc. here. The difference from
+ <structfield>hw_params</structfield> is that the
+ <structfield>prepare</structfield> callback will be called each
+ time
+ <function>snd_pcm_prepare()</function> is called, i.e. when
+ recovering after underruns, etc.
+ </para>
+
+ <para>
+ Note that this callback is now non-atomic.
+ You can use schedule-related functions safely in this callback.
+ </para>
+
+ <para>
+ In this and the following callbacks, you can refer to the
+ values via the runtime record,
+ substream->runtime.
+ For example, to get the current
+ rate, format or channels, access to
+ runtime->rate,
+ runtime->format or
+ runtime->channels, respectively.
+ The physical address of the allocated buffer is set to
+ runtime->dma_area. The buffer and period sizes are
+ in runtime->buffer_size and runtime->period_size,
+ respectively.
+ </para>
+
+ <para>
+ Be careful that this callback will be called many times at
+ each setup, too.
+ </para>
+ </section>
+
+ <section id="pcm-interface-operators-trigger-callback">
+ <title>trigger callback</title>
+ <para>
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static int snd_xxx_trigger(struct snd_pcm_substream *substream, int cmd);
+]]>
+ </programlisting>
+ </informalexample>
+
+ This is called when the pcm is started, stopped or paused.
+ </para>
+
+ <para>
+ Which action is specified in the second argument,
+ <constant>SNDRV_PCM_TRIGGER_XXX</constant> in
+ <filename><sound/pcm.h></filename>. At least,
+ the <constant>START</constant> and <constant>STOP</constant>
+ commands must be defined in this callback.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ switch (cmd) {
+ case SNDRV_PCM_TRIGGER_START:
+ /* do something to start the PCM engine */
+ break;
+ case SNDRV_PCM_TRIGGER_STOP:
+ /* do something to stop the PCM engine */
+ break;
+ default:
+ return -EINVAL;
+ }
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ When the pcm supports the pause operation (given in the info
+ field of the hardware table), the <constant>PAUSE_PUSH</constant>
+ and <constant>PAUSE_RELEASE</constant> commands must be
+ handled here, too. The former is the command to pause the pcm,
+ and the latter to restart the pcm again.
+ </para>
+
+ <para>
+ When the pcm supports the suspend/resume operation,
+ regardless of full or partial suspend/resume support,
+ the <constant>SUSPEND</constant> and <constant>RESUME</constant>
+ commands must be handled, too.
+ These commands are issued when the power-management status is
+ changed. Obviously, the <constant>SUSPEND</constant> and
+ <constant>RESUME</constant> commands
+ suspend and resume the pcm substream, and usually, they
+ are identical to the <constant>STOP</constant> and
+ <constant>START</constant> commands, respectively.
+ See the <link linkend="power-management"><citetitle>
+ Power Management</citetitle></link> section for details.
+ </para>
+
+ <para>
+ As mentioned, this callback is atomic as default unless
+ <structfield>nonatomic</structfield> flag set, and
+ you cannot call functions which may sleep.
+ The trigger callback should be as minimal as possible,
+ just really triggering the DMA. The other stuff should be
+ initialized hw_params and prepare callbacks properly
+ beforehand.
+ </para>
+ </section>
+
+ <section id="pcm-interface-operators-pointer-callback">
+ <title>pointer callback</title>
+ <para>
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static snd_pcm_uframes_t snd_xxx_pointer(struct snd_pcm_substream *substream)
+]]>
+ </programlisting>
+ </informalexample>
+
+ This callback is called when the PCM middle layer inquires
+ the current hardware position on the buffer. The position must
+ be returned in frames,
+ ranging from 0 to buffer_size - 1.
+ </para>
+
+ <para>
+ This is called usually from the buffer-update routine in the
+ pcm middle layer, which is invoked when
+ <function>snd_pcm_period_elapsed()</function> is called in the
+ interrupt routine. Then the pcm middle layer updates the
+ position and calculates the available space, and wakes up the
+ sleeping poll threads, etc.
+ </para>
+
+ <para>
+ This callback is also atomic as default.
+ </para>
+ </section>
+
+ <section id="pcm-interface-operators-copy-silence">
+ <title>copy and silence callbacks</title>
+ <para>
+ These callbacks are not mandatory, and can be omitted in
+ most cases. These callbacks are used when the hardware buffer
+ cannot be in the normal memory space. Some chips have their
+ own buffer on the hardware which is not mappable. In such a
+ case, you have to transfer the data manually from the memory
+ buffer to the hardware buffer. Or, if the buffer is
+ non-contiguous on both physical and virtual memory spaces,
+ these callbacks must be defined, too.
+ </para>
+
+ <para>
+ If these two callbacks are defined, copy and set-silence
+ operations are done by them. The detailed will be described in
+ the later section <link
+ linkend="buffer-and-memory"><citetitle>Buffer and Memory
+ Management</citetitle></link>.
+ </para>
+ </section>
+
+ <section id="pcm-interface-operators-ack">
+ <title>ack callback</title>
+ <para>
+ This callback is also not mandatory. This callback is called
+ when the appl_ptr is updated in read or write operations.
+ Some drivers like emu10k1-fx and cs46xx need to track the
+ current appl_ptr for the internal buffer, and this callback
+ is useful only for such a purpose.
+ </para>
+ <para>
+ This callback is atomic as default.
+ </para>
+ </section>
+
+ <section id="pcm-interface-operators-page-callback">
+ <title>page callback</title>
+
+ <para>
+ This callback is optional too. This callback is used
+ mainly for non-contiguous buffers. The mmap calls this
+ callback to get the page address. Some examples will be
+ explained in the later section <link
+ linkend="buffer-and-memory"><citetitle>Buffer and Memory
+ Management</citetitle></link>, too.
+ </para>
+ </section>
+ </section>
+
+ <section id="pcm-interface-interrupt-handler">
+ <title>Interrupt Handler</title>
+ <para>
+ The rest of pcm stuff is the PCM interrupt handler. The
+ role of PCM interrupt handler in the sound driver is to update
+ the buffer position and to tell the PCM middle layer when the
+ buffer position goes across the prescribed period size. To
+ inform this, call the <function>snd_pcm_period_elapsed()</function>
+ function.
+ </para>
+
+ <para>
+ There are several types of sound chips to generate the interrupts.
+ </para>
+
+ <section id="pcm-interface-interrupt-handler-boundary">
+ <title>Interrupts at the period (fragment) boundary</title>
+ <para>
+ This is the most frequently found type: the hardware
+ generates an interrupt at each period boundary.
+ In this case, you can call
+ <function>snd_pcm_period_elapsed()</function> at each
+ interrupt.
+ </para>
+
+ <para>
+ <function>snd_pcm_period_elapsed()</function> takes the
+ substream pointer as its argument. Thus, you need to keep the
+ substream pointer accessible from the chip instance. For
+ example, define substream field in the chip record to hold the
+ current running substream pointer, and set the pointer value
+ at open callback (and reset at close callback).
+ </para>
+
+ <para>
+ If you acquire a spinlock in the interrupt handler, and the
+ lock is used in other pcm callbacks, too, then you have to
+ release the lock before calling
+ <function>snd_pcm_period_elapsed()</function>, because
+ <function>snd_pcm_period_elapsed()</function> calls other pcm
+ callbacks inside.
+ </para>
+
+ <para>
+ Typical code would be like:
+
+ <example>
+ <title>Interrupt Handler Case #1</title>
+ <programlisting>
+<![CDATA[
+ static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id)
+ {
+ struct mychip *chip = dev_id;
+ spin_lock(&chip->lock);
+ ....
+ if (pcm_irq_invoked(chip)) {
+ /* call updater, unlock before it */
+ spin_unlock(&chip->lock);
+ snd_pcm_period_elapsed(chip->substream);
+ spin_lock(&chip->lock);
+ /* acknowledge the interrupt if necessary */
+ }
+ ....
+ spin_unlock(&chip->lock);
+ return IRQ_HANDLED;
+ }
+]]>
+ </programlisting>
+ </example>
+ </para>
+ </section>
+
+ <section id="pcm-interface-interrupt-handler-timer">
+ <title>High frequency timer interrupts</title>
+ <para>
+ This happens when the hardware doesn't generate interrupts
+ at the period boundary but issues timer interrupts at a fixed
+ timer rate (e.g. es1968 or ymfpci drivers).
+ In this case, you need to check the current hardware
+ position and accumulate the processed sample length at each
+ interrupt. When the accumulated size exceeds the period
+ size, call
+ <function>snd_pcm_period_elapsed()</function> and reset the
+ accumulator.
+ </para>
+
+ <para>
+ Typical code would be like the following.
+
+ <example>
+ <title>Interrupt Handler Case #2</title>
+ <programlisting>
+<![CDATA[
+ static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id)
+ {
+ struct mychip *chip = dev_id;
+ spin_lock(&chip->lock);
+ ....
+ if (pcm_irq_invoked(chip)) {
+ unsigned int last_ptr, size;
+ /* get the current hardware pointer (in frames) */
+ last_ptr = get_hw_ptr(chip);
+ /* calculate the processed frames since the
+ * last update
+ */
+ if (last_ptr < chip->last_ptr)
+ size = runtime->buffer_size + last_ptr
+ - chip->last_ptr;
+ else
+ size = last_ptr - chip->last_ptr;
+ /* remember the last updated point */
+ chip->last_ptr = last_ptr;
+ /* accumulate the size */
+ chip->size += size;
+ /* over the period boundary? */
+ if (chip->size >= runtime->period_size) {
+ /* reset the accumulator */
+ chip->size %= runtime->period_size;
+ /* call updater */
+ spin_unlock(&chip->lock);
+ snd_pcm_period_elapsed(substream);
+ spin_lock(&chip->lock);
+ }
+ /* acknowledge the interrupt if necessary */
+ }
+ ....
+ spin_unlock(&chip->lock);
+ return IRQ_HANDLED;
+ }
+]]>
+ </programlisting>
+ </example>
+ </para>
+ </section>
+
+ <section id="pcm-interface-interrupt-handler-both">
+ <title>On calling <function>snd_pcm_period_elapsed()</function></title>
+ <para>
+ In both cases, even if more than one period are elapsed, you
+ don't have to call
+ <function>snd_pcm_period_elapsed()</function> many times. Call
+ only once. And the pcm layer will check the current hardware
+ pointer and update to the latest status.
+ </para>
+ </section>
+ </section>
+
+ <section id="pcm-interface-atomicity">
+ <title>Atomicity</title>
+ <para>
+ One of the most important (and thus difficult to debug) problems
+ in kernel programming are race conditions.
+ In the Linux kernel, they are usually avoided via spin-locks, mutexes
+ or semaphores. In general, if a race condition can happen
+ in an interrupt handler, it has to be managed atomically, and you
+ have to use a spinlock to protect the critical session. If the
+ critical section is not in interrupt handler code and
+ if taking a relatively long time to execute is acceptable, you
+ should use mutexes or semaphores instead.
+ </para>
+
+ <para>
+ As already seen, some pcm callbacks are atomic and some are
+ not. For example, the <parameter>hw_params</parameter> callback is
+ non-atomic, while <parameter>trigger</parameter> callback is
+ atomic. This means, the latter is called already in a spinlock
+ held by the PCM middle layer. Please take this atomicity into
+ account when you choose a locking scheme in the callbacks.
+ </para>
+
+ <para>
+ In the atomic callbacks, you cannot use functions which may call
+ <function>schedule</function> or go to
+ <function>sleep</function>. Semaphores and mutexes can sleep,
+ and hence they cannot be used inside the atomic callbacks
+ (e.g. <parameter>trigger</parameter> callback).
+ To implement some delay in such a callback, please use
+ <function>udelay()</function> or <function>mdelay()</function>.
+ </para>
+
+ <para>
+ All three atomic callbacks (trigger, pointer, and ack) are
+ called with local interrupts disabled.
+ </para>
+
+ <para>
+ The recent changes in PCM core code, however, allow all PCM
+ operations to be non-atomic. This assumes that the all caller
+ sides are in non-atomic contexts. For example, the function
+ <function>snd_pcm_period_elapsed()</function> is called
+ typically from the interrupt handler. But, if you set up the
+ driver to use a threaded interrupt handler, this call can be in
+ non-atomic context, too. In such a case, you can set
+ <structfield>nonatomic</structfield> filed of
+ <structname>snd_pcm</structname> object after creating it.
+ When this flag is set, mutex and rwsem are used internally in
+ the PCM core instead of spin and rwlocks, so that you can call
+ all PCM functions safely in a non-atomic context.
