--- /dev/null
+ <title>Input/Output</title>
+
+ <para>The V4L2 API defines several different methods to read from or
+write to a device. All drivers exchanging data with applications must
+support at least one of them.</para>
+
+ <para>The classic I/O method using the <function>read()</function>
+and <function>write()</function> function is automatically selected
+after opening a V4L2 device. When the driver does not support this
+method attempts to read or write will fail at any time.</para>
+
+ <para>Other methods must be negotiated. To select the streaming I/O
+method with memory mapped or user buffers applications call the
+&VIDIOC-REQBUFS; ioctl. The asynchronous I/O method is not defined
+yet.</para>
+
+ <para>Video overlay can be considered another I/O method, although
+the application does not directly receive the image data. It is
+selected by initiating video overlay with the &VIDIOC-S-FMT; ioctl.
+For more information see <xref linkend="overlay" />.</para>
+
+ <para>Generally exactly one I/O method, including overlay, is
+associated with each file descriptor. The only exceptions are
+applications not exchanging data with a driver ("panel applications",
+see <xref linkend="open" />) and drivers permitting simultaneous video capturing
+and overlay using the same file descriptor, for compatibility with V4L
+and earlier versions of V4L2.</para>
+
+ <para><constant>VIDIOC_S_FMT</constant> and
+<constant>VIDIOC_REQBUFS</constant> would permit this to some degree,
+but for simplicity drivers need not support switching the I/O method
+(after first switching away from read/write) other than by closing
+and reopening the device.</para>
+
+ <para>The following sections describe the various I/O methods in
+more detail.</para>
+
+ <section id="rw">
+ <title>Read/Write</title>
+
+ <para>Input and output devices support the
+<function>read()</function> and <function>write()</function> function,
+respectively, when the <constant>V4L2_CAP_READWRITE</constant> flag in
+the <structfield>capabilities</structfield> field of &v4l2-capability;
+returned by the &VIDIOC-QUERYCAP; ioctl is set.</para>
+
+ <para>Drivers may need the CPU to copy the data, but they may also
+support DMA to or from user memory, so this I/O method is not
+necessarily less efficient than other methods merely exchanging buffer
+pointers. It is considered inferior though because no meta-information
+like frame counters or timestamps are passed. This information is
+necessary to recognize frame dropping and to synchronize with other
+data streams. However this is also the simplest I/O method, requiring
+little or no setup to exchange data. It permits command line stunts
+like this (the <application>vidctrl</application> tool is
+fictitious):</para>
+
+ <informalexample>
+ <screen>
+> vidctrl /dev/video --input=0 --format=YUYV --size=352x288
+> dd if=/dev/video of=myimage.422 bs=202752 count=1
+</screen>
+ </informalexample>
+
+ <para>To read from the device applications use the
+&func-read; function, to write the &func-write; function.
+Drivers must implement one I/O method if they
+exchange data with applications, but it need not be this.<footnote>
+ <para>It would be desirable if applications could depend on
+drivers supporting all I/O interfaces, but as much as the complex
+memory mapping I/O can be inadequate for some devices we have no
+reason to require this interface, which is most useful for simple
+applications capturing still images.</para>
+ </footnote> When reading or writing is supported, the driver
+must also support the &func-select; and &func-poll;
+function.<footnote>
+ <para>At the driver level <function>select()</function> and
+<function>poll()</function> are the same, and
+<function>select()</function> is too important to be optional.</para>
+ </footnote></para>
+ </section>
+
+ <section id="mmap">
+ <title>Streaming I/O (Memory Mapping)</title>
+
+ <para>Input and output devices support this I/O method when the
+<constant>V4L2_CAP_STREAMING</constant> flag in the
+<structfield>capabilities</structfield> field of &v4l2-capability;
+returned by the &VIDIOC-QUERYCAP; ioctl is set. There are two
+streaming methods, to determine if the memory mapping flavor is
+supported applications must call the &VIDIOC-REQBUFS; ioctl.</para>
+
+ <para>Streaming is an I/O method where only pointers to buffers
+are exchanged between application and driver, the data itself is not
+copied. Memory mapping is primarily intended to map buffers in device
+memory into the application's address space. Device memory can be for
+example the video memory on a graphics card with a video capture
+add-on. However, being the most efficient I/O method available for a
+long time, many other drivers support streaming as well, allocating
+buffers in DMA-able main memory.</para>
+
+ <para>A driver can support many sets of buffers. Each set is
+identified by a unique buffer type value. The sets are independent and
+each set can hold a different type of data. To access different sets
+at the same time different file descriptors must be used.<footnote>
+ <para>One could use one file descriptor and set the buffer
+type field accordingly when calling &VIDIOC-QBUF; etc., but it makes
+the <function>select()</function> function ambiguous. We also like the
+clean approach of one file descriptor per logical stream. Video
+overlay for example is also a logical stream, although the CPU is not
+needed for continuous operation.</para>
+ </footnote></para>
+
+ <para>To allocate device buffers applications call the
+&VIDIOC-REQBUFS; ioctl with the desired number of buffers and buffer
+type, for example <constant>V4L2_BUF_TYPE_VIDEO_CAPTURE</constant>.
+This ioctl can also be used to change the number of buffers or to free
+the allocated memory, provided none of the buffers are still
+mapped.</para>
+
+ <para>Before applications can access the buffers they must map
+them into their address space with the &func-mmap; function. The
+location of the buffers in device memory can be determined with the
+&VIDIOC-QUERYBUF; ioctl. In the single-planar API case, the
+<structfield>m.offset</structfield> and <structfield>length</structfield>
+returned in a &v4l2-buffer; are passed as sixth and second parameter to the
+<function>mmap()</function> function. When using the multi-planar API,
+&v4l2-buffer; contains an array of &v4l2-plane; structures, each
+containing its own <structfield>m.offset</structfield> and
+<structfield>length</structfield>. When using the multi-planar API, every
+plane of every buffer has to be mapped separately, so the number of
+calls to &func-mmap; should be equal to number of buffers times number of
+planes in each buffer. The offset and length values must not be modified.
