Streaming I/O (Memory Mapping)

Video for Linux Two API Specification: Revision 0.24
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# 3.2. 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 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.

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.[1]

To allocate device buffers applications call the `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 `mmap()` function. The location of the buffers in device memory can be determined with the `VIDIOC_QUERYBUF` ioctl. The `m.offset` and `length` returned in a struct v4l2_buffer are passed as sixth and second parameter to the `mmap()` function. 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 `munmap()` function.

Example 3-1. Mapping buffers

    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);

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. [2] The index number of a buffer (struct 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 `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 `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 `VIDIOC_DQBUF` blocks when no buffer is in the outgoing queue. When the `O_NONBLOCK` flag was given to the `open()` function, `VIDIOC_DQBUF` returns immediately with an EAGAIN error code when no buffer is available. The `select()` or `poll()` function are always available.

To start and stop capturing or output applications call the `VIDIOC_STREAMON` and `VIDIOC_STREAMOFF` ioctl. Note `VIDIOC_STREAMOFF` 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 v4l2_buffer `timestamp` of captured buffers, or set the field before enqueuing buffers for output.

Drivers implementing memory mapping I/O must support the `VIDIOC_REQBUFS`, `VIDIOC_QUERYBUF`, `VIDIOC_QBUF`, `VIDIOC_DQBUF`, `VIDIOC_STREAMON` and `VIDIOC_STREAMOFF` ioctl, the `mmap()`, `munmap()`, `select()` and `poll()` function.[3]

[capture example]

### Notes

[1] | One could use one file descriptor and set the buffer type field accordingly when calling `VIDIOC_QBUF` etc., but it makes the `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.
[2] | 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.
[3] | At the driver level `select()` and `poll()` are the same, and `select()` is too important to be optional. The rest should be evident.

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