+ </para>
+
+ </section>
+ <section id="pcm-interface-constraints">
+ <title>Constraints</title>
+ <para>
+ If your chip supports unconventional sample rates, or only the
+ limited samples, you need to set a constraint for the
+ condition.
+ </para>
+
+ <para>
+ For example, in order to restrict the sample rates in the some
+ supported values, use
+ <function>snd_pcm_hw_constraint_list()</function>.
+ You need to call this function in the open callback.
+
+ <example>
+ <title>Example of Hardware Constraints</title>
+ <programlisting>
+<![CDATA[
+ static unsigned int rates[] =
+ {4000, 10000, 22050, 44100};
+ static struct snd_pcm_hw_constraint_list constraints_rates = {
+ .count = ARRAY_SIZE(rates),
+ .list = rates,
+ .mask = 0,
+ };
+
+ static int snd_mychip_pcm_open(struct snd_pcm_substream *substream)
+ {
+ int err;
+ ....
+ err = snd_pcm_hw_constraint_list(substream->runtime, 0,
+ SNDRV_PCM_HW_PARAM_RATE,
+ &constraints_rates);
+ if (err < 0)
+ return err;
+ ....
+ }
+]]>
+ </programlisting>
+ </example>
+ </para>
+
+ <para>
+ There are many different constraints.
+ Look at <filename>sound/pcm.h</filename> for a complete list.
+ You can even define your own constraint rules.
+ For example, let's suppose my_chip can manage a substream of 1 channel
+ if and only if the format is S16_LE, otherwise it supports any format
+ specified in the <structname>snd_pcm_hardware</structname> structure (or in any
+ other constraint_list). You can build a rule like this:
+
+ <example>
+ <title>Example of Hardware Constraints for Channels</title>
+ <programlisting>
+<![CDATA[
+ static int hw_rule_channels_by_format(struct snd_pcm_hw_params *params,
+ struct snd_pcm_hw_rule *rule)
+ {
+ struct snd_interval *c = hw_param_interval(params,
+ SNDRV_PCM_HW_PARAM_CHANNELS);
+ struct snd_mask *f = hw_param_mask(params, SNDRV_PCM_HW_PARAM_FORMAT);
+ struct snd_interval ch;
+
+ snd_interval_any(&ch);
+ if (f->bits[0] == SNDRV_PCM_FMTBIT_S16_LE) {
+ ch.min = ch.max = 1;
+ ch.integer = 1;
+ return snd_interval_refine(c, &ch);
+ }
+ return 0;
+ }
+]]>
+ </programlisting>
+ </example>
+ </para>
+
+ <para>
+ Then you need to call this function to add your rule:
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ snd_pcm_hw_rule_add(substream->runtime, 0, SNDRV_PCM_HW_PARAM_CHANNELS,
+ hw_rule_channels_by_format, NULL,
+ SNDRV_PCM_HW_PARAM_FORMAT, -1);
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ The rule function is called when an application sets the PCM
+ format, and it refines the number of channels accordingly.
+ But an application may set the number of channels before
+ setting the format. Thus you also need to define the inverse rule:
+
+ <example>
+ <title>Example of Hardware Constraints for Formats</title>
+ <programlisting>
+<![CDATA[
+ static int hw_rule_format_by_channels(struct snd_pcm_hw_params *params,
+ struct snd_pcm_hw_rule *rule)
+ {
+ struct snd_interval *c = hw_param_interval(params,
+ SNDRV_PCM_HW_PARAM_CHANNELS);
+ struct snd_mask *f = hw_param_mask(params, SNDRV_PCM_HW_PARAM_FORMAT);
+ struct snd_mask fmt;
+
+ snd_mask_any(&fmt); /* Init the struct */
+ if (c->min < 2) {
+ fmt.bits[0] &= SNDRV_PCM_FMTBIT_S16_LE;
+ return snd_mask_refine(f, &fmt);
+ }
+ return 0;
+ }
+]]>
+ </programlisting>
+ </example>
+ </para>
+
+ <para>
+ ...and in the open callback:
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ snd_pcm_hw_rule_add(substream->runtime, 0, SNDRV_PCM_HW_PARAM_FORMAT,
+ hw_rule_format_by_channels, NULL,
+ SNDRV_PCM_HW_PARAM_CHANNELS, -1);
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ I won't give more details here, rather I
+ would like to say, <quote>Luke, use the source.</quote>
+ </para>
+ </section>
+
+ </chapter>
+
+
+<!-- ****************************************************** -->
+<!-- Control Interface -->
+<!-- ****************************************************** -->
+ <chapter id="control-interface">
+ <title>Control Interface</title>
+
+ <section id="control-interface-general">
+ <title>General</title>
+ <para>
+ The control interface is used widely for many switches,
+ sliders, etc. which are accessed from user-space. Its most
+ important use is the mixer interface. In other words, since ALSA
+ 0.9.x, all the mixer stuff is implemented on the control kernel API.
+ </para>
+
+ <para>
+ ALSA has a well-defined AC97 control module. If your chip
+ supports only the AC97 and nothing else, you can skip this
+ section.
+ </para>
+
+ <para>
+ The control API is defined in
+ <filename><sound/control.h></filename>.
+ Include this file if you want to add your own controls.
+ </para>
+ </section>
+
+ <section id="control-interface-definition">
+ <title>Definition of Controls</title>
+ <para>
+ To create a new control, you need to define the
+ following three
+ callbacks: <structfield>info</structfield>,
+ <structfield>get</structfield> and
+ <structfield>put</structfield>. Then, define a
+ struct <structname>snd_kcontrol_new</structname> record, such as:
+
+ <example>
+ <title>Definition of a Control</title>
+ <programlisting>
+<![CDATA[
+ static struct snd_kcontrol_new my_control = {
+ .iface = SNDRV_CTL_ELEM_IFACE_MIXER,
+ .name = "PCM Playback Switch",
+ .index = 0,
+ .access = SNDRV_CTL_ELEM_ACCESS_READWRITE,
+ .private_value = 0xffff,
+ .info = my_control_info,
+ .get = my_control_get,
+ .put = my_control_put
+ };
+]]>
+ </programlisting>
+ </example>
+ </para>
+
+ <para>
+ The <structfield>iface</structfield> field specifies the control
+ type, <constant>SNDRV_CTL_ELEM_IFACE_XXX</constant>, which
+ is usually <constant>MIXER</constant>.
+ Use <constant>CARD</constant> for global controls that are not
+ logically part of the mixer.
+ If the control is closely associated with some specific device on
+ the sound card, use <constant>HWDEP</constant>,
+ <constant>PCM</constant>, <constant>RAWMIDI</constant>,
+ <constant>TIMER</constant>, or <constant>SEQUENCER</constant>, and
+ specify the device number with the
+ <structfield>device</structfield> and
+ <structfield>subdevice</structfield> fields.
+ </para>
+
+ <para>
+ The <structfield>name</structfield> is the name identifier
+ string. Since ALSA 0.9.x, the control name is very important,
+ because its role is classified from its name. There are
+ pre-defined standard control names. The details are described in
+ the <link linkend="control-interface-control-names"><citetitle>
+ Control Names</citetitle></link> subsection.
+ </para>
+
+ <para>
+ The <structfield>index</structfield> field holds the index number
+ of this control. If there are several different controls with
+ the same name, they can be distinguished by the index
+ number. This is the case when
+ several codecs exist on the card. If the index is zero, you can
+ omit the definition above.
+ </para>
+
+ <para>
+ The <structfield>access</structfield> field contains the access
+ type of this control. Give the combination of bit masks,
+ <constant>SNDRV_CTL_ELEM_ACCESS_XXX</constant>, there.
+ The details will be explained in
+ the <link linkend="control-interface-access-flags"><citetitle>
+ Access Flags</citetitle></link> subsection.
+ </para>
+
+ <para>
+ The <structfield>private_value</structfield> field contains
+ an arbitrary long integer value for this record. When using
+ the generic <structfield>info</structfield>,
+ <structfield>get</structfield> and
+ <structfield>put</structfield> callbacks, you can pass a value
+ through this field. If several small numbers are necessary, you can
+ combine them in bitwise. Or, it's possible to give a pointer
+ (casted to unsigned long) of some record to this field, too.
+ </para>
+
+ <para>
+ The <structfield>tlv</structfield> field can be used to provide
+ metadata about the control; see the
+ <link linkend="control-interface-tlv">
+ <citetitle>Metadata</citetitle></link> subsection.
+ </para>
+
+ <para>
+ The other three are
+ <link linkend="control-interface-callbacks"><citetitle>
+ callback functions</citetitle></link>.
+ </para>
+ </section>
+
+ <section id="control-interface-control-names">
+ <title>Control Names</title>
+ <para>
+ There are some standards to define the control names. A
+ control is usually defined from the three parts as
+ <quote>SOURCE DIRECTION FUNCTION</quote>.
+ </para>
+
+ <para>
+ The first, <constant>SOURCE</constant>, specifies the source
+ of the control, and is a string such as <quote>Master</quote>,
+ <quote>PCM</quote>, <quote>CD</quote> and
+ <quote>Line</quote>. There are many pre-defined sources.
+ </para>
+
+ <para>
+ The second, <constant>DIRECTION</constant>, is one of the
+ following strings according to the direction of the control:
+ <quote>Playback</quote>, <quote>Capture</quote>, <quote>Bypass
+ Playback</quote> and <quote>Bypass Capture</quote>. Or, it can
+ be omitted, meaning both playback and capture directions.
+ </para>
+
+ <para>
+ The third, <constant>FUNCTION</constant>, is one of the
+ following strings according to the function of the control:
+ <quote>Switch</quote>, <quote>Volume</quote> and
+ <quote>Route</quote>.
+ </para>
+
+ <para>
+ The example of control names are, thus, <quote>Master Capture
+ Switch</quote> or <quote>PCM Playback Volume</quote>.
+ </para>
+
+ <para>
+ There are some exceptions:
+ </para>
+
+ <section id="control-interface-control-names-global">
+ <title>Global capture and playback</title>
+ <para>
+ <quote>Capture Source</quote>, <quote>Capture Switch</quote>
+ and <quote>Capture Volume</quote> are used for the global
+ capture (input) source, switch and volume. Similarly,
+ <quote>Playback Switch</quote> and <quote>Playback
+ Volume</quote> are used for the global output gain switch and
+ volume.
+ </para>
+ </section>
+
+ <section id="control-interface-control-names-tone">
+ <title>Tone-controls</title>
+ <para>
+ tone-control switch and volumes are specified like
+ <quote>Tone Control - XXX</quote>, e.g. <quote>Tone Control -
+ Switch</quote>, <quote>Tone Control - Bass</quote>,
+ <quote>Tone Control - Center</quote>.
+ </para>
+ </section>
+
+ <section id="control-interface-control-names-3d">
+ <title>3D controls</title>
+ <para>
+ 3D-control switches and volumes are specified like <quote>3D
+ Control - XXX</quote>, e.g. <quote>3D Control -
+ Switch</quote>, <quote>3D Control - Center</quote>, <quote>3D
+ Control - Space</quote>.
+ </para>
+ </section>
+
+ <section id="control-interface-control-names-mic">
+ <title>Mic boost</title>
+ <para>
+ Mic-boost switch is set as <quote>Mic Boost</quote> or
+ <quote>Mic Boost (6dB)</quote>.
+ </para>
+
+ <para>
+ More precise information can be found in
+ <filename>Documentation/sound/alsa/ControlNames.txt</filename>.
+ </para>
+ </section>
+ </section>
+
+ <section id="control-interface-access-flags">
+ <title>Access Flags</title>
+
+ <para>
+ The access flag is the bitmask which specifies the access type
+ of the given control. The default access type is
+ <constant>SNDRV_CTL_ELEM_ACCESS_READWRITE</constant>,
+ which means both read and write are allowed to this control.
+ When the access flag is omitted (i.e. = 0), it is
+ considered as <constant>READWRITE</constant> access as default.
+ </para>
+
+ <para>
+ When the control is read-only, pass
+ <constant>SNDRV_CTL_ELEM_ACCESS_READ</constant> instead.
+ In this case, you don't have to define
+ the <structfield>put</structfield> callback.
+ Similarly, when the control is write-only (although it's a rare
+ case), you can use the <constant>WRITE</constant> flag instead, and
+ you don't need the <structfield>get</structfield> callback.
+ </para>
+
+ <para>
+ If the control value changes frequently (e.g. the VU meter),
+ <constant>VOLATILE</constant> flag should be given. This means
+ that the control may be changed without
+ <link linkend="control-interface-change-notification"><citetitle>
+ notification</citetitle></link>. Applications should poll such
+ a control constantly.
