286 lines
11 KiB
ReStructuredText
286 lines
11 KiB
ReStructuredText
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.. -*- coding: utf-8; mode: rst -*-
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.. _mmap:
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******************************
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Streaming I/O (Memory Mapping)
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******************************
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Input and output devices support this I/O method when the
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``V4L2_CAP_STREAMING`` flag in the ``capabilities`` field of struct
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:c:type:`v4l2_capability` returned by the
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:ref:`VIDIOC_QUERYCAP` ioctl is set. There are two
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streaming methods, to determine if the memory mapping flavor is
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supported applications must call the :ref:`VIDIOC_REQBUFS` ioctl
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with the memory type set to ``V4L2_MEMORY_MMAP``.
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Streaming is an I/O method where only pointers to buffers are exchanged
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between application and driver, the data itself is not copied. Memory
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mapping is primarily intended to map buffers in device memory into the
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application's address space. Device memory can be for example the video
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memory on a graphics card with a video capture add-on. However, being
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the most efficient I/O method available for a long time, many other
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drivers support streaming as well, allocating buffers in DMA-able main
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memory.
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A driver can support many sets of buffers. Each set is identified by a
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unique buffer type value. The sets are independent and each set can hold
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a different type of data. To access different sets at the same time
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different file descriptors must be used. [#f1]_
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To allocate device buffers applications call the
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:ref:`VIDIOC_REQBUFS` ioctl with the desired number
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of buffers and buffer type, for example ``V4L2_BUF_TYPE_VIDEO_CAPTURE``.
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This ioctl can also be used to change the number of buffers or to free
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the allocated memory, provided none of the buffers are still mapped.
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Before applications can access the buffers they must map them into their
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address space with the :ref:`mmap() <func-mmap>` function. The
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location of the buffers in device memory can be determined with the
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:ref:`VIDIOC_QUERYBUF` ioctl. In the single-planar
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API case, the ``m.offset`` and ``length`` returned in a struct
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:c:type:`v4l2_buffer` are passed as sixth and second
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parameter to the :ref:`mmap() <func-mmap>` function. When using the
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multi-planar API, struct :c:type:`v4l2_buffer` contains an
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array of struct :c:type:`v4l2_plane` structures, each
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containing its own ``m.offset`` and ``length``. When using the
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multi-planar API, every plane of every buffer has to be mapped
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separately, so the number of calls to :ref:`mmap() <func-mmap>` should
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be equal to number of buffers times number of planes in each buffer. The
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offset and length values must not be modified. Remember, the buffers are
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allocated in physical memory, as opposed to virtual memory, which can be
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swapped out to disk. Applications should free the buffers as soon as
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possible with the :ref:`munmap() <func-munmap>` function.
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Example: Mapping buffers in the single-planar API
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=================================================
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.. code-block:: c
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struct v4l2_requestbuffers reqbuf;
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struct {
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void *start;
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size_t length;
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} *buffers;
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unsigned int i;
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memset(&reqbuf, 0, sizeof(reqbuf));
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reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
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reqbuf.memory = V4L2_MEMORY_MMAP;
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reqbuf.count = 20;
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if (-1 == ioctl (fd, VIDIOC_REQBUFS, &reqbuf)) {
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if (errno == EINVAL)
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printf("Video capturing or mmap-streaming is not supported\\n");
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else
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perror("VIDIOC_REQBUFS");
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exit(EXIT_FAILURE);
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}
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/* We want at least five buffers. */
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if (reqbuf.count < 5) {
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/* You may need to free the buffers here. */
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printf("Not enough buffer memory\\n");
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exit(EXIT_FAILURE);
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}
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buffers = calloc(reqbuf.count, sizeof(*buffers));
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assert(buffers != NULL);
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for (i = 0; i < reqbuf.count; i++) {
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struct v4l2_buffer buffer;
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memset(&buffer, 0, sizeof(buffer));
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buffer.type = reqbuf.type;
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buffer.memory = V4L2_MEMORY_MMAP;
