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]
[1] | One could use one file descriptor and set the buffer
type field accordingly when calling |
[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 |