+ </para>
+
+ <para>
+ When the control is inactive, set
+ the <constant>INACTIVE</constant> flag, too.
+ There are <constant>LOCK</constant> and
+ <constant>OWNER</constant> flags to change the write
+ permissions.
+ </para>
+
+ </section>
+
+ <section id="control-interface-callbacks">
+ <title>Callbacks</title>
+
+ <section id="control-interface-callbacks-info">
+ <title>info callback</title>
+ <para>
+ The <structfield>info</structfield> callback is used to get
+ detailed information on this control. This must store the
+ values of the given struct <structname>snd_ctl_elem_info</structname>
+ object. For example, for a boolean control with a single
+ element:
+
+ <example>
+ <title>Example of info callback</title>
+ <programlisting>
+<![CDATA[
+ static int snd_myctl_mono_info(struct snd_kcontrol *kcontrol,
+ struct snd_ctl_elem_info *uinfo)
+ {
+ uinfo->type = SNDRV_CTL_ELEM_TYPE_BOOLEAN;
+ uinfo->count = 1;
+ uinfo->value.integer.min = 0;
+ uinfo->value.integer.max = 1;
+ return 0;
+ }
+]]>
+ </programlisting>
+ </example>
+ </para>
+
+ <para>
+ The <structfield>type</structfield> field specifies the type
+ of the control. There are <constant>BOOLEAN</constant>,
+ <constant>INTEGER</constant>, <constant>ENUMERATED</constant>,
+ <constant>BYTES</constant>, <constant>IEC958</constant> and
+ <constant>INTEGER64</constant>. The
+ <structfield>count</structfield> field specifies the
+ number of elements in this control. For example, a stereo
+ volume would have count = 2. The
+ <structfield>value</structfield> field is a union, and
+ the values stored are depending on the type. The boolean and
+ integer types are identical.
+ </para>
+
+ <para>
+ The enumerated type is a bit different from others. You'll
+ need to set the string for the currently given item index.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static int snd_myctl_enum_info(struct snd_kcontrol *kcontrol,
+ struct snd_ctl_elem_info *uinfo)
+ {
+ static char *texts[4] = {
+ "First", "Second", "Third", "Fourth"
+ };
+ uinfo->type = SNDRV_CTL_ELEM_TYPE_ENUMERATED;
+ uinfo->count = 1;
+ uinfo->value.enumerated.items = 4;
+ if (uinfo->value.enumerated.item > 3)
+ uinfo->value.enumerated.item = 3;
+ strcpy(uinfo->value.enumerated.name,
+ texts[uinfo->value.enumerated.item]);
+ return 0;
+ }
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ The above callback can be simplified with a helper function,
+ <function>snd_ctl_enum_info</function>. The final code
+ looks like below.
+ (You can pass ARRAY_SIZE(texts) instead of 4 in the third
+ argument; it's a matter of taste.)
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static int snd_myctl_enum_info(struct snd_kcontrol *kcontrol,
+ struct snd_ctl_elem_info *uinfo)
+ {
+ static char *texts[4] = {
+ "First", "Second", "Third", "Fourth"
+ };
+ return snd_ctl_enum_info(uinfo, 1, 4, texts);
+ }
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ Some common info callbacks are available for your convenience:
+ <function>snd_ctl_boolean_mono_info()</function> and
+ <function>snd_ctl_boolean_stereo_info()</function>.
+ Obviously, the former is an info callback for a mono channel
+ boolean item, just like <function>snd_myctl_mono_info</function>
+ above, and the latter is for a stereo channel boolean item.
+ </para>
+
+ </section>
+
+ <section id="control-interface-callbacks-get">
+ <title>get callback</title>
+
+ <para>
+ This callback is used to read the current value of the
+ control and to return to user-space.
+ </para>
+
+ <para>
+ For example,
+
+ <example>
+ <title>Example of get callback</title>
+ <programlisting>
+<![CDATA[
+ static int snd_myctl_get(struct snd_kcontrol *kcontrol,
+ struct snd_ctl_elem_value *ucontrol)
+ {
+ struct mychip *chip = snd_kcontrol_chip(kcontrol);
+ ucontrol->value.integer.value[0] = get_some_value(chip);
+ return 0;
+ }
+]]>
+ </programlisting>
+ </example>
+ </para>
+
+ <para>
+ The <structfield>value</structfield> field depends on
+ the type of control as well as on the info callback. For example,
+ the sb driver uses this field to store the register offset,
+ the bit-shift and the bit-mask. The
+ <structfield>private_value</structfield> field is set as follows:
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ .private_value = reg | (shift << 16) | (mask << 24)
+]]>
+ </programlisting>
+ </informalexample>
+ and is retrieved in callbacks like
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static int snd_sbmixer_get_single(struct snd_kcontrol *kcontrol,
+ struct snd_ctl_elem_value *ucontrol)
+ {
+ int reg = kcontrol->private_value & 0xff;
+ int shift = (kcontrol->private_value >> 16) & 0xff;
+ int mask = (kcontrol->private_value >> 24) & 0xff;
+ ....
+ }
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ In the <structfield>get</structfield> callback,
+ you have to fill all the elements if the
+ control has more than one elements,
+ i.e. <structfield>count</structfield> > 1.
+ In the example above, we filled only one element
+ (<structfield>value.integer.value[0]</structfield>) since it's
+ assumed as <structfield>count</structfield> = 1.
+ </para>
+ </section>
+
+ <section id="control-interface-callbacks-put">
+ <title>put callback</title>
+
+ <para>
+ This callback is used to write a value from user-space.
+ </para>
+
+ <para>
+ For example,
+
+ <example>
+ <title>Example of put callback</title>
+ <programlisting>
+<![CDATA[
+ static int snd_myctl_put(struct snd_kcontrol *kcontrol,
+ struct snd_ctl_elem_value *ucontrol)
+ {
+ struct mychip *chip = snd_kcontrol_chip(kcontrol);
+ int changed = 0;
+ if (chip->current_value !=
+ ucontrol->value.integer.value[0]) {
+ change_current_value(chip,
+ ucontrol->value.integer.value[0]);
+ changed = 1;
+ }
+ return changed;
+ }
+]]>
+ </programlisting>
+ </example>
+
+ As seen above, you have to return 1 if the value is
+ changed. If the value is not changed, return 0 instead.
+ If any fatal error happens, return a negative error code as
+ usual.
+ </para>
+
+ <para>
+ As in the <structfield>get</structfield> callback,
+ when the control has more than one elements,
+ all elements must be evaluated in this callback, too.
+ </para>
+ </section>
+
+ <section id="control-interface-callbacks-all">
+ <title>Callbacks are not atomic</title>
+ <para>
+ All these three callbacks are basically not atomic.
+ </para>
+ </section>
+ </section>
+
+ <section id="control-interface-constructor">
+ <title>Constructor</title>
+ <para>
+ When everything is ready, finally we can create a new
+ control. To create a control, there are two functions to be
+ called, <function>snd_ctl_new1()</function> and
+ <function>snd_ctl_add()</function>.
+ </para>
+
+ <para>
+ In the simplest way, you can do like this:
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ err = snd_ctl_add(card, snd_ctl_new1(&my_control, chip));
+ if (err < 0)
+ return err;
+]]>
+ </programlisting>
+ </informalexample>
+
+ where <parameter>my_control</parameter> is the
+ struct <structname>snd_kcontrol_new</structname> object defined above, and chip
+ is the object pointer to be passed to
+ kcontrol->private_data
+ which can be referred to in callbacks.
+ </para>
+
+ <para>
+ <function>snd_ctl_new1()</function> allocates a new
+ <structname>snd_kcontrol</structname> instance,
+ and <function>snd_ctl_add</function> assigns the given
+ control component to the card.
+ </para>
+ </section>
+
+ <section id="control-interface-change-notification">
+ <title>Change Notification</title>
+ <para>
+ If you need to change and update a control in the interrupt
+ routine, you can call <function>snd_ctl_notify()</function>. For
+ example,
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ snd_ctl_notify(card, SNDRV_CTL_EVENT_MASK_VALUE, id_pointer);
+]]>
+ </programlisting>
+ </informalexample>
+
+ This function takes the card pointer, the event-mask, and the
+ control id pointer for the notification. The event-mask
+ specifies the types of notification, for example, in the above
+ example, the change of control values is notified.
+ The id pointer is the pointer of struct <structname>snd_ctl_elem_id</structname>
+ to be notified.
+ You can find some examples in <filename>es1938.c</filename> or
+ <filename>es1968.c</filename> for hardware volume interrupts.
+ </para>
+ </section>
+
+ <section id="control-interface-tlv">
+ <title>Metadata</title>
+ <para>
+ To provide information about the dB values of a mixer control, use
+ on of the <constant>DECLARE_TLV_xxx</constant> macros from
+ <filename><sound/tlv.h></filename> to define a variable
+ containing this information, set the<structfield>tlv.p
+ </structfield> field to point to this variable, and include the
+ <constant>SNDRV_CTL_ELEM_ACCESS_TLV_READ</constant> flag in the
+ <structfield>access</structfield> field; like this:
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static DECLARE_TLV_DB_SCALE(db_scale_my_control, -4050, 150, 0);
+
+ static struct snd_kcontrol_new my_control = {
+ ...
+ .access = SNDRV_CTL_ELEM_ACCESS_READWRITE |
+ SNDRV_CTL_ELEM_ACCESS_TLV_READ,
+ ...
+ .tlv.p = db_scale_my_control,
+ };
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ The <function>DECLARE_TLV_DB_SCALE</function> macro defines
+ information about a mixer control where each step in the control's
+ value changes the dB value by a constant dB amount.
+ The first parameter is the name of the variable to be defined.
+ The second parameter is the minimum value, in units of 0.01 dB.
+ The third parameter is the step size, in units of 0.01 dB.
+ Set the fourth parameter to 1 if the minimum value actually mutes
+ the control.
+ </para>
+
+ <para>
+ The <function>DECLARE_TLV_DB_LINEAR</function> macro defines
+ information about a mixer control where the control's value affects
+ the output linearly.
+ The first parameter is the name of the variable to be defined.
+ The second parameter is the minimum value, in units of 0.01 dB.
+ The third parameter is the maximum value, in units of 0.01 dB.
+ If the minimum value mutes the control, set the second parameter to
+ <constant>TLV_DB_GAIN_MUTE</constant>.
+ </para>
+ </section>
+
+ </chapter>
+
+
+<!-- ****************************************************** -->
+<!-- API for AC97 Codec -->
+<!-- ****************************************************** -->
+ <chapter id="api-ac97">
+ <title>API for AC97 Codec</title>
+
+ <section>
+ <title>General</title>
+ <para>
+ The ALSA AC97 codec layer is a well-defined one, and you don't
+ have to write much code to control it. Only low-level control
+ routines are necessary. The AC97 codec API is defined in
+ <filename><sound/ac97_codec.h></filename>.
+ </para>
+ </section>
+
+ <section id="api-ac97-example">
+ <title>Full Code Example</title>
+ <para>
+ <example>
+ <title>Example of AC97 Interface</title>
+ <programlisting>
+<![CDATA[
+ struct mychip {
+ ....
+ struct snd_ac97 *ac97;
+ ....
+ };
+
+ static unsigned short snd_mychip_ac97_read(struct snd_ac97 *ac97,
+ unsigned short reg)
+ {
+ struct mychip *chip = ac97->private_data;
+ ....
+ /* read a register value here from the codec */
+ return the_register_value;
+ }
+
+ static void snd_mychip_ac97_write(struct snd_ac97 *ac97,
+ unsigned short reg, unsigned short val)
+ {
+ struct mychip *chip = ac97->private_data;
+ ....
+ /* write the given register value to the codec */
+ }
+
+ static int snd_mychip_ac97(struct mychip *chip)
+ {
+ struct snd_ac97_bus *bus;
+ struct snd_ac97_template ac97;
+ int err;
+ static struct snd_ac97_bus_ops ops = {
+ .write = snd_mychip_ac97_write,
+ .read = snd_mychip_ac97_read,
+ };
+
+ err = snd_ac97_bus(chip->card, 0, &ops, NULL, &bus);
+ if (err < 0)
+ return err;
+ memset(&ac97, 0, sizeof(ac97));
+ ac97.private_data = chip;
+ return snd_ac97_mixer(bus, &ac97, &chip->ac97);
+ }
+
+]]>
+ </programlisting>
+ </example>
+ </para>
+ </section>
+
+ <section id="api-ac97-constructor">
+ <title>Constructor</title>
+ <para>
+ To create an ac97 instance, first call <function>snd_ac97_bus</function>
+ with an <type>ac97_bus_ops_t</type> record with callback functions.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ struct snd_ac97_bus *bus;
+ static struct snd_ac97_bus_ops ops = {
+ .write = snd_mychip_ac97_write,
+ .read = snd_mychip_ac97_read,
+ };
+
+ snd_ac97_bus(card, 0, &ops, NULL, &pbus);
+]]>
+ </programlisting>
+ </informalexample>
+
+ The bus record is shared among all belonging ac97 instances.
