Add the rt linux 4.1.3-rt3 as base

[kvmfornfv.git] / kernel / Documentation / i2c / i2c-protocol

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+This document describes the i2c protocol. Or will, when it is finished :-)
+
+Key to symbols
+==============
+
+S     (1 bit) : Start bit
+P     (1 bit) : Stop bit
+Rd/Wr (1 bit) : Read/Write bit. Rd equals 1, Wr equals 0.
+A, NA (1 bit) : Accept and reverse accept bit.
+Addr  (7 bits): I2C 7 bit address. Note that this can be expanded as usual to
+                get a 10 bit I2C address.
+Comm  (8 bits): Command byte, a data byte which often selects a register on
+                the device.
+Data  (8 bits): A plain data byte. Sometimes, I write DataLow, DataHigh
+                for 16 bit data.
+Count (8 bits): A data byte containing the length of a block operation.
+
+[..]: Data sent by I2C device, as opposed to data sent by the host adapter.
+
+
+Simple send transaction
+======================
+
+This corresponds to i2c_master_send.
+
+  S Addr Wr [A] Data [A] Data [A] ... [A] Data [A] P
+
+
+Simple receive transaction
+===========================
+
+This corresponds to i2c_master_recv
+
+  S Addr Rd [A] [Data] A [Data] A ... A [Data] NA P
+
+
+Combined transactions
+====================
+
+This corresponds to i2c_transfer
+
+They are just like the above transactions, but instead of a stop bit P
+a start bit S is sent and the transaction continues. An example of
+a byte read, followed by a byte write:
+
+  S Addr Rd [A] [Data] NA S Addr Wr [A] Data [A] P
+
+
+Modified transactions
+=====================
+
+The following modifications to the I2C protocol can also be generated by
+setting these flags for i2c messages. With the exception of I2C_M_NOSTART, they
+are usually only needed to work around device issues:
+
+I2C_M_IGNORE_NAK:
+    Normally message is interrupted immediately if there is [NA] from the
+    client. Setting this flag treats any [NA] as [A], and all of
+    message is sent.
+    These messages may still fail to SCL lo->hi timeout.
+
+I2C_M_NO_RD_ACK:
+    In a read message, master A/NA bit is skipped.
+
+I2C_M_NOSTART:
+    In a combined transaction, no 'S Addr Wr/Rd [A]' is generated at some
+    point. For example, setting I2C_M_NOSTART on the second partial message
+    generates something like:
+      S Addr Rd [A] [Data] NA Data [A] P
+    If you set the I2C_M_NOSTART variable for the first partial message,
+    we do not generate Addr, but we do generate the startbit S. This will
+    probably confuse all other clients on your bus, so don't try this.
+
+    This is often used to gather transmits from multiple data buffers in
+    system memory into something that appears as a single transfer to the
+    I2C device but may also be used between direction changes by some
+    rare devices.
+
+I2C_M_REV_DIR_ADDR:
+    This toggles the Rd/Wr flag. That is, if you want to do a write, but
+    need to emit an Rd instead of a Wr, or vice versa, you set this
+    flag. For example:
+      S Addr Rd [A] Data [A] Data [A] ... [A] Data [A] P
+
+I2C_M_STOP:
+    Force a stop condition (P) after the message. Some I2C related protocols
+    like SCCB require that. Normally, you really don't want to get interrupted
+    between the messages of one transfer.