+Remember, the buffers are allocated in physical memory, as opposed to virtual
+memory, which can be swapped out to disk. Applications should free the buffers
+as soon as possible with the &func-munmap; function.</para>
+
+ <example>
+ <title>Mapping buffers in the single-planar API</title>
+ <programlisting>
+&v4l2-requestbuffers; reqbuf;
+struct {
+ void *start;
+ size_t length;
+} *buffers;
+unsigned int i;
+
+memset(&reqbuf, 0, sizeof(reqbuf));
+reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
+reqbuf.memory = V4L2_MEMORY_MMAP;
+reqbuf.count = 20;
+
+if (-1 == ioctl (fd, &VIDIOC-REQBUFS;, &reqbuf)) {
+ if (errno == EINVAL)
+ printf("Video capturing or mmap-streaming is not supported\n");
+ else
+ perror("VIDIOC_REQBUFS");
+
+ exit(EXIT_FAILURE);
+}
+
+/* We want at least five buffers. */
+
+if (reqbuf.count < 5) {
+ /* You may need to free the buffers here. */
+ printf("Not enough buffer memory\n");
+ exit(EXIT_FAILURE);
+}
+
+buffers = calloc(reqbuf.count, sizeof(*buffers));
+assert(buffers != NULL);
+
+for (i = 0; i < reqbuf.count; i++) {
+ &v4l2-buffer; buffer;
+
+ memset(&buffer, 0, sizeof(buffer));
+ buffer.type = reqbuf.type;
+ buffer.memory = V4L2_MEMORY_MMAP;
+ buffer.index = i;
+
+ if (-1 == ioctl (fd, &VIDIOC-QUERYBUF;, &buffer)) {
+ perror("VIDIOC_QUERYBUF");
+ exit(EXIT_FAILURE);
+ }
+
+ buffers[i].length = buffer.length; /* remember for munmap() */
+
+ buffers[i].start = mmap(NULL, buffer.length,
+ PROT_READ | PROT_WRITE, /* recommended */
+ MAP_SHARED, /* recommended */
+ fd, buffer.m.offset);
+
+ if (MAP_FAILED == buffers[i].start) {
+ /* If you do not exit here you should unmap() and free()
+ the buffers mapped so far. */
+ perror("mmap");
+ exit(EXIT_FAILURE);
+ }
+}
+
+/* Cleanup. */
+
+for (i = 0; i < reqbuf.count; i++)
+ munmap(buffers[i].start, buffers[i].length);
+ </programlisting>
+ </example>
+
+ <example>
+ <title>Mapping buffers in the multi-planar API</title>
+ <programlisting>
+&v4l2-requestbuffers; reqbuf;
+/* Our current format uses 3 planes per buffer */
+#define FMT_NUM_PLANES = 3
+
+struct {
+ void *start[FMT_NUM_PLANES];
+ size_t length[FMT_NUM_PLANES];
+} *buffers;
+unsigned int i, j;
+
+memset(&reqbuf, 0, sizeof(reqbuf));
+reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE;
+reqbuf.memory = V4L2_MEMORY_MMAP;
+reqbuf.count = 20;
+
+if (ioctl(fd, &VIDIOC-REQBUFS;, &reqbuf) < 0) {
+ if (errno == EINVAL)
+ printf("Video capturing or mmap-streaming is not supported\n");
+ else
+ perror("VIDIOC_REQBUFS");
+
+ exit(EXIT_FAILURE);
+}
+
+/* We want at least five buffers. */
+
+if (reqbuf.count < 5) {
+ /* You may need to free the buffers here. */
+ printf("Not enough buffer memory\n");
+ exit(EXIT_FAILURE);
+}
+
+buffers = calloc(reqbuf.count, sizeof(*buffers));
+assert(buffers != NULL);
+
+for (i = 0; i < reqbuf.count; i++) {
+ &v4l2-buffer; buffer;
+ &v4l2-plane; planes[FMT_NUM_PLANES];
+
+ memset(&buffer, 0, sizeof(buffer));
+ buffer.type = reqbuf.type;
+ buffer.memory = V4L2_MEMORY_MMAP;
+ buffer.index = i;
+ /* length in struct v4l2_buffer in multi-planar API stores the size
+ * of planes array. */
+ buffer.length = FMT_NUM_PLANES;
+ buffer.m.planes = planes;
+
+ if (ioctl(fd, &VIDIOC-QUERYBUF;, &buffer) < 0) {
+ perror("VIDIOC_QUERYBUF");
+ exit(EXIT_FAILURE);
+ }
+
+ /* Every plane has to be mapped separately */
+ for (j = 0; j < FMT_NUM_PLANES; j++) {
+ buffers[i].length[j] = buffer.m.planes[j].length; /* remember for munmap() */
+
+ buffers[i].start[j] = mmap(NULL, buffer.m.planes[j].length,
+ PROT_READ | PROT_WRITE, /* recommended */
+ MAP_SHARED, /* recommended */
+ fd, buffer.m.planes[j].m.offset);
+
+ if (MAP_FAILED == buffers[i].start[j]) {
+ /* If you do not exit here you should unmap() and free()
+ the buffers and planes mapped so far. */
+ perror("mmap");
+ exit(EXIT_FAILURE);
+ }
+ }
+}
+
+/* Cleanup. */
+
+for (i = 0; i < reqbuf.count; i++)
+ for (j = 0; j < FMT_NUM_PLANES; j++)
+ munmap(buffers[i].start[j], buffers[i].length[j]);
+ </programlisting>
+ </example>
+
+ <para>Conceptually streaming drivers maintain two buffer queues, an incoming
+and an outgoing queue. They separate the synchronous capture or output
+operation locked to a video clock from the application which is
+subject to random disk or network delays and preemption by
+other processes, thereby reducing the probability of data loss.
+The queues are organized as FIFOs, buffers will be
+output in the order enqueued in the incoming FIFO, and were
+captured in the order dequeued from the outgoing FIFO.</para>
+
+ <para>The driver may require a minimum number of buffers enqueued
+at all times to function, apart of this no limit exists on the number
+of buffers applications can enqueue in advance, or dequeue and
+process. They can also enqueue in a different order than buffers have
+been dequeued, and the driver can <emphasis>fill</emphasis> enqueued
+<emphasis>empty</emphasis> buffers in any order. <footnote>
+ <para>Random enqueue order permits applications processing
+images out of order (such as video codecs) to return buffers earlier,
+reducing the probability of data loss. Random fill order allows
+drivers to reuse buffers on a LIFO-basis, taking advantage of caches
+holding scatter-gather lists and the like.</para>
+ </footnote> The index number of a buffer (&v4l2-buffer;
+<structfield>index</structfield>) plays no role here, it only
+identifies the buffer.</para>
+
+ <para>Initially all mapped buffers are in dequeued state,
+inaccessible by the driver. For capturing applications it is customary
+to first enqueue all mapped buffers, then to start capturing and enter
+the read loop. Here the application waits until a filled buffer can be
+dequeued, and re-enqueues the buffer when the data is no longer
+needed. Output applications fill and enqueue buffers, when enough
+buffers are stacked up the output is started with
+<constant>VIDIOC_STREAMON</constant>. In the write loop, when
+the application runs out of free buffers, it must wait until an empty
+buffer can be dequeued and reused.</para>
+
+ <para>To enqueue and dequeue a buffer applications use the
+&VIDIOC-QBUF; and &VIDIOC-DQBUF; ioctl. The status of a buffer being
+mapped, enqueued, full or empty can be determined at any time using the
+&VIDIOC-QUERYBUF; ioctl. Two methods exist to suspend execution of the
+application until one or more buffers can be dequeued. By default
+<constant>VIDIOC_DQBUF</constant> blocks when no buffer is in the
+outgoing queue. When the <constant>O_NONBLOCK</constant> flag was
+given to the &func-open; function, <constant>VIDIOC_DQBUF</constant>
+returns immediately with an &EAGAIN; when no buffer is available. The
+&func-select; or &func-poll; functions are always available.</para>
+
+ <para>To start and stop capturing or output applications call the
+&VIDIOC-STREAMON; and &VIDIOC-STREAMOFF; ioctl. Note
+<constant>VIDIOC_STREAMOFF</constant> removes all buffers from both
+queues as a side effect. Since there is no notion of doing anything
+"now" on a multitasking system, if an application needs to synchronize
+with another event it should examine the &v4l2-buffer;
+<structfield>timestamp</structfield> of captured or outputted buffers.
+</para>
+
+ <para>Drivers implementing memory mapping I/O must
+support the <constant>VIDIOC_REQBUFS</constant>,
+<constant>VIDIOC_QUERYBUF</constant>,
+<constant>VIDIOC_QBUF</constant>, <constant>VIDIOC_DQBUF</constant>,
+<constant>VIDIOC_STREAMON</constant> and
+<constant>VIDIOC_STREAMOFF</constant> ioctl, the
+<function>mmap()</function>, <function>munmap()</function>,
+<function>select()</function> and <function>poll()</function>
+function.<footnote>
+ <para>At the driver level <function>select()</function> and
+<function>poll()</function> are the same, and
+<function>select()</function> is too important to be optional. The
+rest should be evident.</para>
+ </footnote></para>
+
+ <para>[capture example]</para>
+
+ </section>
+
+ <section id="userp">
+ <title>Streaming I/O (User Pointers)</title>
+
+ <para>Input and output devices support this I/O method when the
+<constant>V4L2_CAP_STREAMING</constant> flag in the
+<structfield>capabilities</structfield> field of &v4l2-capability;
+returned by the &VIDIOC-QUERYCAP; ioctl is set. If the particular user
+pointer method (not only memory mapping) is supported must be
+determined by calling the &VIDIOC-REQBUFS; ioctl.</para>
+
+ <para>This I/O method combines advantages of the read/write and
+memory mapping methods. Buffers (planes) are allocated by the application
+itself, and can reside for example in virtual or shared memory. Only
+pointers to data are exchanged, these pointers and meta-information
+are passed in &v4l2-buffer; (or in &v4l2-plane; in the multi-planar API case).
