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+.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
+.. c:namespace:: V4L
+
+.. _mmap:
+
+******************************
+Streaming I/O (Memory Mapping)
+******************************
+
+Input and output devices support this I/O method when the
+``V4L2_CAP_STREAMING`` flag in the ``capabilities`` field of struct
+:c:type:`v4l2_capability` returned by the
+:ref:`VIDIOC_QUERYCAP` ioctl is set. There are two
+streaming methods, to determine if the memory mapping flavor is
+supported applications must call the :ref:`VIDIOC_REQBUFS` ioctl
+with the memory type set to ``V4L2_MEMORY_MMAP``.
+
+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.
+
+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. [#f1]_
+
+To allocate device buffers applications call the
+:ref:`VIDIOC_REQBUFS` ioctl with the desired number
+of buffers and buffer type, for example ``V4L2_BUF_TYPE_VIDEO_CAPTURE``.
+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.
+
+Before applications can access the buffers they must map them into their
+address space with the :c:func:`mmap()` function. The
+location of the buffers in device memory can be determined with the
+:ref:`VIDIOC_QUERYBUF` ioctl. In the single-planar
+API case, the ``m.offset`` and ``length`` returned in a struct
+:c:type:`v4l2_buffer` are passed as sixth and second
+parameter to the :c:func:`mmap()` function. When using the
+multi-planar API, struct :c:type:`v4l2_buffer` contains an
+array of struct :c:type:`v4l2_plane` structures, each
+containing its own ``m.offset`` and ``length``. When using the
+multi-planar API, every plane of every buffer has to be mapped
+separately, so the number of calls to :c: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 :c:func:`munmap()` function.
+
+Example: Mapping buffers in the single-planar API
+=================================================
+
+.. code-block:: c
+
+    struct 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++) {
+	struct 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);
+
+Example: Mapping buffers in the multi-planar API
+================================================
+
+.. code-block:: c
+
+    struct 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++) {
+	struct v4l2_buffer buffer;
+	struct 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]);
+
+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.
+
+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 *fill* enqueued *empty* buffers in any order.  [#f2]_ The
+index number of a buffer (struct :c:type:`v4l2_buffer`
+``index``) plays no role here, it only identifies the buffer.
+
+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 :ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>`.
+In the write loop, when the application runs out of free buffers, it
+must wait until an empty buffer can be dequeued and reused.
+
+To enqueue and dequeue a buffer applications use the
+:ref:`VIDIOC_QBUF <VIDIOC_QBUF>` and :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>`
+ioctl. The status of a buffer being mapped, enqueued, full or empty can
+be determined at any time using the :ref:`VIDIOC_QUERYBUF` ioctl. Two
+methods exist to suspend execution of the application until one or more
+buffers can be dequeued.  By default :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>`
+blocks when no buffer is in the outgoing queue. When the ``O_NONBLOCK``
+flag was given to the :c:func:`open()` function,
+:ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` returns immediately with an ``EAGAIN``
+error code when no buffer is available. The :c:func:`select()`
+or :c:func:`poll()` functions are always available.
+
+To start and stop capturing or output applications call the
+:ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>` and :ref:`VIDIOC_STREAMOFF
+<VIDIOC_STREAMON>` ioctl.
+
+.. note:::ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>`
+   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 struct ::c:type:`v4l2_buffer` ``timestamp`` of captured
+   or outputted buffers.
+
+Drivers implementing memory mapping I/O must support the
+:ref:`VIDIOC_REQBUFS <VIDIOC_REQBUFS>`, :ref:`VIDIOC_QUERYBUF
+<VIDIOC_QUERYBUF>`, :ref:`VIDIOC_QBUF <VIDIOC_QBUF>`, :ref:`VIDIOC_DQBUF
+<VIDIOC_QBUF>`, :ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>`
+and :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` ioctls, the :ref:`mmap()
+<func-mmap>`, :c:func:`munmap()`, :ref:`select()
+<func-select>` and :c:func:`poll()` function. [#f3]_
+
+[capture example]
+
+.. [#f1]
+   One could use one file descriptor and set the buffer type field
+   accordingly when calling :ref:`VIDIOC_QBUF` etc.,
+   but it makes the :c:func:`select()` 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.
+
+.. [#f2]
+   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.
+
+.. [#f3]
+   At the driver level :c:func:`select()` and :c:func:`poll()` are
+   the same, and :c:func:`select()` is too important to be optional.
+   The rest should be evident.