+ </para>
+
+ <para>
+ And then call <function>snd_ac97_mixer()</function> with an
+ struct <structname>snd_ac97_template</structname>
+ record together with the bus pointer created above.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ struct snd_ac97_template ac97;
+ int err;
+
+ memset(&ac97, 0, sizeof(ac97));
+ ac97.private_data = chip;
+ snd_ac97_mixer(bus, &ac97, &chip->ac97);
+]]>
+ </programlisting>
+ </informalexample>
+
+ where chip->ac97 is a pointer to a newly created
+ <type>ac97_t</type> instance.
+ In this case, the chip pointer is set as the private data, so that
+ the read/write callback functions can refer to this chip instance.
+ This instance is not necessarily stored in the chip
+ record. If you need to change the register values from the
+ driver, or need the suspend/resume of ac97 codecs, keep this
+ pointer to pass to the corresponding functions.
+ </para>
+ </section>
+
+ <section id="api-ac97-callbacks">
+ <title>Callbacks</title>
+ <para>
+ The standard callbacks are <structfield>read</structfield> and
+ <structfield>write</structfield>. Obviously they
+ correspond to the functions for read and write accesses to the
+ hardware low-level codes.
+ </para>
+
+ <para>
+ The <structfield>read</structfield> callback returns the
+ register value specified in the argument.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static unsigned short snd_mychip_ac97_read(struct snd_ac97 *ac97,
+ unsigned short reg)
+ {
+ struct mychip *chip = ac97->private_data;
+ ....
+ return the_register_value;
+ }
+]]>
+ </programlisting>
+ </informalexample>
+
+ Here, the chip can be cast from ac97->private_data.
+ </para>
+
+ <para>
+ Meanwhile, the <structfield>write</structfield> callback is
+ used to set the register value.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static void snd_mychip_ac97_write(struct snd_ac97 *ac97,
+ unsigned short reg, unsigned short val)
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ These callbacks are non-atomic like the control API callbacks.
+ </para>
+
+ <para>
+ There are also other callbacks:
+ <structfield>reset</structfield>,
+ <structfield>wait</structfield> and
+ <structfield>init</structfield>.
+ </para>
+
+ <para>
+ The <structfield>reset</structfield> callback is used to reset
+ the codec. If the chip requires a special kind of reset, you can
+ define this callback.
+ </para>
+
+ <para>
+ The <structfield>wait</structfield> callback is used to
+ add some waiting time in the standard initialization of the codec. If the
+ chip requires the extra waiting time, define this callback.
+ </para>
+
+ <para>
+ The <structfield>init</structfield> callback is used for
+ additional initialization of the codec.
+ </para>
+ </section>
+
+ <section id="api-ac97-updating-registers">
+ <title>Updating Registers in The Driver</title>
+ <para>
+ If you need to access to the codec from the driver, you can
+ call the following functions:
+ <function>snd_ac97_write()</function>,
+ <function>snd_ac97_read()</function>,
+ <function>snd_ac97_update()</function> and
+ <function>snd_ac97_update_bits()</function>.
+ </para>
+
+ <para>
+ Both <function>snd_ac97_write()</function> and
+ <function>snd_ac97_update()</function> functions are used to
+ set a value to the given register
+ (<constant>AC97_XXX</constant>). The difference between them is
+ that <function>snd_ac97_update()</function> doesn't write a
+ value if the given value has been already set, while
+ <function>snd_ac97_write()</function> always rewrites the
+ value.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ snd_ac97_write(ac97, AC97_MASTER, 0x8080);
+ snd_ac97_update(ac97, AC97_MASTER, 0x8080);
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ <function>snd_ac97_read()</function> is used to read the value
+ of the given register. For example,
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ value = snd_ac97_read(ac97, AC97_MASTER);
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ <function>snd_ac97_update_bits()</function> is used to update
+ some bits in the given register.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ snd_ac97_update_bits(ac97, reg, mask, value);
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ Also, there is a function to change the sample rate (of a
+ given register such as
+ <constant>AC97_PCM_FRONT_DAC_RATE</constant>) when VRA or
+ DRA is supported by the codec:
+ <function>snd_ac97_set_rate()</function>.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ snd_ac97_set_rate(ac97, AC97_PCM_FRONT_DAC_RATE, 44100);
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ The following registers are available to set the rate:
+ <constant>AC97_PCM_MIC_ADC_RATE</constant>,
+ <constant>AC97_PCM_FRONT_DAC_RATE</constant>,
+ <constant>AC97_PCM_LR_ADC_RATE</constant>,
+ <constant>AC97_SPDIF</constant>. When
+ <constant>AC97_SPDIF</constant> is specified, the register is
+ not really changed but the corresponding IEC958 status bits will
+ be updated.
+ </para>
+ </section>
+
+ <section id="api-ac97-clock-adjustment">
+ <title>Clock Adjustment</title>
+ <para>
+ In some chips, the clock of the codec isn't 48000 but using a
+ PCI clock (to save a quartz!). In this case, change the field
+ bus->clock to the corresponding
+ value. For example, intel8x0
+ and es1968 drivers have their own function to read from the clock.
+ </para>
+ </section>
+
+ <section id="api-ac97-proc-files">
+ <title>Proc Files</title>
+ <para>
+ The ALSA AC97 interface will create a proc file such as
+ <filename>/proc/asound/card0/codec97#0/ac97#0-0</filename> and
+ <filename>ac97#0-0+regs</filename>. You can refer to these files to
+ see the current status and registers of the codec.
+ </para>
+ </section>
+
+ <section id="api-ac97-multiple-codecs">
+ <title>Multiple Codecs</title>
+ <para>
+ When there are several codecs on the same card, you need to
+ call <function>snd_ac97_mixer()</function> multiple times with
+ ac97.num=1 or greater. The <structfield>num</structfield> field
+ specifies the codec number.
+ </para>
+
+ <para>
+ If you set up multiple codecs, you either need to write
+ different callbacks for each codec or check
+ ac97->num in the callback routines.
+ </para>
+ </section>
+
+ </chapter>
+
+
+<!-- ****************************************************** -->
+<!-- MIDI (MPU401-UART) Interface -->
+<!-- ****************************************************** -->
+ <chapter id="midi-interface">
+ <title>MIDI (MPU401-UART) Interface</title>
+
+ <section id="midi-interface-general">
+ <title>General</title>
+ <para>
+ Many soundcards have built-in MIDI (MPU401-UART)
+ interfaces. When the soundcard supports the standard MPU401-UART
+ interface, most likely you can use the ALSA MPU401-UART API. The
+ MPU401-UART API is defined in
+ <filename><sound/mpu401.h></filename>.
+ </para>
+
+ <para>
+ Some soundchips have a similar but slightly different
+ implementation of mpu401 stuff. For example, emu10k1 has its own
+ mpu401 routines.
+ </para>
+ </section>
+
+ <section id="midi-interface-constructor">
+ <title>Constructor</title>
+ <para>
+ To create a rawmidi object, call
+ <function>snd_mpu401_uart_new()</function>.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ struct snd_rawmidi *rmidi;
+ snd_mpu401_uart_new(card, 0, MPU401_HW_MPU401, port, info_flags,
+ irq, &rmidi);
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ The first argument is the card pointer, and the second is the
+ index of this component. You can create up to 8 rawmidi
+ devices.
+ </para>
+
+ <para>
+ The third argument is the type of the hardware,
+ <constant>MPU401_HW_XXX</constant>. If it's not a special one,
+ you can use <constant>MPU401_HW_MPU401</constant>.
+ </para>
+
+ <para>
+ The 4th argument is the I/O port address. Many
+ backward-compatible MPU401 have an I/O port such as 0x330. Or, it
+ might be a part of its own PCI I/O region. It depends on the
+ chip design.
+ </para>
+
+ <para>
+ The 5th argument is a bitflag for additional information.
+ When the I/O port address above is part of the PCI I/O
+ region, the MPU401 I/O port might have been already allocated
+ (reserved) by the driver itself. In such a case, pass a bit flag
+ <constant>MPU401_INFO_INTEGRATED</constant>,
+ and the mpu401-uart layer will allocate the I/O ports by itself.
+ </para>
+
+ <para>
+ When the controller supports only the input or output MIDI stream,
+ pass the <constant>MPU401_INFO_INPUT</constant> or
+ <constant>MPU401_INFO_OUTPUT</constant> bitflag, respectively.
+ Then the rawmidi instance is created as a single stream.
+ </para>
+
+ <para>
+ <constant>MPU401_INFO_MMIO</constant> bitflag is used to change
+ the access method to MMIO (via readb and writeb) instead of
+ iob and outb. In this case, you have to pass the iomapped address
+ to <function>snd_mpu401_uart_new()</function>.
+ </para>
+
+ <para>
+ When <constant>MPU401_INFO_TX_IRQ</constant> is set, the output
+ stream isn't checked in the default interrupt handler. The driver
+ needs to call <function>snd_mpu401_uart_interrupt_tx()</function>
+ by itself to start processing the output stream in the irq handler.
+ </para>
+
+ <para>
+ If the MPU-401 interface shares its interrupt with the other logical
+ devices on the card, set <constant>MPU401_INFO_IRQ_HOOK</constant>
+ (see <link linkend="midi-interface-interrupt-handler"><citetitle>
+ below</citetitle></link>).
+ </para>
+
+ <para>
+ Usually, the port address corresponds to the command port and
+ port + 1 corresponds to the data port. If not, you may change
+ the <structfield>cport</structfield> field of
+ struct <structname>snd_mpu401</structname> manually
+ afterward. However, <structname>snd_mpu401</structname> pointer is not
+ returned explicitly by
+ <function>snd_mpu401_uart_new()</function>. You need to cast
+ rmidi->private_data to
+ <structname>snd_mpu401</structname> explicitly,
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ struct snd_mpu401 *mpu;
+ mpu = rmidi->private_data;
+]]>
+ </programlisting>
+ </informalexample>
+
+ and reset the cport as you like:
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ mpu->cport = my_own_control_port;
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ The 6th argument specifies the ISA irq number that will be
+ allocated. If no interrupt is to be allocated (because your
+ code is already allocating a shared interrupt, or because the
+ device does not use interrupts), pass -1 instead.
+ For a MPU-401 device without an interrupt, a polling timer
+ will be used instead.
+ </para>
+ </section>
+
+ <section id="midi-interface-interrupt-handler">
+ <title>Interrupt Handler</title>
+ <para>
+ When the interrupt is allocated in
+ <function>snd_mpu401_uart_new()</function>, an exclusive ISA
+ interrupt handler is automatically used, hence you don't have
+ anything else to do than creating the mpu401 stuff. Otherwise, you
+ have to set <constant>MPU401_INFO_IRQ_HOOK</constant>, and call
+ <function>snd_mpu401_uart_interrupt()</function> explicitly from your
+ own interrupt handler when it has determined that a UART interrupt
+ has occurred.
+ </para>
+
+ <para>
+ In this case, you need to pass the private_data of the
+ returned rawmidi object from
+ <function>snd_mpu401_uart_new()</function> as the second
+ argument of <function>snd_mpu401_uart_interrupt()</function>.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ snd_mpu401_uart_interrupt(irq, rmidi->private_data, regs);
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+ </section>
+
+ </chapter>
+
+
+<!-- ****************************************************** -->
+<!-- RawMIDI Interface -->
+<!-- ****************************************************** -->
+ <chapter id="rawmidi-interface">
+ <title>RawMIDI Interface</title>
+
+ <section id="rawmidi-interface-overview">
+ <title>Overview</title>
+
+ <para>
+ The raw MIDI interface is used for hardware MIDI ports that can
+ be accessed as a byte stream. It is not used for synthesizer
+ chips that do not directly understand MIDI.
+ </para>
+
+ <para>
+ ALSA handles file and buffer management. All you have to do is
+ to write some code to move data between the buffer and the
+ hardware.
+ </para>
+
+ <para>
+ The rawmidi API is defined in
+ <filename><sound/rawmidi.h></filename>.