+The driver must be switched into user pointer I/O mode by calling the
+&VIDIOC-REQBUFS; with the desired buffer type. No buffers (planes) are allocated
+beforehand, consequently they are not indexed and cannot be queried like mapped
+buffers with the <constant>VIDIOC_QUERYBUF</constant> ioctl.</para>
+
+ <example>
+ <title>Initiating streaming I/O with user pointers</title>
+
+ <programlisting>
+&v4l2-requestbuffers; reqbuf;
+
+memset (&reqbuf, 0, sizeof (reqbuf));
+reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
+reqbuf.memory = V4L2_MEMORY_USERPTR;
+
+if (ioctl (fd, &VIDIOC-REQBUFS;, &reqbuf) == -1) {
+ if (errno == EINVAL)
+ printf ("Video capturing or user pointer streaming is not supported\n");
+ else
+ perror ("VIDIOC_REQBUFS");
+
+ exit (EXIT_FAILURE);
+}
+ </programlisting>
+ </example>
+
+ <para>Buffer (plane) addresses and sizes are passed on the fly with the
+&VIDIOC-QBUF; ioctl. Although buffers are commonly cycled,
+applications can pass different addresses and sizes at each
+<constant>VIDIOC_QBUF</constant> call. If required by the hardware the
+driver swaps memory pages within physical memory to create a
+continuous area of memory. This happens transparently to the
+application in the virtual memory subsystem of the kernel. When buffer
+pages have been swapped out to disk they are brought back and finally
+locked in physical memory for DMA.<footnote>
+ <para>We expect that frequently used buffers are typically not
+swapped out. Anyway, the process of swapping, locking or generating
+scatter-gather lists may be time consuming. The delay can be masked by
+the depth of the incoming buffer queue, and perhaps by maintaining
+caches assuming a buffer will be soon enqueued again. On the other
+hand, to optimize memory usage drivers can limit the number of buffers
+locked in advance and recycle the most recently used buffers first. Of
+course, the pages of empty buffers in the incoming queue need not be
+saved to disk. Output buffers must be saved on the incoming and
+outgoing queue because an application may share them with other
+processes.</para>
+ </footnote></para>
+
+ <para>Filled or displayed buffers are dequeued with the
+&VIDIOC-DQBUF; ioctl. The driver can unlock the memory pages at any
+time between the completion of the DMA and this ioctl. The memory is
+also unlocked when &VIDIOC-STREAMOFF; is called, &VIDIOC-REQBUFS;, or
+when the device is closed. Applications must take care not to free
+buffers without dequeuing. For once, the buffers remain locked until
+further, wasting physical memory. Second the driver will not be
+notified when the memory is returned to the application's free list
+and subsequently reused for other purposes, possibly completing the
+requested DMA and overwriting valuable data.</para>
+
+ <para>For capturing applications it is customary to enqueue a
+number of empty buffers, to start capturing and enter the read loop.
+Here the application waits until a filled buffer can be dequeued, and
+re-enqueues the buffer when the data is no longer needed. Output
+applications fill and enqueue buffers, when enough buffers are stacked
+up output is started. In the write loop, when the application
+runs out of free buffers it must wait until an empty buffer can be
+dequeued and reused. Two methods exist to suspend execution of the
+application until one or more buffers can be dequeued. By default
+<constant>VIDIOC_DQBUF</constant> blocks when no buffer is in the
+outgoing queue. When the <constant>O_NONBLOCK</constant> flag was
+given to the &func-open; function, <constant>VIDIOC_DQBUF</constant>
+returns immediately with an &EAGAIN; when no buffer is available. The
+&func-select; or &func-poll; function are always available.</para>
+
+ <para>To start and stop capturing or output applications call the
+&VIDIOC-STREAMON; and &VIDIOC-STREAMOFF; ioctl. Note
+<constant>VIDIOC_STREAMOFF</constant> removes all buffers from both
+queues and unlocks all buffers as a side effect. Since there is no
+notion of doing anything "now" on a multitasking system, if an
+application needs to synchronize with another event it should examine
+the &v4l2-buffer; <structfield>timestamp</structfield> of captured
+or outputted buffers.</para>
+
+ <para>Drivers implementing user pointer I/O must
+support the <constant>VIDIOC_REQBUFS</constant>,
+<constant>VIDIOC_QBUF</constant>, <constant>VIDIOC_DQBUF</constant>,
+<constant>VIDIOC_STREAMON</constant> and
+<constant>VIDIOC_STREAMOFF</constant> ioctl, the
+<function>select()</function> and <function>poll()</function> function.<footnote>
+ <para>At the driver level <function>select()</function> and
+<function>poll()</function> are the same, and
+<function>select()</function> is too important to be optional. The
+rest should be evident.</para>
+ </footnote></para>
+ </section>
+
+ <section id="dmabuf">
+ <title>Streaming I/O (DMA buffer importing)</title>
+
+ <note>
+ <title>Experimental</title>
+ <para>This is an <link linkend="experimental">experimental</link>
+ interface and may change in the future.</para>
+ </note>
+
+<para>The DMABUF framework provides a generic method for sharing buffers
+between multiple devices. Device drivers that support DMABUF can export a DMA
+buffer to userspace as a file descriptor (known as the exporter role), import a
+DMA buffer from userspace using a file descriptor previously exported for a
+different or the same device (known as the importer role), or both. This
+section describes the DMABUF importer role API in V4L2.</para>
+
+ <para>Refer to <link linkend="vidioc-expbuf">DMABUF exporting</link> for
+details about exporting V4L2 buffers as DMABUF file descriptors.</para>
+
+<para>Input and output devices support the streaming I/O method when the
+<constant>V4L2_CAP_STREAMING</constant> flag in the
+<structfield>capabilities</structfield> field of &v4l2-capability; returned by
+the &VIDIOC-QUERYCAP; ioctl is set. Whether importing DMA buffers through
+DMABUF file descriptors is supported is determined by calling the
+&VIDIOC-REQBUFS; ioctl with the memory type set to
+<constant>V4L2_MEMORY_DMABUF</constant>.</para>
+
+ <para>This I/O method is dedicated to sharing DMA buffers between different
+devices, which may be V4L devices or other video-related devices (e.g. DRM).
+Buffers (planes) are allocated by a driver on behalf of an application. Next,
+these buffers are exported to the application as file descriptors using an API
+which is specific for an allocator driver. Only such file descriptor are
+exchanged. The descriptors and meta-information are passed in &v4l2-buffer; (or
+in &v4l2-plane; in the multi-planar API case). The driver must be switched
+into DMABUF I/O mode by calling the &VIDIOC-REQBUFS; with the desired buffer
+type.</para>
+
+ <example>
+ <title>Initiating streaming I/O with DMABUF file descriptors</title>
+
+ <programlisting>
+&v4l2-requestbuffers; reqbuf;
+
+memset(&reqbuf, 0, sizeof (reqbuf));
+reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
+reqbuf.memory = V4L2_MEMORY_DMABUF;
+reqbuf.count = 1;
+
+if (ioctl(fd, &VIDIOC-REQBUFS;, &reqbuf) == -1) {
+ if (errno == EINVAL)
+ printf("Video capturing or DMABUF streaming is not supported\n");
+ else
+ perror("VIDIOC_REQBUFS");
+
+ exit(EXIT_FAILURE);
+}
+ </programlisting>
+ </example>
+
+ <para>The buffer (plane) file descriptor is passed on the fly with the
+&VIDIOC-QBUF; ioctl. In case of multiplanar buffers, every plane can be
+associated with a different DMABUF descriptor. Although buffers are commonly
+cycled, applications can pass a different DMABUF descriptor at each
+<constant>VIDIOC_QBUF</constant> call.</para>
+
+ <example>
+ <title>Queueing DMABUF using single plane API</title>
+
+ <programlisting>
+int buffer_queue(int v4lfd, int index, int dmafd)
+{
+ &v4l2-buffer; buf;
+
+ memset(&buf, 0, sizeof buf);
+ buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
+ buf.memory = V4L2_MEMORY_DMABUF;
+ buf.index = index;
+ buf.m.fd = dmafd;
+
+ if (ioctl(v4lfd, &VIDIOC-QBUF;, &buf) == -1) {
+ perror("VIDIOC_QBUF");
+ return -1;
+ }
+
+ return 0;
+}
+ </programlisting>
+ </example>
+
+ <example>
+ <title>Queueing DMABUF using multi plane API</title>
+
+ <programlisting>
+int buffer_queue_mp(int v4lfd, int index, int dmafd[], int n_planes)
+{
+ &v4l2-buffer; buf;
+ &v4l2-plane; planes[VIDEO_MAX_PLANES];
+ int i;
+
+ memset(&buf, 0, sizeof buf);
+ buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE;
+ buf.memory = V4L2_MEMORY_DMABUF;
+ buf.index = index;
+ buf.m.planes = planes;
+ buf.length = n_planes;
+
+ memset(&planes, 0, sizeof planes);
+
+ for (i = 0; i < n_planes; ++i)
+ buf.m.planes[i].m.fd = dmafd[i];
+
+ if (ioctl(v4lfd, &VIDIOC-QBUF;, &buf) == -1) {
+ perror("VIDIOC_QBUF");
+ return -1;
+ }
+
+ return 0;
+}
+ </programlisting>
+ </example>
+
+ <para>Captured or displayed buffers are dequeued with the
+&VIDIOC-DQBUF; ioctl. The driver can unlock the buffer at any
+time between the completion of the DMA and this ioctl. The memory is
+also unlocked when &VIDIOC-STREAMOFF; is called, &VIDIOC-REQBUFS;, or
+when the device is closed.</para>
+
+ <para>For capturing applications it is customary to enqueue a
+number of empty buffers, to start capturing and enter the read loop.