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buffer.index = i;
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if (-1 == ioctl (fd, VIDIOC_QUERYBUF, &buffer)) {
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perror("VIDIOC_QUERYBUF");
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exit(EXIT_FAILURE);
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}
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buffers[i].length = buffer.length; /* remember for munmap() */
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buffers[i].start = mmap(NULL, buffer.length,
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PROT_READ | PROT_WRITE, /* recommended */
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MAP_SHARED, /* recommended */
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fd, buffer.m.offset);
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if (MAP_FAILED == buffers[i].start) {
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/* If you do not exit here you should unmap() and free()
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the buffers mapped so far. */
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perror("mmap");
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exit(EXIT_FAILURE);
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}
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}
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/* Cleanup. */
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for (i = 0; i < reqbuf.count; i++)
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munmap(buffers[i].start, buffers[i].length);
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Example: Mapping buffers in the multi-planar API
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================================================
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.. code-block:: c
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struct v4l2_requestbuffers reqbuf;
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/* Our current format uses 3 planes per buffer */
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#define FMT_NUM_PLANES = 3
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struct {
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void *start[FMT_NUM_PLANES];
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size_t length[FMT_NUM_PLANES];
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} *buffers;
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unsigned int i, j;
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memset(&reqbuf, 0, sizeof(reqbuf));
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reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE;
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reqbuf.memory = V4L2_MEMORY_MMAP;
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reqbuf.count = 20;
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if (ioctl(fd, VIDIOC_REQBUFS, &reqbuf) < 0) {
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if (errno == EINVAL)
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printf("Video capturing or mmap-streaming is not supported\\n");
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else
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perror("VIDIOC_REQBUFS");
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exit(EXIT_FAILURE);
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}
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/* We want at least five buffers. */
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if (reqbuf.count < 5) {
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/* You may need to free the buffers here. */
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printf("Not enough buffer memory\\n");
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exit(EXIT_FAILURE);
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}
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buffers = calloc(reqbuf.count, sizeof(*buffers));
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assert(buffers != NULL);
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for (i = 0; i < reqbuf.count; i++) {
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struct v4l2_buffer buffer;
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struct v4l2_plane planes[FMT_NUM_PLANES];
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memset(&buffer, 0, sizeof(buffer));
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buffer.type = reqbuf.type;
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buffer.memory = V4L2_MEMORY_MMAP;
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buffer.index = i;
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/* length in struct v4l2_buffer in multi-planar API stores the size
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* of planes array. */
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buffer.length = FMT_NUM_PLANES;
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buffer.m.planes = planes;
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if (ioctl(fd, VIDIOC_QUERYBUF, &buffer) < 0) {
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perror("VIDIOC_QUERYBUF");
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exit(EXIT_FAILURE);
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}
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/* Every plane has to be mapped separately */
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for (j = 0; j < FMT_NUM_PLANES; j++) {
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buffers[i].length[j] = buffer.m.planes[j].length; /* remember for munmap() */
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buffers[i].start[j] = mmap(NULL, buffer.m.planes[j].length,
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PROT_READ | PROT_WRITE, /* recommended */
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MAP_SHARED, /* recommended */
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fd, buffer.m.planes[j].m.offset);
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if (MAP_FAILED == buffers[i].start[j]) {
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/* If you do not exit here you should unmap() and free()
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the buffers and planes mapped so far. */
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perror("mmap");
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exit(EXIT_FAILURE);
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}
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}
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}
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/* Cleanup. */
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for (i = 0; i < reqbuf.count; i++)
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for (j = 0; j < FMT_NUM_PLANES; j++)
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munmap(buffers[i].start[j], buffers[i].length[j]);
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Conceptually streaming drivers maintain two buffer queues, an incoming
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and an outgoing queue. They separate the synchronous capture or output
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operation locked to a video clock from the application which is subject
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to random disk or network delays and preemption by other processes,
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thereby reducing the probability of data loss. The queues are organized
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as FIFOs, buffers will be output in the order enqueued in the incoming
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FIFO, and were captured in the order dequeued from the outgoing FIFO.