+ </para>
+ </section>
+
+ <section id="rawmidi-interface-constructor">
+ <title>Constructor</title>
+
+ <para>
+ To create a rawmidi device, call the
+ <function>snd_rawmidi_new</function> function:
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ struct snd_rawmidi *rmidi;
+ err = snd_rawmidi_new(chip->card, "MyMIDI", 0, outs, ins, &rmidi);
+ if (err < 0)
+ return err;
+ rmidi->private_data = chip;
+ strcpy(rmidi->name, "My MIDI");
+ rmidi->info_flags = SNDRV_RAWMIDI_INFO_OUTPUT |
+ SNDRV_RAWMIDI_INFO_INPUT |
+ SNDRV_RAWMIDI_INFO_DUPLEX;
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ The first argument is the card pointer, the second argument is
+ the ID string.
+ </para>
+
+ <para>
+ The third argument is the index of this component. You can
+ create up to 8 rawmidi devices.
+ </para>
+
+ <para>
+ The fourth and fifth arguments are the number of output and
+ input substreams, respectively, of this device (a substream is
+ the equivalent of a MIDI port).
+ </para>
+
+ <para>
+ Set the <structfield>info_flags</structfield> field to specify
+ the capabilities of the device.
+ Set <constant>SNDRV_RAWMIDI_INFO_OUTPUT</constant> if there is
+ at least one output port,
+ <constant>SNDRV_RAWMIDI_INFO_INPUT</constant> if there is at
+ least one input port,
+ and <constant>SNDRV_RAWMIDI_INFO_DUPLEX</constant> if the device
+ can handle output and input at the same time.
+ </para>
+
+ <para>
+ After the rawmidi device is created, you need to set the
+ operators (callbacks) for each substream. There are helper
+ functions to set the operators for all the substreams of a device:
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ snd_rawmidi_set_ops(rmidi, SNDRV_RAWMIDI_STREAM_OUTPUT, &snd_mymidi_output_ops);
+ snd_rawmidi_set_ops(rmidi, SNDRV_RAWMIDI_STREAM_INPUT, &snd_mymidi_input_ops);
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ The operators are usually defined like this:
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static struct snd_rawmidi_ops snd_mymidi_output_ops = {
+ .open = snd_mymidi_output_open,
+ .close = snd_mymidi_output_close,
+ .trigger = snd_mymidi_output_trigger,
+ };
+]]>
+ </programlisting>
+ </informalexample>
+ These callbacks are explained in the <link
+ linkend="rawmidi-interface-callbacks"><citetitle>Callbacks</citetitle></link>
+ section.
+ </para>
+
+ <para>
+ If there are more than one substream, you should give a
+ unique name to each of them:
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ struct snd_rawmidi_substream *substream;
+ list_for_each_entry(substream,
+ &rmidi->streams[SNDRV_RAWMIDI_STREAM_OUTPUT].substreams,
+ list {
+ sprintf(substream->name, "My MIDI Port %d", substream->number + 1);
+ }
+ /* same for SNDRV_RAWMIDI_STREAM_INPUT */
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+ </section>
+
+ <section id="rawmidi-interface-callbacks">
+ <title>Callbacks</title>
+
+ <para>
+ In all the callbacks, the private data that you've set for the
+ rawmidi device can be accessed as
+ substream->rmidi->private_data.
+ <!-- <code> isn't available before DocBook 4.3 -->
+ </para>
+
+ <para>
+ If there is more than one port, your callbacks can determine the
+ port index from the struct snd_rawmidi_substream data passed to each
+ callback:
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ struct snd_rawmidi_substream *substream;
+ int index = substream->number;
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <section id="rawmidi-interface-op-open">
+ <title><function>open</function> callback</title>
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static int snd_xxx_open(struct snd_rawmidi_substream *substream);
+]]>
+ </programlisting>
+ </informalexample>
+
+ <para>
+ This is called when a substream is opened.
+ You can initialize the hardware here, but you shouldn't
+ start transmitting/receiving data yet.
+ </para>
+ </section>
+
+ <section id="rawmidi-interface-op-close">
+ <title><function>close</function> callback</title>
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static int snd_xxx_close(struct snd_rawmidi_substream *substream);
+]]>
+ </programlisting>
+ </informalexample>
+
+ <para>
+ Guess what.
+ </para>
+
+ <para>
+ The <function>open</function> and <function>close</function>
+ callbacks of a rawmidi device are serialized with a mutex,
+ and can sleep.
+ </para>
+ </section>
+
+ <section id="rawmidi-interface-op-trigger-out">
+ <title><function>trigger</function> callback for output
+ substreams</title>
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static void snd_xxx_output_trigger(struct snd_rawmidi_substream *substream, int up);
+]]>
+ </programlisting>
+ </informalexample>
+
+ <para>
+ This is called with a nonzero <parameter>up</parameter>
+ parameter when there is some data in the substream buffer that
+ must be transmitted.
+ </para>
+
+ <para>
+ To read data from the buffer, call
+ <function>snd_rawmidi_transmit_peek</function>. It will
+ return the number of bytes that have been read; this will be
+ less than the number of bytes requested when there are no more
+ data in the buffer.
+ After the data have been transmitted successfully, call
+ <function>snd_rawmidi_transmit_ack</function> to remove the
+ data from the substream buffer:
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ unsigned char data;
+ while (snd_rawmidi_transmit_peek(substream, &data, 1) == 1) {
+ if (snd_mychip_try_to_transmit(data))
+ snd_rawmidi_transmit_ack(substream, 1);
+ else
+ break; /* hardware FIFO full */
+ }
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ If you know beforehand that the hardware will accept data, you
+ can use the <function>snd_rawmidi_transmit</function> function
+ which reads some data and removes them from the buffer at once:
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ while (snd_mychip_transmit_possible()) {
+ unsigned char data;
+ if (snd_rawmidi_transmit(substream, &data, 1) != 1)
+ break; /* no more data */
+ snd_mychip_transmit(data);
+ }
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ If you know beforehand how many bytes you can accept, you can
+ use a buffer size greater than one with the
+ <function>snd_rawmidi_transmit*</function> functions.
+ </para>
+
+ <para>
+ The <function>trigger</function> callback must not sleep. If
+ the hardware FIFO is full before the substream buffer has been
+ emptied, you have to continue transmitting data later, either
+ in an interrupt handler, or with a timer if the hardware
+ doesn't have a MIDI transmit interrupt.
+ </para>
+
+ <para>
+ The <function>trigger</function> callback is called with a
+ zero <parameter>up</parameter> parameter when the transmission
+ of data should be aborted.
+ </para>
+ </section>
+
+ <section id="rawmidi-interface-op-trigger-in">
+ <title><function>trigger</function> callback for input
+ substreams</title>
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static void snd_xxx_input_trigger(struct snd_rawmidi_substream *substream, int up);
+]]>
+ </programlisting>
+ </informalexample>
+
+ <para>
+ This is called with a nonzero <parameter>up</parameter>
+ parameter to enable receiving data, or with a zero
+ <parameter>up</parameter> parameter do disable receiving data.
+ </para>
+
+ <para>
+ The <function>trigger</function> callback must not sleep; the
+ actual reading of data from the device is usually done in an
+ interrupt handler.
+ </para>
+
+ <para>
+ When data reception is enabled, your interrupt handler should
+ call <function>snd_rawmidi_receive</function> for all received
+ data:
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ void snd_mychip_midi_interrupt(...)
+ {
+ while (mychip_midi_available()) {
+ unsigned char data;
+ data = mychip_midi_read();
+ snd_rawmidi_receive(substream, &data, 1);
+ }
+ }
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+ </section>
+
+ <section id="rawmidi-interface-op-drain">
+ <title><function>drain</function> callback</title>
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static void snd_xxx_drain(struct snd_rawmidi_substream *substream);
+]]>
+ </programlisting>
+ </informalexample>
+
+ <para>
+ This is only used with output substreams. This function should wait
+ until all data read from the substream buffer have been transmitted.
+ This ensures that the device can be closed and the driver unloaded
+ without losing data.
+ </para>
+
+ <para>
+ This callback is optional. If you do not set
+ <structfield>drain</structfield> in the struct snd_rawmidi_ops
+ structure, ALSA will simply wait for 50 milliseconds
+ instead.
+ </para>
+ </section>
+ </section>
+
+ </chapter>
+
+
+<!-- ****************************************************** -->
+<!-- Miscellaneous Devices -->
+<!-- ****************************************************** -->
+ <chapter id="misc-devices">
+ <title>Miscellaneous Devices</title>
+
+ <section id="misc-devices-opl3">
+ <title>FM OPL3</title>
+ <para>
+ The FM OPL3 is still used in many chips (mainly for backward
+ compatibility). ALSA has a nice OPL3 FM control layer, too. The
+ OPL3 API is defined in
+ <filename><sound/opl3.h></filename>.
+ </para>
+
+ <para>
+ FM registers can be directly accessed through the direct-FM API,
+ defined in <filename><sound/asound_fm.h></filename>. In
+ ALSA native mode, FM registers are accessed through
+ the Hardware-Dependent Device direct-FM extension API, whereas in
+ OSS compatible mode, FM registers can be accessed with the OSS
+ direct-FM compatible API in <filename>/dev/dmfmX</filename> device.
+ </para>
+
+ <para>
+ To create the OPL3 component, you have two functions to
+ call. The first one is a constructor for the <type>opl3_t</type>
+ instance.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ struct snd_opl3 *opl3;
+ snd_opl3_create(card, lport, rport, OPL3_HW_OPL3_XXX,
+ integrated, &opl3);
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ The first argument is the card pointer, the second one is the
+ left port address, and the third is the right port address. In
+ most cases, the right port is placed at the left port + 2.
+ </para>
+
+ <para>
+ The fourth argument is the hardware type.
+ </para>
+
+ <para>
+ When the left and right ports have been already allocated by
+ the card driver, pass non-zero to the fifth argument
+ (<parameter>integrated</parameter>). Otherwise, the opl3 module will
+ allocate the specified ports by itself.
+ </para>
+
+ <para>
+ When the accessing the hardware requires special method
+ instead of the standard I/O access, you can create opl3 instance
+ separately with <function>snd_opl3_new()</function>.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ struct snd_opl3 *opl3;
+ snd_opl3_new(card, OPL3_HW_OPL3_XXX, &opl3);
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ Then set <structfield>command</structfield>,
+ <structfield>private_data</structfield> and
+ <structfield>private_free</structfield> for the private
+ access function, the private data and the destructor.
+ The l_port and r_port are not necessarily set. Only the
+ command must be set properly. You can retrieve the data
+ from the opl3->private_data field.
+ </para>
+
+ <para>
+ After creating the opl3 instance via <function>snd_opl3_new()</function>,
+ call <function>snd_opl3_init()</function> to initialize the chip to the
+ proper state. Note that <function>snd_opl3_create()</function> always
+ calls it internally.
+ </para>
+
+ <para>
+ If the opl3 instance is created successfully, then create a
+ hwdep device for this opl3.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ struct snd_hwdep *opl3hwdep;
+ snd_opl3_hwdep_new(opl3, 0, 1, &opl3hwdep);
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ The first argument is the <type>opl3_t</type> instance you
+ created, and the second is the index number, usually 0.
+ </para>
+
+ <para>
+ The third argument is the index-offset for the sequencer
+ client assigned to the OPL3 port. When there is an MPU401-UART,
+ give 1 for here (UART always takes 0).
+ </para>
+ </section>
+
+ <section id="misc-devices-hardware-dependent">
+ <title>Hardware-Dependent Devices</title>
+ <para>
+ Some chips need user-space access for special
+ controls or for loading the micro code. In such a case, you can
+ create a hwdep (hardware-dependent) device. The hwdep API is
+ defined in <filename><sound/hwdep.h></filename>. You can
+ find examples in opl3 driver or
+ <filename>isa/sb/sb16_csp.c</filename>.
+ </para>
+
+ <para>
+ The creation of the <type>hwdep</type> instance is done via
+ <function>snd_hwdep_new()</function>.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ struct snd_hwdep *hw;
+ snd_hwdep_new(card, "My HWDEP", 0, &hw);
+]]>
+ </programlisting>
+ </informalexample>
+
+ where the third argument is the index number.
+ </para>
+
+ <para>
+ You can then pass any pointer value to the
+ <parameter>private_data</parameter>.