+Here the application waits until a filled buffer can be dequeued, and
+re-enqueues the buffer when the data is no longer needed. Output
+applications fill and enqueue buffers, when enough buffers are stacked
+up output is started. In the write loop, when the application
+runs out of free buffers it must wait until an empty buffer can be
+dequeued and reused. Two methods exist to suspend execution of the
+application until one or more buffers can be dequeued. By default
+<constant>VIDIOC_DQBUF</constant> blocks when no buffer is in the
+outgoing queue. When the <constant>O_NONBLOCK</constant> flag was
+given to the &func-open; function, <constant>VIDIOC_DQBUF</constant>
+returns immediately with an &EAGAIN; when no buffer is available. The
+&func-select; and &func-poll; functions are always available.</para>
+
+ <para>To start and stop capturing or displaying applications call the
+&VIDIOC-STREAMON; and &VIDIOC-STREAMOFF; ioctls. Note that
+<constant>VIDIOC_STREAMOFF</constant> removes all buffers from both queues and
+unlocks all buffers as a side effect. Since there is no notion of doing
+anything "now" on a multitasking system, if an application needs to synchronize
+with another event it should examine the &v4l2-buffer;
+<structfield>timestamp</structfield> of captured or outputted buffers.</para>
+
+ <para>Drivers implementing DMABUF importing I/O must support the
+<constant>VIDIOC_REQBUFS</constant>, <constant>VIDIOC_QBUF</constant>,
+<constant>VIDIOC_DQBUF</constant>, <constant>VIDIOC_STREAMON</constant> and
+<constant>VIDIOC_STREAMOFF</constant> ioctls, and the
+<function>select()</function> and <function>poll()</function> functions.</para>
+
+ </section>
+
+ <section id="async">
+ <title>Asynchronous I/O</title>
+
+ <para>This method is not defined yet.</para>
+ </section>
+
+ <section id="buffer">
+ <title>Buffers</title>
+
+ <para>A buffer contains data exchanged by application and
+driver using one of the Streaming I/O methods. In the multi-planar API, the
+data is held in planes, while the buffer structure acts as a container
+for the planes. Only pointers to buffers (planes) are exchanged, the data
+itself is not copied. These pointers, together with meta-information like
+timestamps or field parity, are stored in a struct
+<structname>v4l2_buffer</structname>, argument to
+the &VIDIOC-QUERYBUF;, &VIDIOC-QBUF; and &VIDIOC-DQBUF; ioctl.
+In the multi-planar API, some plane-specific members of struct
+<structname>v4l2_buffer</structname>, such as pointers and sizes for each
+plane, are stored in struct <structname>v4l2_plane</structname> instead.
+In that case, struct <structname>v4l2_buffer</structname> contains an array of
+plane structures.</para>
+
+ <para>Dequeued video buffers come with timestamps. The driver
+ decides at which part of the frame and with which clock the
+ timestamp is taken. Please see flags in the masks
+ <constant>V4L2_BUF_FLAG_TIMESTAMP_MASK</constant> and
+ <constant>V4L2_BUF_FLAG_TSTAMP_SRC_MASK</constant> in <xref
+ linkend="buffer-flags" />. These flags are always valid and constant
+ across all buffers during the whole video stream. Changes in these
+ flags may take place as a side effect of &VIDIOC-S-INPUT; or
+ &VIDIOC-S-OUTPUT; however. The
+ <constant>V4L2_BUF_FLAG_TIMESTAMP_COPY</constant> timestamp type
+ which is used by e.g. on mem-to-mem devices is an exception to the
+ rule: the timestamp source flags are copied from the OUTPUT video
+ buffer to the CAPTURE video buffer.</para>
+
+ <table frame="none" pgwide="1" id="v4l2-buffer">
+ <title>struct <structname>v4l2_buffer</structname></title>
+ <tgroup cols="4">
+ &cs-ustr;
+ <tbody valign="top">
+ <row>
+ <entry>__u32</entry>
+ <entry><structfield>index</structfield></entry>
+ <entry></entry>
+ <entry>Number of the buffer, set by the application except
+when calling &VIDIOC-DQBUF;, then it is set by the driver.
+This field can range from zero to the number of buffers allocated
+with the &VIDIOC-REQBUFS; ioctl (&v4l2-requestbuffers; <structfield>count</structfield>),
+plus any buffers allocated with &VIDIOC-CREATE-BUFS; minus one.</entry>
+ </row>
+ <row>
+ <entry>__u32</entry>
+ <entry><structfield>type</structfield></entry>
+ <entry></entry>
+ <entry>Type of the buffer, same as &v4l2-format;
+<structfield>type</structfield> or &v4l2-requestbuffers;
+<structfield>type</structfield>, set by the application. See <xref
+linkend="v4l2-buf-type" /></entry>
+ </row>
+ <row>
+ <entry>__u32</entry>
+ <entry><structfield>bytesused</structfield></entry>
+ <entry></entry>
+ <entry>The number of bytes occupied by the data in the
+buffer. It depends on the negotiated data format and may change with
+each buffer for compressed variable size data like JPEG images.
+Drivers must set this field when <structfield>type</structfield>
+refers to an input stream, applications when it refers to an output stream.
