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diff --git a/Documentation/media/uapi/v4l/mmap.rst b/Documentation/media/uapi/v4l/mmap.rst new file mode 100644 index 000000000..670596c1a --- /dev/null +++ b/Documentation/media/uapi/v4l/mmap.rst @@ -0,0 +1,285 @@ +.. -*- coding: utf-8; mode: rst -*- + +.. _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 :ref:`mmap() <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 :ref:`mmap() <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 :ref:`mmap() <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 :ref:`munmap() <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` +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 +:ref:`open() <func-open>` function, :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` +returns immediately with an ``EAGAIN`` error code when no buffer is +available. The :ref:`select() <func-select>` or :ref:`poll() +<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>`, :ref:`munmap() <func-munmap>`, :ref:`select() +<func-select>` and :ref:`poll() <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 :ref:`select() <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 :ref:`select() <func-select>` and :ref:`poll() <func-poll>` are + the same, and :ref:`select() <func-select>` is too important to be optional. + The rest should be evident. |