+ If you assign a private data, you should define the
+ destructor, too. The destructor function is set in
+ the <structfield>private_free</structfield> field.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ struct mydata *p = kmalloc(sizeof(*p), GFP_KERNEL);
+ hw->private_data = p;
+ hw->private_free = mydata_free;
+]]>
+ </programlisting>
+ </informalexample>
+
+ and the implementation of the destructor would be:
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static void mydata_free(struct snd_hwdep *hw)
+ {
+ struct mydata *p = hw->private_data;
+ kfree(p);
+ }
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ The arbitrary file operations can be defined for this
+ instance. The file operators are defined in
+ the <parameter>ops</parameter> table. For example, assume that
+ this chip needs an ioctl.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ hw->ops.open = mydata_open;
+ hw->ops.ioctl = mydata_ioctl;
+ hw->ops.release = mydata_release;
+]]>
+ </programlisting>
+ </informalexample>
+
+ And implement the callback functions as you like.
+ </para>
+ </section>
+
+ <section id="misc-devices-IEC958">
+ <title>IEC958 (S/PDIF)</title>
+ <para>
+ Usually the controls for IEC958 devices are implemented via
+ the control interface. There is a macro to compose a name string for
+ IEC958 controls, <function>SNDRV_CTL_NAME_IEC958()</function>
+ defined in <filename><include/asound.h></filename>.
+ </para>
+
+ <para>
+ There are some standard controls for IEC958 status bits. These
+ controls use the type <type>SNDRV_CTL_ELEM_TYPE_IEC958</type>,
+ and the size of element is fixed as 4 bytes array
+ (value.iec958.status[x]). For the <structfield>info</structfield>
+ callback, you don't specify
+ the value field for this type (the count field must be set,
+ though).
+ </para>
+
+ <para>
+ <quote>IEC958 Playback Con Mask</quote> is used to return the
+ bit-mask for the IEC958 status bits of consumer mode. Similarly,
+ <quote>IEC958 Playback Pro Mask</quote> returns the bitmask for
+ professional mode. They are read-only controls, and are defined
+ as MIXER controls (iface =
+ <constant>SNDRV_CTL_ELEM_IFACE_MIXER</constant>).
+ </para>
+
+ <para>
+ Meanwhile, <quote>IEC958 Playback Default</quote> control is
+ defined for getting and setting the current default IEC958
+ bits. Note that this one is usually defined as a PCM control
+ (iface = <constant>SNDRV_CTL_ELEM_IFACE_PCM</constant>),
+ although in some places it's defined as a MIXER control.
+ </para>
+
+ <para>
+ In addition, you can define the control switches to
+ enable/disable or to set the raw bit mode. The implementation
+ will depend on the chip, but the control should be named as
+ <quote>IEC958 xxx</quote>, preferably using
+ the <function>SNDRV_CTL_NAME_IEC958()</function> macro.
+ </para>
+
+ <para>
+ You can find several cases, for example,
+ <filename>pci/emu10k1</filename>,
+ <filename>pci/ice1712</filename>, or
+ <filename>pci/cmipci.c</filename>.
+ </para>
+ </section>
+
+ </chapter>
+
+
+<!-- ****************************************************** -->
+<!-- Buffer and Memory Management -->
+<!-- ****************************************************** -->
+ <chapter id="buffer-and-memory">
+ <title>Buffer and Memory Management</title>
+
+ <section id="buffer-and-memory-buffer-types">
+ <title>Buffer Types</title>
+ <para>
+ ALSA provides several different buffer allocation functions
+ depending on the bus and the architecture. All these have a
+ consistent API. The allocation of physically-contiguous pages is
+ done via
+ <function>snd_malloc_xxx_pages()</function> function, where xxx
+ is the bus type.
+ </para>
+
+ <para>
+ The allocation of pages with fallback is
+ <function>snd_malloc_xxx_pages_fallback()</function>. This
+ function tries to allocate the specified pages but if the pages
+ are not available, it tries to reduce the page sizes until
+ enough space is found.
+ </para>
+
+ <para>
+ The release the pages, call
+ <function>snd_free_xxx_pages()</function> function.
+ </para>
+
+ <para>
+ Usually, ALSA drivers try to allocate and reserve
+ a large contiguous physical space
+ at the time the module is loaded for the later use.
+ This is called <quote>pre-allocation</quote>.
+ As already written, you can call the following function at
+ pcm instance construction time (in the case of PCI bus).
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_DEV,
+ snd_dma_pci_data(pci), size, max);
+]]>
+ </programlisting>
+ </informalexample>
+
+ where <parameter>size</parameter> is the byte size to be
+ pre-allocated and the <parameter>max</parameter> is the maximum
+ size to be changed via the <filename>prealloc</filename> proc file.
+ The allocator will try to get an area as large as possible
+ within the given size.
+ </para>
+
+ <para>
+ The second argument (type) and the third argument (device pointer)
+ are dependent on the bus.
+ In the case of the ISA bus, pass <function>snd_dma_isa_data()</function>
+ as the third argument with <constant>SNDRV_DMA_TYPE_DEV</constant> type.
+ For the continuous buffer unrelated to the bus can be pre-allocated
+ with <constant>SNDRV_DMA_TYPE_CONTINUOUS</constant> type and the
+ <function>snd_dma_continuous_data(GFP_KERNEL)</function> device pointer,
+ where <constant>GFP_KERNEL</constant> is the kernel allocation flag to
+ use.
+ For the PCI scatter-gather buffers, use
+ <constant>SNDRV_DMA_TYPE_DEV_SG</constant> with
+ <function>snd_dma_pci_data(pci)</function>
+ (see the
+ <link linkend="buffer-and-memory-non-contiguous"><citetitle>Non-Contiguous Buffers
+ </citetitle></link> section).
+ </para>
+
+ <para>
+ Once the buffer is pre-allocated, you can use the
+ allocator in the <structfield>hw_params</structfield> callback:
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ snd_pcm_lib_malloc_pages(substream, size);
+]]>
+ </programlisting>
+ </informalexample>
+
+ Note that you have to pre-allocate to use this function.
+ </para>
+ </section>
+
+ <section id="buffer-and-memory-external-hardware">
+ <title>External Hardware Buffers</title>
+ <para>
+ Some chips have their own hardware buffers and the DMA
+ transfer from the host memory is not available. In such a case,
+ you need to either 1) copy/set the audio data directly to the
+ external hardware buffer, or 2) make an intermediate buffer and
+ copy/set the data from it to the external hardware buffer in
+ interrupts (or in tasklets, preferably).
+ </para>
+
+ <para>
+ The first case works fine if the external hardware buffer is large
+ enough. This method doesn't need any extra buffers and thus is
+ more effective. You need to define the
+ <structfield>copy</structfield> and
+ <structfield>silence</structfield> callbacks for
+ the data transfer. However, there is a drawback: it cannot
+ be mmapped. The examples are GUS's GF1 PCM or emu8000's
+ wavetable PCM.
+ </para>
+
+ <para>
+ The second case allows for mmap on the buffer, although you have
+ to handle an interrupt or a tasklet to transfer the data
+ from the intermediate buffer to the hardware buffer. You can find an
+ example in the vxpocket driver.
+ </para>
+
+ <para>
+ Another case is when the chip uses a PCI memory-map
+ region for the buffer instead of the host memory. In this case,
+ mmap is available only on certain architectures like the Intel one.
+ In non-mmap mode, the data cannot be transferred as in the normal
+ way. Thus you need to define the <structfield>copy</structfield> and
+ <structfield>silence</structfield> callbacks as well,
+ as in the cases above. The examples are found in
+ <filename>rme32.c</filename> and <filename>rme96.c</filename>.
+ </para>
+
+ <para>
+ The implementation of the <structfield>copy</structfield> and
+ <structfield>silence</structfield> callbacks depends upon
+ whether the hardware supports interleaved or non-interleaved
+ samples. The <structfield>copy</structfield> callback is
+ defined like below, a bit
+ differently depending whether the direction is playback or
+ capture:
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static int playback_copy(struct snd_pcm_substream *substream, int channel,
+ snd_pcm_uframes_t pos, void *src, snd_pcm_uframes_t count);
+ static int capture_copy(struct snd_pcm_substream *substream, int channel,
+ snd_pcm_uframes_t pos, void *dst, snd_pcm_uframes_t count);
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ In the case of interleaved samples, the second argument
+ (<parameter>channel</parameter>) is not used. The third argument
+ (<parameter>pos</parameter>) points the
+ current position offset in frames.
+ </para>
+
+ <para>
+ The meaning of the fourth argument is different between
+ playback and capture. For playback, it holds the source data
+ pointer, and for capture, it's the destination data pointer.
+ </para>
+
+ <para>
+ The last argument is the number of frames to be copied.
+ </para>
+
+ <para>
+ What you have to do in this callback is again different
+ between playback and capture directions. In the
+ playback case, you copy the given amount of data
+ (<parameter>count</parameter>) at the specified pointer
+ (<parameter>src</parameter>) to the specified offset
+ (<parameter>pos</parameter>) on the hardware buffer. When
+ coded like memcpy-like way, the copy would be like:
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ my_memcpy(my_buffer + frames_to_bytes(runtime, pos), src,
+ frames_to_bytes(runtime, count));
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ For the capture direction, you copy the given amount of
+ data (<parameter>count</parameter>) at the specified offset
+ (<parameter>pos</parameter>) on the hardware buffer to the
+ specified pointer (<parameter>dst</parameter>).
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ my_memcpy(dst, my_buffer + frames_to_bytes(runtime, pos),
+ frames_to_bytes(runtime, count));
+]]>
+ </programlisting>
+ </informalexample>
+
+ Note that both the position and the amount of data are given
+ in frames.
+ </para>
+
+ <para>
+ In the case of non-interleaved samples, the implementation
+ will be a bit more complicated.
+ </para>
+
+ <para>
+ You need to check the channel argument, and if it's -1, copy
+ the whole channels. Otherwise, you have to copy only the
+ specified channel. Please check
+ <filename>isa/gus/gus_pcm.c</filename> as an example.
+ </para>
+
+ <para>
+ The <structfield>silence</structfield> callback is also
+ implemented in a similar way.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static int silence(struct snd_pcm_substream *substream, int channel,
+ snd_pcm_uframes_t pos, snd_pcm_uframes_t count);
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ The meanings of arguments are the same as in the
+ <structfield>copy</structfield>
+ callback, although there is no <parameter>src/dst</parameter>
+ argument. In the case of interleaved samples, the channel
+ argument has no meaning, as well as on
+ <structfield>copy</structfield> callback.
+ </para>
+
+ <para>
+ The role of <structfield>silence</structfield> callback is to
+ set the given amount
+ (<parameter>count</parameter>) of silence data at the
+ specified offset (<parameter>pos</parameter>) on the hardware
+ buffer. Suppose that the data format is signed (that is, the
+ silent-data is 0), and the implementation using a memset-like
+ function would be like:
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ my_memcpy(my_buffer + frames_to_bytes(runtime, pos), 0,
+ frames_to_bytes(runtime, count));
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ In the case of non-interleaved samples, again, the
+ implementation becomes a bit more complicated. See, for example,
+ <filename>isa/gus/gus_pcm.c</filename>.
+ </para>
+ </section>
+
+ <section id="buffer-and-memory-non-contiguous">
+ <title>Non-Contiguous Buffers</title>
+ <para>
+ If your hardware supports the page table as in emu10k1 or the
+ buffer descriptors as in via82xx, you can use the scatter-gather
+ (SG) DMA. ALSA provides an interface for handling SG-buffers.
+ The API is provided in <filename><sound/pcm.h></filename>.
+ </para>
+
+ <para>
+ For creating the SG-buffer handler, call
+ <function>snd_pcm_lib_preallocate_pages()</function> or
+ <function>snd_pcm_lib_preallocate_pages_for_all()</function>
+ with <constant>SNDRV_DMA_TYPE_DEV_SG</constant>
+ in the PCM constructor like other PCI pre-allocator.
+ You need to pass <function>snd_dma_pci_data(pci)</function>,
+ where pci is the struct <structname>pci_dev</structname> pointer
+ of the chip as well.
+ The <type>struct snd_sg_buf</type> instance is created as
+ substream->dma_private. You can cast
+ the pointer like:
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ struct snd_sg_buf *sgbuf = (struct snd_sg_buf *)substream->dma_private;
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ Then call <function>snd_pcm_lib_malloc_pages()</function>
+ in the <structfield>hw_params</structfield> callback
+ as well as in the case of normal PCI buffer.
+ The SG-buffer handler will allocate the non-contiguous kernel
+ pages of the given size and map them onto the virtually contiguous
+ memory. The virtual pointer is addressed in runtime->dma_area.
+ The physical address (runtime->dma_addr) is set to zero,
+ because the buffer is physically non-contiguous.