+If the application sets this to 0 for an output stream, then
+<structfield>bytesused</structfield> will be set to the size of the
+buffer (see the <structfield>length</structfield> field of this struct) by
+the driver. For multiplanar formats this field is ignored and the
+<structfield>planes</structfield> pointer is used instead.</entry>
+ </row>
+ <row>
+ <entry>__u32</entry>
+ <entry><structfield>flags</structfield></entry>
+ <entry></entry>
+ <entry>Flags set by the application or driver, see <xref
+linkend="buffer-flags" />.</entry>
+ </row>
+ <row>
+ <entry>__u32</entry>
+ <entry><structfield>field</structfield></entry>
+ <entry></entry>
+ <entry>Indicates the field order of the image in the
+buffer, see <xref linkend="v4l2-field" />. This field is not used when
+the buffer contains VBI data. Drivers must set it when
+<structfield>type</structfield> refers to an input stream,
+applications when it refers to an output stream.</entry>
+ </row>
+ <row>
+ <entry>struct timeval</entry>
+ <entry><structfield>timestamp</structfield></entry>
+ <entry></entry>
+ <entry><para>For input streams this is time when the first data
+ byte was captured, as returned by the
+ <function>clock_gettime()</function> function for the relevant
+ clock id; see <constant>V4L2_BUF_FLAG_TIMESTAMP_*</constant> in
+ <xref linkend="buffer-flags" />. For output streams the driver
+ stores the time at which the last data byte was actually sent out
+ in the <structfield>timestamp</structfield> field. This permits
+ applications to monitor the drift between the video and system
+ clock. For output streams that use <constant>V4L2_BUF_FLAG_TIMESTAMP_COPY</constant>
+ the application has to fill in the timestamp which will be copied
+ by the driver to the capture stream.</para></entry>
+ </row>
+ <row>
+ <entry>&v4l2-timecode;</entry>
+ <entry><structfield>timecode</structfield></entry>
+ <entry></entry>
+ <entry>When <structfield>type</structfield> is
+<constant>V4L2_BUF_TYPE_VIDEO_CAPTURE</constant> and the
+<constant>V4L2_BUF_FLAG_TIMECODE</constant> flag is set in
+<structfield>flags</structfield>, this structure contains a frame
+timecode. In <link linkend="v4l2-field">V4L2_FIELD_ALTERNATE</link>
+mode the top and bottom field contain the same timecode.
+Timecodes are intended to help video editing and are typically recorded on
+video tapes, but also embedded in compressed formats like MPEG. This
+field is independent of the <structfield>timestamp</structfield> and
+<structfield>sequence</structfield> fields.</entry>
+ </row>
+ <row>
+ <entry>__u32</entry>
+ <entry><structfield>sequence</structfield></entry>
+ <entry></entry>
+ <entry>Set by the driver, counting the frames (not fields!) in
+sequence. This field is set for both input and output devices.</entry>
+ </row>
+ <row>
+ <entry spanname="hspan"><para>In <link
+linkend="v4l2-field">V4L2_FIELD_ALTERNATE</link> mode the top and
+bottom field have the same sequence number. The count starts at zero
+and includes dropped or repeated frames. A dropped frame was received
+by an input device but could not be stored due to lack of free buffer
+space. A repeated frame was displayed again by an output device
+because the application did not pass new data in
+time.</para><para>Note this may count the frames received
+e.g. over USB, without taking into account the frames dropped by the
+remote hardware due to limited compression throughput or bus
+bandwidth. These devices identify by not enumerating any video
+standards, see <xref linkend="standard" />.</para></entry>
+ </row>
+ <row>
+ <entry>__u32</entry>
+ <entry><structfield>memory</structfield></entry>
+ <entry></entry>
+ <entry>This field must be set by applications and/or drivers
+in accordance with the selected I/O method. See <xref linkend="v4l2-memory"
+ /></entry>
+ </row>
+ <row>
+ <entry>union</entry>
+ <entry><structfield>m</structfield></entry>
+ </row>
+ <row>
+ <entry></entry>
+ <entry>__u32</entry>
+ <entry><structfield>offset</structfield></entry>
+ <entry>For the single-planar API and when
+<structfield>memory</structfield> is <constant>V4L2_MEMORY_MMAP</constant> this
+is the offset of the buffer from the start of the device memory. The value is
+returned by the driver and apart of serving as parameter to the &func-mmap;
+function not useful for applications. See <xref linkend="mmap" /> for details
+ </entry>
+ </row>
+ <row>
+ <entry></entry>
+ <entry>unsigned long</entry>
+ <entry><structfield>userptr</structfield></entry>
+ <entry>For the single-planar API and when
+<structfield>memory</structfield> is <constant>V4L2_MEMORY_USERPTR</constant>
+this is a pointer to the buffer (casted to unsigned long type) in virtual
+memory, set by the application. See <xref linkend="userp" /> for details.
+ </entry>
+ </row>
+ <row>
+ <entry></entry>
+ <entry>struct v4l2_plane</entry>
+ <entry><structfield>*planes</structfield></entry>
+ <entry>When using the multi-planar API, contains a userspace pointer
+ to an array of &v4l2-plane;. The size of the array should be put
+ in the <structfield>length</structfield> field of this
+ <structname>v4l2_buffer</structname> structure.</entry>
+ </row>
+ <row>
+ <entry></entry>
+ <entry>int</entry>
+ <entry><structfield>fd</structfield></entry>
+ <entry>For the single-plane API and when
+<structfield>memory</structfield> is <constant>V4L2_MEMORY_DMABUF</constant> this
+is the file descriptor associated with a DMABUF buffer.</entry>
+ </row>
+ <row>
+ <entry>__u32</entry>
+ <entry><structfield>length</structfield></entry>
+ <entry></entry>
+ <entry>Size of the buffer (not the payload) in bytes for the
+ single-planar API. This is set by the driver based on the calls to
+ &VIDIOC-REQBUFS; and/or &VIDIOC-CREATE-BUFS;. For the multi-planar API the application sets
+ this to the number of elements in the <structfield>planes</structfield>
+ array. The driver will fill in the actual number of valid elements in
+ that array.
+ </entry>
+ </row>
+ <row>
+ <entry>__u32</entry>
+ <entry><structfield>reserved2</structfield></entry>
+ <entry></entry>
+ <entry>A place holder for future extensions. Applications
+should set this to 0.</entry>
+ </row>
+ <row>
+ <entry>__u32</entry>
+ <entry><structfield>reserved</structfield></entry>
+ <entry></entry>
+ <entry>A place holder for future extensions. Applications
+should set this to 0.</entry>
+ </row>
+ </tbody>
+ </tgroup>
+ </table>
+
+ <table frame="none" pgwide="1" id="v4l2-plane">
+ <title>struct <structname>v4l2_plane</structname></title>
+ <tgroup cols="4">
+ &cs-ustr;
+ <tbody valign="top">
+ <row>
+ <entry>__u32</entry>
+ <entry><structfield>bytesused</structfield></entry>
+ <entry></entry>
+ <entry>The number of bytes occupied by data in the plane
+ (its payload). Drivers must set this field when <structfield>type</structfield>
+ refers to an input stream, applications when it refers to an output stream.
+ If the application sets this to 0 for an output stream, then
+ <structfield>bytesused</structfield> will be set to the size of the
+ plane (see the <structfield>length</structfield> field of this struct)
+ by the driver. Note that the actual image data starts at
+ <structfield>data_offset</structfield> which may not be 0.</entry>
+ </row>
+ <row>
+ <entry>__u32</entry>
+ <entry><structfield>length</structfield></entry>
+ <entry></entry>
+ <entry>Size in bytes of the plane (not its payload). This is set by the driver
+ based on the calls to &VIDIOC-REQBUFS; and/or &VIDIOC-CREATE-BUFS;.</entry>
+ </row>
+ <row>
+ <entry>union</entry>
+ <entry><structfield>m</structfield></entry>
+ <entry></entry>
+ <entry></entry>
+ </row>
+ <row>
+ <entry></entry>
+ <entry>__u32</entry>
+ <entry><structfield>mem_offset</structfield></entry>
+ <entry>When the memory type in the containing &v4l2-buffer; is
+ <constant>V4L2_MEMORY_MMAP</constant>, this is the value that
+ should be passed to &func-mmap;, similar to the
+ <structfield>offset</structfield> field in &v4l2-buffer;.</entry>
+ </row>
+ <row>
+ <entry></entry>
+ <entry>unsigned long</entry>
+ <entry><structfield>userptr</structfield></entry>
+ <entry>When the memory type in the containing &v4l2-buffer; is
+ <constant>V4L2_MEMORY_USERPTR</constant>, this is a userspace
+ pointer to the memory allocated for this plane by an application.
+ </entry>
+ </row>
+ <row>
+ <entry></entry>
+ <entry>int</entry>
+ <entry><structfield>fd</structfield></entry>
+ <entry>When the memory type in the containing &v4l2-buffer; is
+ <constant>V4L2_MEMORY_DMABUF</constant>, this is a file
+ descriptor associated with a DMABUF buffer, similar to the
+ <structfield>fd</structfield> field in &v4l2-buffer;.</entry>
+ </row>
+ <row>
+ <entry>__u32</entry>
+ <entry><structfield>data_offset</structfield></entry>
+ <entry></entry>
+ <entry>Offset in bytes to video data in the plane.