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The driver may require a minimum number of buffers enqueued at all times
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to function, apart of this no limit exists on the number of buffers
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applications can enqueue in advance, or dequeue and process. They can
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also enqueue in a different order than buffers have been dequeued, and
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the driver can *fill* enqueued *empty* buffers in any order. [#f2]_ The
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index number of a buffer (struct :c:type:`v4l2_buffer`
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``index``) plays no role here, it only identifies the buffer.
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Initially all mapped buffers are in dequeued state, inaccessible by the
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driver. For capturing applications it is customary to first enqueue all
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mapped buffers, then to start capturing and enter the read loop. Here
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the application waits until a filled buffer can be dequeued, and
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re-enqueues the buffer when the data is no longer needed. Output
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applications fill and enqueue buffers, when enough buffers are stacked
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up the output is started with :ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>`.
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In the write loop, when the application runs out of free buffers, it
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must wait until an empty buffer can be dequeued and reused.
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To enqueue and dequeue a buffer applications use the :ref:`VIDIOC_QBUF`
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and :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl. The status of a buffer
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being mapped, enqueued, full or empty can be determined at any time
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using the :ref:`VIDIOC_QUERYBUF` ioctl. Two methods exist to suspend
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execution of the application until one or more buffers can be dequeued.
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By default :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` blocks when no buffer is
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in the outgoing queue. When the ``O_NONBLOCK`` flag was given to the
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:ref:`open() <func-open>` function, :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>`
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returns immediately with an ``EAGAIN`` error code when no buffer is
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available. The :ref:`select() <func-select>` or :ref:`poll()
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<func-poll>` functions are always available.
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To start and stop capturing or output applications call the
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:ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>` and :ref:`VIDIOC_STREAMOFF
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<VIDIOC_STREAMON>` ioctl.
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.. note:::ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>`
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removes all buffers from both queues as a side effect. Since there is
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no notion of doing anything "now" on a multitasking system, if an
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application needs to synchronize with another event it should examine
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the struct ::c:type:`v4l2_buffer` ``timestamp`` of captured
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or outputted buffers.
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Drivers implementing memory mapping I/O must support the
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:ref:`VIDIOC_REQBUFS <VIDIOC_REQBUFS>`, :ref:`VIDIOC_QUERYBUF
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<VIDIOC_QUERYBUF>`, :ref:`VIDIOC_QBUF <VIDIOC_QBUF>`, :ref:`VIDIOC_DQBUF
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<VIDIOC_QBUF>`, :ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>`
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and :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` ioctls, the :ref:`mmap()
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<func-mmap>`, :ref:`munmap() <func-munmap>`, :ref:`select()
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<func-select>` and :ref:`poll() <func-poll>` function. [#f3]_
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[capture example]
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.. [#f1]
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One could use one file descriptor and set the buffer type field
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accordingly when calling :ref:`VIDIOC_QBUF` etc.,
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but it makes the :ref:`select() <func-select>` function ambiguous. We also
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like the clean approach of one file descriptor per logical stream.
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Video overlay for example is also a logical stream, although the CPU
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is not needed for continuous operation.
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.. [#f2]
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Random enqueue order permits applications processing images out of
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order (such as video codecs) to return buffers earlier, reducing the
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probability of data loss. Random fill order allows drivers to reuse
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buffers on a LIFO-basis, taking advantage of caches holding
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scatter-gather lists and the like.
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.. [#f3]
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At the driver level :ref:`select() <func-select>` and :ref:`poll() <func-poll>` are
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the same, and :ref:`select() <func-select>` is too important to be optional.
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The rest should be evident.
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