+ The physical address table is set up in sgbuf->table.
+ You can get the physical address at a certain offset via
+ <function>snd_pcm_sgbuf_get_addr()</function>.
+ </para>
+
+ <para>
+ When a SG-handler is used, you need to set
+ <function>snd_pcm_sgbuf_ops_page</function> as
+ the <structfield>page</structfield> callback.
+ (See <link linkend="pcm-interface-operators-page-callback">
+ <citetitle>page callback section</citetitle></link>.)
+ </para>
+
+ <para>
+ To release the data, call
+ <function>snd_pcm_lib_free_pages()</function> in the
+ <structfield>hw_free</structfield> callback as usual.
+ </para>
+ </section>
+
+ <section id="buffer-and-memory-vmalloced">
+ <title>Vmalloc'ed Buffers</title>
+ <para>
+ It's possible to use a buffer allocated via
+ <function>vmalloc</function>, for example, for an intermediate
+ buffer. Since the allocated pages are not contiguous, you need
+ to set the <structfield>page</structfield> callback to obtain
+ the physical address at every offset.
+ </para>
+
+ <para>
+ The implementation of <structfield>page</structfield> callback
+ would be like this:
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ #include <linux/vmalloc.h>
+
+ /* get the physical page pointer on the given offset */
+ static struct page *mychip_page(struct snd_pcm_substream *substream,
+ unsigned long offset)
+ {
+ void *pageptr = substream->runtime->dma_area + offset;
+ return vmalloc_to_page(pageptr);
+ }
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+ </section>
+
+ </chapter>
+
+
+<!-- ****************************************************** -->
+<!-- Proc Interface -->
+<!-- ****************************************************** -->
+ <chapter id="proc-interface">
+ <title>Proc Interface</title>
+ <para>
+ ALSA provides an easy interface for procfs. The proc files are
+ very useful for debugging. I recommend you set up proc files if
+ you write a driver and want to get a running status or register
+ dumps. The API is found in
+ <filename><sound/info.h></filename>.
+ </para>
+
+ <para>
+ To create a proc file, call
+ <function>snd_card_proc_new()</function>.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ struct snd_info_entry *entry;
+ int err = snd_card_proc_new(card, "my-file", &entry);
+]]>
+ </programlisting>
+ </informalexample>
+
+ where the second argument specifies the name of the proc file to be
+ created. The above example will create a file
+ <filename>my-file</filename> under the card directory,
+ e.g. <filename>/proc/asound/card0/my-file</filename>.
+ </para>
+
+ <para>
+ Like other components, the proc entry created via
+ <function>snd_card_proc_new()</function> will be registered and
+ released automatically in the card registration and release
+ functions.
+ </para>
+
+ <para>
+ When the creation is successful, the function stores a new
+ instance in the pointer given in the third argument.
+ It is initialized as a text proc file for read only. To use
+ this proc file as a read-only text file as it is, set the read
+ callback with a private data via
+ <function>snd_info_set_text_ops()</function>.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ snd_info_set_text_ops(entry, chip, my_proc_read);
+]]>
+ </programlisting>
+ </informalexample>
+
+ where the second argument (<parameter>chip</parameter>) is the
+ private data to be used in the callbacks. The third parameter
+ specifies the read buffer size and the fourth
+ (<parameter>my_proc_read</parameter>) is the callback function, which
+ is defined like
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static void my_proc_read(struct snd_info_entry *entry,
+ struct snd_info_buffer *buffer);
+]]>
+ </programlisting>
+ </informalexample>
+
+ </para>
+
+ <para>
+ In the read callback, use <function>snd_iprintf()</function> for
+ output strings, which works just like normal
+ <function>printf()</function>. For example,
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static void my_proc_read(struct snd_info_entry *entry,
+ struct snd_info_buffer *buffer)
+ {
+ struct my_chip *chip = entry->private_data;
+
+ snd_iprintf(buffer, "This is my chip!\n");
+ snd_iprintf(buffer, "Port = %ld\n", chip->port);
+ }
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ The file permissions can be changed afterwards. As default, it's
+ set as read only for all users. If you want to add write
+ permission for the user (root as default), do as follows:
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ entry->mode = S_IFREG | S_IRUGO | S_IWUSR;
+]]>
+ </programlisting>
+ </informalexample>
+
+ and set the write buffer size and the callback
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ entry->c.text.write = my_proc_write;
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ For the write callback, you can use
+ <function>snd_info_get_line()</function> to get a text line, and
+ <function>snd_info_get_str()</function> to retrieve a string from
+ the line. Some examples are found in
+ <filename>core/oss/mixer_oss.c</filename>, core/oss/and
+ <filename>pcm_oss.c</filename>.
+ </para>
+
+ <para>
+ For a raw-data proc-file, set the attributes as follows:
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static struct snd_info_entry_ops my_file_io_ops = {
+ .read = my_file_io_read,
+ };
+
+ entry->content = SNDRV_INFO_CONTENT_DATA;
+ entry->private_data = chip;
+ entry->c.ops = &my_file_io_ops;
+ entry->size = 4096;
+ entry->mode = S_IFREG | S_IRUGO;
+]]>
+ </programlisting>
+ </informalexample>
+
+ For the raw data, <structfield>size</structfield> field must be
+ set properly. This specifies the maximum size of the proc file access.
+ </para>
+
+ <para>
+ The read/write callbacks of raw mode are more direct than the text mode.
+ You need to use a low-level I/O functions such as
+ <function>copy_from/to_user()</function> to transfer the
+ data.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static ssize_t my_file_io_read(struct snd_info_entry *entry,
+ void *file_private_data,
+ struct file *file,
+ char *buf,
+ size_t count,
+ loff_t pos)
+ {
+ if (copy_to_user(buf, local_data + pos, count))
+ return -EFAULT;
+ return count;
+ }
+]]>
+ </programlisting>
+ </informalexample>
+
+ If the size of the info entry has been set up properly,
+ <structfield>count</structfield> and <structfield>pos</structfield> are
+ guaranteed to fit within 0 and the given size.
+ You don't have to check the range in the callbacks unless any
+ other condition is required.
+
+ </para>
+
+ </chapter>
+
+
+<!-- ****************************************************** -->
+<!-- Power Management -->
+<!-- ****************************************************** -->
+ <chapter id="power-management">
+ <title>Power Management</title>
+ <para>
+ If the chip is supposed to work with suspend/resume
+ functions, you need to add power-management code to the
+ driver. The additional code for power-management should be
+ <function>ifdef</function>'ed with
+ <constant>CONFIG_PM</constant>.
+ </para>
+
+ <para>
+ If the driver <emphasis>fully</emphasis> supports suspend/resume
+ that is, the device can be
+ properly resumed to its state when suspend was called,
+ you can set the <constant>SNDRV_PCM_INFO_RESUME</constant> flag
+ in the pcm info field. Usually, this is possible when the
+ registers of the chip can be safely saved and restored to
+ RAM. If this is set, the trigger callback is called with
+ <constant>SNDRV_PCM_TRIGGER_RESUME</constant> after the resume
+ callback completes.
+ </para>
+
+ <para>
+ Even if the driver doesn't support PM fully but
+ partial suspend/resume is still possible, it's still worthy to
+ implement suspend/resume callbacks. In such a case, applications
+ would reset the status by calling
+ <function>snd_pcm_prepare()</function> and restart the stream
+ appropriately. Hence, you can define suspend/resume callbacks
+ below but don't set <constant>SNDRV_PCM_INFO_RESUME</constant>
+ info flag to the PCM.
+ </para>
+
+ <para>
+ Note that the trigger with SUSPEND can always be called when
+ <function>snd_pcm_suspend_all</function> is called,
+ regardless of the <constant>SNDRV_PCM_INFO_RESUME</constant> flag.
+ The <constant>RESUME</constant> flag affects only the behavior
+ of <function>snd_pcm_resume()</function>.
+ (Thus, in theory,
+ <constant>SNDRV_PCM_TRIGGER_RESUME</constant> isn't needed
+ to be handled in the trigger callback when no
+ <constant>SNDRV_PCM_INFO_RESUME</constant> flag is set. But,
+ it's better to keep it for compatibility reasons.)
+ </para>
+ <para>
+ In the earlier version of ALSA drivers, a common
+ power-management layer was provided, but it has been removed.
+ The driver needs to define the suspend/resume hooks according to
+ the bus the device is connected to. In the case of PCI drivers, the
+ callbacks look like below:
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ #ifdef CONFIG_PM
+ static int snd_my_suspend(struct pci_dev *pci, pm_message_t state)
+ {
+ .... /* do things for suspend */
+ return 0;
+ }
+ static int snd_my_resume(struct pci_dev *pci)
+ {
+ .... /* do things for suspend */
+ return 0;
+ }
+ #endif
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ The scheme of the real suspend job is as follows.
+
+ <orderedlist>
+ <listitem><para>Retrieve the card and the chip data.</para></listitem>
+ <listitem><para>Call <function>snd_power_change_state()</function> with
+ <constant>SNDRV_CTL_POWER_D3hot</constant> to change the
+ power status.</para></listitem>
+ <listitem><para>Call <function>snd_pcm_suspend_all()</function> to suspend the running PCM streams.</para></listitem>
+ <listitem><para>If AC97 codecs are used, call
+ <function>snd_ac97_suspend()</function> for each codec.</para></listitem>
+ <listitem><para>Save the register values if necessary.</para></listitem>
+ <listitem><para>Stop the hardware if necessary.</para></listitem>
+ <listitem><para>Disable the PCI device by calling
+ <function>pci_disable_device()</function>. Then, call
+ <function>pci_save_state()</function> at last.</para></listitem>
+ </orderedlist>
+ </para>
+
+ <para>
+ A typical code would be like:
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static int mychip_suspend(struct pci_dev *pci, pm_message_t state)
+ {
+ /* (1) */
+ struct snd_card *card = pci_get_drvdata(pci);
+ struct mychip *chip = card->private_data;
+ /* (2) */
+ snd_power_change_state(card, SNDRV_CTL_POWER_D3hot);
+ /* (3) */
+ snd_pcm_suspend_all(chip->pcm);
+ /* (4) */
+ snd_ac97_suspend(chip->ac97);
+ /* (5) */
+ snd_mychip_save_registers(chip);
+ /* (6) */
+ snd_mychip_stop_hardware(chip);
+ /* (7) */
+ pci_disable_device(pci);
+ pci_save_state(pci);
+ return 0;
+ }
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ The scheme of the real resume job is as follows.
+
+ <orderedlist>
+ <listitem><para>Retrieve the card and the chip data.</para></listitem>
+ <listitem><para>Set up PCI. First, call <function>pci_restore_state()</function>.
+ Then enable the pci device again by calling <function>pci_enable_device()</function>.
+ Call <function>pci_set_master()</function> if necessary, too.</para></listitem>
+ <listitem><para>Re-initialize the chip.</para></listitem>
+ <listitem><para>Restore the saved registers if necessary.</para></listitem>
+ <listitem><para>Resume the mixer, e.g. calling
+ <function>snd_ac97_resume()</function>.</para></listitem>
+ <listitem><para>Restart the hardware (if any).</para></listitem>
+ <listitem><para>Call <function>snd_power_change_state()</function> with
+ <constant>SNDRV_CTL_POWER_D0</constant> to notify the processes.</para></listitem>
+ </orderedlist>
+ </para>
+
+ <para>
+ A typical code would be like:
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static int mychip_resume(struct pci_dev *pci)
+ {
+ /* (1) */
+ struct snd_card *card = pci_get_drvdata(pci);
+ struct mychip *chip = card->private_data;
+ /* (2) */
+ pci_restore_state(pci);
+ pci_enable_device(pci);
+ pci_set_master(pci);
+ /* (3) */
+ snd_mychip_reinit_chip(chip);
+ /* (4) */
+ snd_mychip_restore_registers(chip);
+ /* (5) */
+ snd_ac97_resume(chip->ac97);
+ /* (6) */
+ snd_mychip_restart_chip(chip);
+ /* (7) */
+ snd_power_change_state(card, SNDRV_CTL_POWER_D0);
+ return 0;
+ }
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ As shown in the above, it's better to save registers after
+ suspending the PCM operations via
+ <function>snd_pcm_suspend_all()</function> or
+ <function>snd_pcm_suspend()</function>. It means that the PCM
+ streams are already stopped when the register snapshot is
+ taken. But, remember that you don't have to restart the PCM
+ stream in the resume callback. It'll be restarted via
+ trigger call with <constant>SNDRV_PCM_TRIGGER_RESUME</constant>
+ when necessary.