+ Drivers must set this field when <structfield>type</structfield>
+ refers to an input stream, applications when it refers to an output stream.
+ Note that data_offset is included in <structfield>bytesused</structfield>.
+ So the size of the image in the plane is
+ <structfield>bytesused</structfield>-<structfield>data_offset</structfield> at
+ offset <structfield>data_offset</structfield> from the start of the plane.
+ </entry>
+ </row>
+ <row>
+ <entry>__u32</entry>
+ <entry><structfield>reserved[11]</structfield></entry>
+ <entry></entry>
+ <entry>Reserved for future use. Should be zeroed by an
+ application.</entry>
+ </row>
+ </tbody>
+ </tgroup>
+ </table>
+
+ <table frame="none" pgwide="1" id="v4l2-buf-type">
+ <title>enum v4l2_buf_type</title>
+ <tgroup cols="3">
+ &cs-def;
+ <tbody valign="top">
+ <row>
+ <entry><constant>V4L2_BUF_TYPE_VIDEO_CAPTURE</constant></entry>
+ <entry>1</entry>
+ <entry>Buffer of a single-planar video capture stream, see <xref
+ linkend="capture" />.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE</constant>
+ </entry>
+ <entry>9</entry>
+ <entry>Buffer of a multi-planar video capture stream, see <xref
+ linkend="capture" />.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_BUF_TYPE_VIDEO_OUTPUT</constant></entry>
+ <entry>2</entry>
+ <entry>Buffer of a single-planar video output stream, see <xref
+ linkend="output" />.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE</constant>
+ </entry>
+ <entry>10</entry>
+ <entry>Buffer of a multi-planar video output stream, see <xref
+ linkend="output" />.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_BUF_TYPE_VIDEO_OVERLAY</constant></entry>
+ <entry>3</entry>
+ <entry>Buffer for video overlay, see <xref linkend="overlay" />.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_BUF_TYPE_VBI_CAPTURE</constant></entry>
+ <entry>4</entry>
+ <entry>Buffer of a raw VBI capture stream, see <xref
+ linkend="raw-vbi" />.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_BUF_TYPE_VBI_OUTPUT</constant></entry>
+ <entry>5</entry>
+ <entry>Buffer of a raw VBI output stream, see <xref
+ linkend="raw-vbi" />.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_BUF_TYPE_SLICED_VBI_CAPTURE</constant></entry>
+ <entry>6</entry>
+ <entry>Buffer of a sliced VBI capture stream, see <xref
+ linkend="sliced" />.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_BUF_TYPE_SLICED_VBI_OUTPUT</constant></entry>
+ <entry>7</entry>
+ <entry>Buffer of a sliced VBI output stream, see <xref
+ linkend="sliced" />.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY</constant></entry>
+ <entry>8</entry>
+ <entry>Buffer for video output overlay (OSD), see <xref
+ linkend="osd" />.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_BUF_TYPE_SDR_CAPTURE</constant></entry>
+ <entry>11</entry>
+ <entry>Buffer for Software Defined Radio (SDR), see <xref
+ linkend="sdr" />.</entry>
+ </row>
+ </tbody>
+ </tgroup>
+ </table>
+
+ <table frame="none" pgwide="1" id="buffer-flags">
+ <title>Buffer Flags</title>
+ <tgroup cols="3">
+ &cs-def;
+ <tbody valign="top">
+ <row>
+ <entry><constant>V4L2_BUF_FLAG_MAPPED</constant></entry>
+ <entry>0x00000001</entry>
+ <entry>The buffer resides in device memory and has been mapped
+into the application's address space, see <xref linkend="mmap" /> for details.
+Drivers set or clear this flag when the
+<link linkend="vidioc-querybuf">VIDIOC_QUERYBUF</link>, <link
+ linkend="vidioc-qbuf">VIDIOC_QBUF</link> or <link
+ linkend="vidioc-qbuf">VIDIOC_DQBUF</link> ioctl is called. Set by the driver.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_BUF_FLAG_QUEUED</constant></entry>
+ <entry>0x00000002</entry>
+ <entry>Internally drivers maintain two buffer queues, an
+incoming and outgoing queue. When this flag is set, the buffer is
+currently on the incoming queue. It automatically moves to the
+outgoing queue after the buffer has been filled (capture devices) or
+displayed (output devices). Drivers set or clear this flag when the
+<constant>VIDIOC_QUERYBUF</constant> ioctl is called. After
+(successful) calling the <constant>VIDIOC_QBUF </constant>ioctl it is
+always set and after <constant>VIDIOC_DQBUF</constant> always
+cleared.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_BUF_FLAG_DONE</constant></entry>
+ <entry>0x00000004</entry>
+ <entry>When this flag is set, the buffer is currently on
+the outgoing queue, ready to be dequeued from the driver. Drivers set
+or clear this flag when the <constant>VIDIOC_QUERYBUF</constant> ioctl
+is called. After calling the <constant>VIDIOC_QBUF</constant> or
+<constant>VIDIOC_DQBUF</constant> it is always cleared. Of course a
+buffer cannot be on both queues at the same time, the
+<constant>V4L2_BUF_FLAG_QUEUED</constant> and
+<constant>V4L2_BUF_FLAG_DONE</constant> flag are mutually exclusive.
+They can be both cleared however, then the buffer is in "dequeued"
+state, in the application domain so to say.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_BUF_FLAG_ERROR</constant></entry>
+ <entry>0x00000040</entry>
+ <entry>When this flag is set, the buffer has been dequeued
+ successfully, although the data might have been corrupted.
+ This is recoverable, streaming may continue as normal and
+ the buffer may be reused normally.
+ Drivers set this flag when the <constant>VIDIOC_DQBUF</constant>
+ ioctl is called.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_BUF_FLAG_KEYFRAME</constant></entry>
+ <entry>0x00000008</entry>
+ <entry>Drivers set or clear this flag when calling the
+<constant>VIDIOC_DQBUF</constant> ioctl. It may be set by video
+capture devices when the buffer contains a compressed image which is a
+key frame (or field), &ie; can be decompressed on its own. Also known as
+an I-frame. Applications can set this bit when <structfield>type</structfield>
+refers to an output stream.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_BUF_FLAG_PFRAME</constant></entry>
+ <entry>0x00000010</entry>
+ <entry>Similar to <constant>V4L2_BUF_FLAG_KEYFRAME</constant>
+this flags predicted frames or fields which contain only differences to a
+previous key frame. Applications can set this bit when <structfield>type</structfield>
+refers to an output stream.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_BUF_FLAG_BFRAME</constant></entry>
+ <entry>0x00000020</entry>
+ <entry>Similar to <constant>V4L2_BUF_FLAG_KEYFRAME</constant>
+this flags a bi-directional predicted frame or field which contains only
+the differences between the current frame and both the preceding and following
+key frames to specify its content. Applications can set this bit when
+<structfield>type</structfield> refers to an output stream.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_BUF_FLAG_TIMECODE</constant></entry>
+ <entry>0x00000100</entry>
+ <entry>The <structfield>timecode</structfield> field is valid.
+Drivers set or clear this flag when the <constant>VIDIOC_DQBUF</constant>
+ioctl is called. Applications can set this bit and the corresponding
+<structfield>timecode</structfield> structure when <structfield>type</structfield>
+refers to an output stream.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_BUF_FLAG_PREPARED</constant></entry>
+ <entry>0x00000400</entry>
+ <entry>The buffer has been prepared for I/O and can be queued by the
+application. Drivers set or clear this flag when the
+<link linkend="vidioc-querybuf">VIDIOC_QUERYBUF</link>, <link
+ linkend="vidioc-qbuf">VIDIOC_PREPARE_BUF</link>, <link
+ linkend="vidioc-qbuf">VIDIOC_QBUF</link> or <link
+ linkend="vidioc-qbuf">VIDIOC_DQBUF</link> ioctl is called.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_BUF_FLAG_NO_CACHE_INVALIDATE</constant></entry>
+ <entry>0x00000800</entry>
+ <entry>Caches do not have to be invalidated for this buffer.
+Typically applications shall use this flag if the data captured in the buffer
+is not going to be touched by the CPU, instead the buffer will, probably, be
+passed on to a DMA-capable hardware unit for further processing or output.
+</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_BUF_FLAG_NO_CACHE_CLEAN</constant></entry>
+ <entry>0x00001000</entry>
+ <entry>Caches do not have to be cleaned for this buffer.
+Typically applications shall use this flag for output buffers if the data
+in this buffer has not been created by the CPU but by some DMA-capable unit,
+in which case caches have not been used.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_BUF_FLAG_TIMESTAMP_MASK</constant></entry>
+ <entry>0x0000e000</entry>
+ <entry>Mask for timestamp types below. To test the
+ timestamp type, mask out bits not belonging to timestamp
+ type by performing a logical and operation with buffer
+ flags and timestamp mask.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_BUF_FLAG_TIMESTAMP_UNKNOWN</constant></entry>
+ <entry>0x00000000</entry>
+ <entry>Unknown timestamp type. This type is used by
+ drivers before Linux 3.9 and may be either monotonic (see
+ below) or realtime (wall clock). Monotonic clock has been
+ favoured in embedded systems whereas most of the drivers
+ use the realtime clock. Either kinds of timestamps are
+ available in user space via
+ <function>clock_gettime(2)</function> using clock IDs
+ <constant>CLOCK_MONOTONIC</constant> and
+ <constant>CLOCK_REALTIME</constant>, respectively.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_BUF_FLAG_TIMESTAMP_MONOTONIC</constant></entry>
+ <entry>0x00002000</entry>
+ <entry>The buffer timestamp has been taken from the
+ <constant>CLOCK_MONOTONIC</constant> clock. To access the
+ same clock outside V4L2, use
+ <function>clock_gettime(2)</function> .</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_BUF_FLAG_TIMESTAMP_COPY</constant></entry>
+ <entry>0x00004000</entry>
+ <entry>The CAPTURE buffer timestamp has been taken from the
+ corresponding OUTPUT buffer. This flag applies only to mem2mem devices.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_BUF_FLAG_TSTAMP_SRC_MASK</constant></entry>
+ <entry>0x00070000</entry>
+ <entry>Mask for timestamp sources below. The timestamp source
+ defines the point of time the timestamp is taken in relation to
+ the frame. Logical 'and' operation between the
+ <structfield>flags</structfield> field and
+ <constant>V4L2_BUF_FLAG_TSTAMP_SRC_MASK</constant> produces the
+ value of the timestamp source. Applications must set the timestamp
+ source when <structfield>type</structfield> refers to an output stream
+ and <constant>V4L2_BUF_FLAG_TIMESTAMP_COPY</constant> is set.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_BUF_FLAG_TSTAMP_SRC_EOF</constant></entry>
+ <entry>0x00000000</entry>
+ <entry>End Of Frame. The buffer timestamp has been taken
+ when the last pixel of the frame has been received or the
+ last pixel of the frame has been transmitted. In practice,
+ software generated timestamps will typically be read from
+ the clock a small amount of time after the last pixel has
+ been received or transmitten, depending on the system and
+ other activity in it.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_BUF_FLAG_TSTAMP_SRC_SOE</constant></entry>
+ <entry>0x00010000</entry>
+ <entry>Start Of Exposure. The buffer timestamp has been
+ taken when the exposure of the frame has begun. This is
+ only valid for the
+ <constant>V4L2_BUF_TYPE_VIDEO_CAPTURE</constant> buffer
+ type.</entry>
+ </row>
+ </tbody>
+ </tgroup>
+ </table>
+
+ <table pgwide="1" frame="none" id="v4l2-memory">
+ <title>enum v4l2_memory</title>
+ <tgroup cols="3">
+ &cs-def;
+ <tbody valign="top">
+ <row>
+ <entry><constant>V4L2_MEMORY_MMAP</constant></entry>
+ <entry>1</entry>
+ <entry>The buffer is used for <link linkend="mmap">memory
+mapping</link> I/O.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_MEMORY_USERPTR</constant></entry>
+ <entry>2</entry>
+ <entry>The buffer is used for <link linkend="userp">user
+pointer</link> I/O.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_MEMORY_OVERLAY</constant></entry>
+ <entry>3</entry>
+ <entry>[to do]</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_MEMORY_DMABUF</constant></entry>
+ <entry>4</entry>
+ <entry>The buffer is used for <link linkend="dmabuf">DMA shared
+buffer</link> I/O.</entry>
+ </row>
+ </tbody>
+ </tgroup>
+ </table>
+
+ <section>
+ <title>Timecodes</title>
+
+ <para>The <structname>v4l2_timecode</structname> structure is
+designed to hold a <xref linkend="smpte12m" /> or similar timecode.
+(struct <structname>timeval</structname> timestamps are stored in
+&v4l2-buffer; field <structfield>timestamp</structfield>.)</para>
+
+ <table frame="none" pgwide="1" id="v4l2-timecode">
+ <title>struct <structname>v4l2_timecode</structname></title>
+ <tgroup cols="3">
+ &cs-str;
+ <tbody valign="top">
+ <row>
+ <entry>__u32</entry>
+ <entry><structfield>type</structfield></entry>
+ <entry>Frame rate the timecodes are based on, see <xref
+ linkend="timecode-type" />.</entry>
+ </row>
+ <row>
+ <entry>__u32</entry>
+ <entry><structfield>flags</structfield></entry>
+ <entry>Timecode flags, see <xref linkend="timecode-flags" />.</entry>
+ </row>
+ <row>
+ <entry>__u8</entry>
+ <entry><structfield>frames</structfield></entry>
+ <entry>Frame count, 0 ... 23/24/29/49/59, depending on the
+ type of timecode.</entry>
+ </row>
+ <row>
+ <entry>__u8</entry>
+ <entry><structfield>seconds</structfield></entry>
+ <entry>Seconds count, 0 ... 59. This is a binary, not BCD number.</entry>
+ </row>
+ <row>
+ <entry>__u8</entry>
+ <entry><structfield>minutes</structfield></entry>
+ <entry>Minutes count, 0 ... 59. This is a binary, not BCD number.</entry>
+ </row>
+ <row>
+ <entry>__u8</entry>
+ <entry><structfield>hours</structfield></entry>
+ <entry>Hours count, 0 ... 29. This is a binary, not BCD number.</entry>
+ </row>
+ <row>
+ <entry>__u8</entry>
+ <entry><structfield>userbits</structfield>[4]</entry>
+ <entry>The "user group" bits from the timecode.</entry>
+ </row>
+ </tbody>
+ </tgroup>
+ </table>
+
+ <table frame="none" pgwide="1" id="timecode-type">
+ <title>Timecode Types</title>
+ <tgroup cols="3">
+ &cs-def;
+ <tbody valign="top">
+ <row>
+ <entry><constant>V4L2_TC_TYPE_24FPS</constant></entry>
+ <entry>1</entry>
+ <entry>24 frames per second, i. e. film.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_TC_TYPE_25FPS</constant></entry>
+ <entry>2</entry>
+ <entry>25 frames per second, &ie; PAL or SECAM video.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_TC_TYPE_30FPS</constant></entry>
+ <entry>3</entry>
+ <entry>30 frames per second, &ie; NTSC video.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_TC_TYPE_50FPS</constant></entry>
+ <entry>4</entry>
+ <entry></entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_TC_TYPE_60FPS</constant></entry>
+ <entry>5</entry>
+ <entry></entry>
+ </row>
+ </tbody>
+ </tgroup>
+ </table>
+
+ <table frame="none" pgwide="1" id="timecode-flags">
+ <title>Timecode Flags</title>
+ <tgroup cols="3">
+ &cs-def;
+ <tbody valign="top">
+ <row>
+ <entry><constant>V4L2_TC_FLAG_DROPFRAME</constant></entry>
+ <entry>0x0001</entry>
+ <entry>Indicates "drop frame" semantics for counting frames
+in 29.97 fps material. When set, frame numbers 0 and 1 at the start of
+each minute, except minutes 0, 10, 20, 30, 40, 50 are omitted from the
+count.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_TC_FLAG_COLORFRAME</constant></entry>
+ <entry>0x0002</entry>
+ <entry>The "color frame" flag.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_TC_USERBITS_field</constant></entry>
+ <entry>0x000C</entry>
+ <entry>Field mask for the "binary group flags".</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_TC_USERBITS_USERDEFINED</constant></entry>
+ <entry>0x0000</entry>
+ <entry>Unspecified format.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_TC_USERBITS_8BITCHARS</constant></entry>
+ <entry>0x0008</entry>
+ <entry>8-bit ISO characters.</entry>
+ </row>
+ </tbody>
+ </tgroup>
+ </table>
+ </section>
+ </section>
+
+ <section id="field-order">
+ <title>Field Order</title>
+
+ <para>We have to distinguish between progressive and interlaced
+video. Progressive video transmits all lines of a video image
+sequentially. Interlaced video divides an image into two fields,
+containing only the odd and even lines of the image, respectively.