+ </para>
+
+ <para>
+ OK, we have all callbacks now. Let's set them up. In the
+ initialization of the card, make sure that you can get the chip
+ data from the card instance, typically via
+ <structfield>private_data</structfield> field, in case you
+ created the chip data individually.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static int snd_mychip_probe(struct pci_dev *pci,
+ const struct pci_device_id *pci_id)
+ {
+ ....
+ struct snd_card *card;
+ struct mychip *chip;
+ int err;
+ ....
+ err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
+ 0, &card);
+ ....
+ chip = kzalloc(sizeof(*chip), GFP_KERNEL);
+ ....
+ card->private_data = chip;
+ ....
+ }
+]]>
+ </programlisting>
+ </informalexample>
+
+ When you created the chip data with
+ <function>snd_card_new()</function>, it's anyway accessible
+ via <structfield>private_data</structfield> field.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static int snd_mychip_probe(struct pci_dev *pci,
+ const struct pci_device_id *pci_id)
+ {
+ ....
+ struct snd_card *card;
+ struct mychip *chip;
+ int err;
+ ....
+ err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
+ sizeof(struct mychip), &card);
+ ....
+ chip = card->private_data;
+ ....
+ }
+]]>
+ </programlisting>
+ </informalexample>
+
+ </para>
+
+ <para>
+ If you need a space to save the registers, allocate the
+ buffer for it here, too, since it would be fatal
+ if you cannot allocate a memory in the suspend phase.
+ The allocated buffer should be released in the corresponding
+ destructor.
+ </para>
+
+ <para>
+ And next, set suspend/resume callbacks to the pci_driver.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static struct pci_driver driver = {
+ .name = KBUILD_MODNAME,
+ .id_table = snd_my_ids,
+ .probe = snd_my_probe,
+ .remove = snd_my_remove,
+ #ifdef CONFIG_PM
+ .suspend = snd_my_suspend,
+ .resume = snd_my_resume,
+ #endif
+ };
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ </chapter>
+
+
+<!-- ****************************************************** -->
+<!-- Module Parameters -->
+<!-- ****************************************************** -->
+ <chapter id="module-parameters">
+ <title>Module Parameters</title>
+ <para>
+ There are standard module options for ALSA. At least, each
+ module should have the <parameter>index</parameter>,
+ <parameter>id</parameter> and <parameter>enable</parameter>
+ options.
+ </para>
+
+ <para>
+ If the module supports multiple cards (usually up to
+ 8 = <constant>SNDRV_CARDS</constant> cards), they should be
+ arrays. The default initial values are defined already as
+ constants for easier programming:
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ static int index[SNDRV_CARDS] = SNDRV_DEFAULT_IDX;
+ static char *id[SNDRV_CARDS] = SNDRV_DEFAULT_STR;
+ static int enable[SNDRV_CARDS] = SNDRV_DEFAULT_ENABLE_PNP;
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ If the module supports only a single card, they could be single
+ variables, instead. <parameter>enable</parameter> option is not
+ always necessary in this case, but it would be better to have a
+ dummy option for compatibility.
+ </para>
+
+ <para>
+ The module parameters must be declared with the standard
+ <function>module_param()()</function>,
+ <function>module_param_array()()</function> and
+ <function>MODULE_PARM_DESC()</function> macros.
+ </para>
+
+ <para>
+ The typical coding would be like below:
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ #define CARD_NAME "My Chip"
+
+ module_param_array(index, int, NULL, 0444);
+ MODULE_PARM_DESC(index, "Index value for " CARD_NAME " soundcard.");
+ module_param_array(id, charp, NULL, 0444);
+ MODULE_PARM_DESC(id, "ID string for " CARD_NAME " soundcard.");
+ module_param_array(enable, bool, NULL, 0444);
+ MODULE_PARM_DESC(enable, "Enable " CARD_NAME " soundcard.");
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ Also, don't forget to define the module description, classes,
+ license and devices. Especially, the recent modprobe requires to
+ define the module license as GPL, etc., otherwise the system is
+ shown as <quote>tainted</quote>.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ MODULE_DESCRIPTION("My Chip");
+ MODULE_LICENSE("GPL");
+ MODULE_SUPPORTED_DEVICE("{{Vendor,My Chip Name}}");
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ </chapter>
+
+
+<!-- ****************************************************** -->
+<!-- How To Put Your Driver -->
+<!-- ****************************************************** -->
+ <chapter id="how-to-put-your-driver">
+ <title>How To Put Your Driver Into ALSA Tree</title>
+ <section>
+ <title>General</title>
+ <para>
+ So far, you've learned how to write the driver codes.
+ And you might have a question now: how to put my own
+ driver into the ALSA driver tree?
+ Here (finally :) the standard procedure is described briefly.
+ </para>
+
+ <para>
+ Suppose that you create a new PCI driver for the card
+ <quote>xyz</quote>. The card module name would be
+ snd-xyz. The new driver is usually put into the alsa-driver
+ tree, <filename>alsa-driver/pci</filename> directory in
+ the case of PCI cards.
+ Then the driver is evaluated, audited and tested
+ by developers and users. After a certain time, the driver
+ will go to the alsa-kernel tree (to the corresponding directory,
+ such as <filename>alsa-kernel/pci</filename>) and eventually
+ will be integrated into the Linux 2.6 tree (the directory would be
+ <filename>linux/sound/pci</filename>).
+ </para>
+
+ <para>
+ In the following sections, the driver code is supposed
+ to be put into alsa-driver tree. The two cases are covered:
+ a driver consisting of a single source file and one consisting
+ of several source files.
+ </para>
+ </section>
+
+ <section>
+ <title>Driver with A Single Source File</title>
+ <para>
+ <orderedlist>
+ <listitem>
+ <para>
+ Modify alsa-driver/pci/Makefile
+ </para>
+
+ <para>
+ Suppose you have a file xyz.c. Add the following
+ two lines
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ snd-xyz-objs := xyz.o
+ obj-$(CONFIG_SND_XYZ) += snd-xyz.o
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Create the Kconfig entry
+ </para>
+
+ <para>
+ Add the new entry of Kconfig for your xyz driver.
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ config SND_XYZ
+ tristate "Foobar XYZ"
+ depends on SND
+ select SND_PCM
+ help
+ Say Y here to include support for Foobar XYZ soundcard.
+
+ To compile this driver as a module, choose M here: the module
+ will be called snd-xyz.
+]]>
+ </programlisting>
+ </informalexample>
+
+ the line, select SND_PCM, specifies that the driver xyz supports
+ PCM. In addition to SND_PCM, the following components are
+ supported for select command:
+ SND_RAWMIDI, SND_TIMER, SND_HWDEP, SND_MPU401_UART,
+ SND_OPL3_LIB, SND_OPL4_LIB, SND_VX_LIB, SND_AC97_CODEC.
+ Add the select command for each supported component.
+ </para>
+
+ <para>
+ Note that some selections imply the lowlevel selections.
+ For example, PCM includes TIMER, MPU401_UART includes RAWMIDI,
+ AC97_CODEC includes PCM, and OPL3_LIB includes HWDEP.
+ You don't need to give the lowlevel selections again.
+ </para>
+
+ <para>
+ For the details of Kconfig script, refer to the kbuild
+ documentation.
+ </para>
+
+ </listitem>
+
+ <listitem>
+ <para>
+ Run cvscompile script to re-generate the configure script and
+ build the whole stuff again.
+ </para>
+ </listitem>
+ </orderedlist>
+ </para>
+ </section>
+
+ <section>
+ <title>Drivers with Several Source Files</title>
+ <para>
+ Suppose that the driver snd-xyz have several source files.
+ They are located in the new subdirectory,
+ pci/xyz.
+
+ <orderedlist>
+ <listitem>
+ <para>
+ Add a new directory (<filename>xyz</filename>) in
+ <filename>alsa-driver/pci/Makefile</filename> as below
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ obj-$(CONFIG_SND) += xyz/
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Under the directory <filename>xyz</filename>, create a Makefile
+
+ <example>
+ <title>Sample Makefile for a driver xyz</title>
+ <programlisting>
+<![CDATA[
+ ifndef SND_TOPDIR
+ SND_TOPDIR=../..
+ endif
+
+ include $(SND_TOPDIR)/toplevel.config
+ include $(SND_TOPDIR)/Makefile.conf
+
+ snd-xyz-objs := xyz.o abc.o def.o
+
+ obj-$(CONFIG_SND_XYZ) += snd-xyz.o
+
+ include $(SND_TOPDIR)/Rules.make
+]]>
+ </programlisting>
+ </example>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Create the Kconfig entry
+ </para>
+
+ <para>
+ This procedure is as same as in the last section.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Run cvscompile script to re-generate the configure script and
+ build the whole stuff again.
+ </para>
+ </listitem>
+ </orderedlist>
+ </para>
+ </section>
+
+ </chapter>
+
+<!-- ****************************************************** -->
+<!-- Useful Functions -->
+<!-- ****************************************************** -->
+ <chapter id="useful-functions">
+ <title>Useful Functions</title>
+
+ <section id="useful-functions-snd-printk">
+ <title><function>snd_printk()</function> and friends</title>
+ <para>
+ ALSA provides a verbose version of the
+ <function>printk()</function> function. If a kernel config
+ <constant>CONFIG_SND_VERBOSE_PRINTK</constant> is set, this
+ function prints the given message together with the file name
+ and the line of the caller. The <constant>KERN_XXX</constant>
+ prefix is processed as
+ well as the original <function>printk()</function> does, so it's
+ recommended to add this prefix, e.g.
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ snd_printk(KERN_ERR "Oh my, sorry, it's extremely bad!\n");
+]]>
+ </programlisting>
+ </informalexample>
+ </para>
+
+ <para>
+ There are also <function>printk()</function>'s for
+ debugging. <function>snd_printd()</function> can be used for
+ general debugging purposes. If
+ <constant>CONFIG_SND_DEBUG</constant> is set, this function is
+ compiled, and works just like
+ <function>snd_printk()</function>. If the ALSA is compiled
+ without the debugging flag, it's ignored.
+ </para>
+
+ <para>
+ <function>snd_printdd()</function> is compiled in only when
+ <constant>CONFIG_SND_DEBUG_VERBOSE</constant> is set. Please note
+ that <constant>CONFIG_SND_DEBUG_VERBOSE</constant> is not set as default
+ even if you configure the alsa-driver with
+ <option>--with-debug=full</option> option. You need to give
+ explicitly <option>--with-debug=detect</option> option instead.
+ </para>
+ </section>
+
+ <section id="useful-functions-snd-bug">
+ <title><function>snd_BUG()</function></title>
+ <para>
+ It shows the <computeroutput>BUG?</computeroutput> message and
+ stack trace as well as <function>snd_BUG_ON</function> at the point.
+ It's useful to show that a fatal error happens there.
+ </para>
+ <para>
+ When no debug flag is set, this macro is ignored.
+ </para>
+ </section>
+
+ <section id="useful-functions-snd-bug-on">
+ <title><function>snd_BUG_ON()</function></title>
+ <para>
+ <function>snd_BUG_ON()</function> macro is similar with
+ <function>WARN_ON()</function> macro. For example,
+
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ snd_BUG_ON(!pointer);
+]]>
+ </programlisting>
+ </informalexample>
+
+ or it can be used as the condition,
+ <informalexample>
+ <programlisting>
+<![CDATA[
+ if (snd_BUG_ON(non_zero_is_bug))
+ return -EINVAL;
+]]>
+ </programlisting>
+ </informalexample>
+
+ </para>
+
+ <para>
+ The macro takes an conditional expression to evaluate.
+ When <constant>CONFIG_SND_DEBUG</constant>, is set, if the
+ expression is non-zero, it shows the warning message such as
+ <computeroutput>BUG? (xxx)</computeroutput>
+ normally followed by stack trace.
+
+ In both cases it returns the evaluated value.
+ </para>
+
+ </section>
+
+ </chapter>
+
+
+<!-- ****************************************************** -->
+<!-- Acknowledgments -->
+<!-- ****************************************************** -->
+ <chapter id="acknowledgments">
+ <title>Acknowledgments</title>
+ <para>
+ I would like to thank Phil Kerr for his help for improvement and
+ corrections of this document.
+ </para>
+ <para>
+ Kevin Conder reformatted the original plain-text to the
+ DocBook format.
+ </para>
+ <para>
+ Giuliano Pochini corrected typos and contributed the example codes
+ in the hardware constraints section.
+ </para>
+ </chapter>
+</book>
Code Review / kvmfornfv.git / blobdiff