+Alternating the so called odd and even field are transmitted, and due
+to a small delay between fields a cathode ray TV displays the lines
+interleaved, yielding the original frame. This curious technique was
+invented because at refresh rates similar to film the image would
+fade out too quickly. Transmitting fields reduces the flicker without
+the necessity of doubling the frame rate and with it the bandwidth
+required for each channel.</para>
+
+ <para>It is important to understand a video camera does not expose
+one frame at a time, merely transmitting the frames separated into
+fields. The fields are in fact captured at two different instances in
+time. An object on screen may well move between one field and the
+next. For applications analysing motion it is of paramount importance
+to recognize which field of a frame is older, the <emphasis>temporal
+order</emphasis>.</para>
+
+ <para>When the driver provides or accepts images field by field
+rather than interleaved, it is also important applications understand
+how the fields combine to frames. We distinguish between top (aka odd) and
+bottom (aka even) fields, the <emphasis>spatial order</emphasis>: The first line
+of the top field is the first line of an interlaced frame, the first
+line of the bottom field is the second line of that frame.</para>
+
+ <para>However because fields were captured one after the other,
+arguing whether a frame commences with the top or bottom field is
+pointless. Any two successive top and bottom, or bottom and top fields
+yield a valid frame. Only when the source was progressive to begin
+with, ⪚ when transferring film to video, two fields may come from
+the same frame, creating a natural order.</para>
+
+ <para>Counter to intuition the top field is not necessarily the
+older field. Whether the older field contains the top or bottom lines
+is a convention determined by the video standard. Hence the
+distinction between temporal and spatial order of fields. The diagrams
+below should make this clearer.</para>
+
+ <para>All video capture and output devices must report the current
+field order. Some drivers may permit the selection of a different
+order, to this end applications initialize the
+<structfield>field</structfield> field of &v4l2-pix-format; before
+calling the &VIDIOC-S-FMT; ioctl. If this is not desired it should
+have the value <constant>V4L2_FIELD_ANY</constant> (0).</para>
+
+ <table frame="none" pgwide="1" id="v4l2-field">
+ <title>enum v4l2_field</title>
+ <tgroup cols="3">
+ &cs-def;
+ <tbody valign="top">
+ <row>
+ <entry><constant>V4L2_FIELD_ANY</constant></entry>
+ <entry>0</entry>
+ <entry>Applications request this field order when any
+one of the <constant>V4L2_FIELD_NONE</constant>,
+<constant>V4L2_FIELD_TOP</constant>,
+<constant>V4L2_FIELD_BOTTOM</constant>, or
+<constant>V4L2_FIELD_INTERLACED</constant> formats is acceptable.
+Drivers choose depending on hardware capabilities or e. g. the
+requested image size, and return the actual field order. Drivers must
+never return <constant>V4L2_FIELD_ANY</constant>. If multiple
+field orders are possible the driver must choose one of the possible
+field orders during &VIDIOC-S-FMT; or &VIDIOC-TRY-FMT;. &v4l2-buffer;
+<structfield>field</structfield> can never be
+<constant>V4L2_FIELD_ANY</constant>.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_FIELD_NONE</constant></entry>
+ <entry>1</entry>
+ <entry>Images are in progressive format, not interlaced.
+The driver may also indicate this order when it cannot distinguish
+between <constant>V4L2_FIELD_TOP</constant> and
+<constant>V4L2_FIELD_BOTTOM</constant>.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_FIELD_TOP</constant></entry>
+ <entry>2</entry>
+ <entry>Images consist of the top (aka odd) field only.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_FIELD_BOTTOM</constant></entry>
+ <entry>3</entry>
+ <entry>Images consist of the bottom (aka even) field only.
+Applications may wish to prevent a device from capturing interlaced
+images because they will have "comb" or "feathering" artefacts around
+moving objects.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_FIELD_INTERLACED</constant></entry>
+ <entry>4</entry>
+ <entry>Images contain both fields, interleaved line by
+line. The temporal order of the fields (whether the top or bottom
+field is first transmitted) depends on the current video standard.
+M/NTSC transmits the bottom field first, all other standards the top
+field first.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_FIELD_SEQ_TB</constant></entry>
+ <entry>5</entry>
+ <entry>Images contain both fields, the top field lines
+are stored first in memory, immediately followed by the bottom field
+lines. Fields are always stored in temporal order, the older one first
+in memory. Image sizes refer to the frame, not fields.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_FIELD_SEQ_BT</constant></entry>
+ <entry>6</entry>
+ <entry>Images contain both fields, the bottom field
+lines are stored first in memory, immediately followed by the top
+field lines. Fields are always stored in temporal order, the older one
+first in memory. Image sizes refer to the frame, not fields.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_FIELD_ALTERNATE</constant></entry>
+ <entry>7</entry>
+ <entry>The two fields of a frame are passed in separate
+buffers, in temporal order, &ie; the older one first. To indicate the field
+parity (whether the current field is a top or bottom field) the driver
+or application, depending on data direction, must set &v4l2-buffer;
+<structfield>field</structfield> to
+<constant>V4L2_FIELD_TOP</constant> or
+<constant>V4L2_FIELD_BOTTOM</constant>. Any two successive fields pair
+to build a frame. If fields are successive, without any dropped fields
+between them (fields can drop individually), can be determined from
+the &v4l2-buffer; <structfield>sequence</structfield> field. This format
+cannot be selected when using the read/write I/O method since there
+is no way to communicate if a field was a top or bottom field.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_FIELD_INTERLACED_TB</constant></entry>
+ <entry>8</entry>
+ <entry>Images contain both fields, interleaved line by
+line, top field first. The top field is transmitted first.</entry>
+ </row>
+ <row>
+ <entry><constant>V4L2_FIELD_INTERLACED_BT</constant></entry>
+ <entry>9</entry>
+ <entry>Images contain both fields, interleaved line by
+line, top field first. The bottom field is transmitted first.</entry>
+ </row>
+ </tbody>
+ </tgroup>
+ </table>
+
+ <figure id="fieldseq-tb">
+ <title>Field Order, Top Field First Transmitted</title>
+ <mediaobject>
+ <imageobject>
+ <imagedata fileref="fieldseq_tb.pdf" format="PS" />
+ </imageobject>
+ <imageobject>
+ <imagedata fileref="fieldseq_tb.gif" format="GIF" />
+ </imageobject>
+ </mediaobject>
+ </figure>
+
+ <figure id="fieldseq-bt">
+ <title>Field Order, Bottom Field First Transmitted</title>
+ <mediaobject>
+ <imageobject>
+ <imagedata fileref="fieldseq_bt.pdf" format="PS" />
+ </imageobject>
+ <imageobject>
+ <imagedata fileref="fieldseq_bt.gif" format="GIF" />
+ </imageobject>
+ </mediaobject>
+ </figure>
+ </section>
Code Review / kvmfornfv.git / blobdiff