293 lines
11 KiB
ReStructuredText
293 lines
11 KiB
ReStructuredText
|
.. Permission is granted to copy, distribute and/or modify this
|
||
|
|
.. document under the terms of the GNU Free Documentation License,
|
||
|
|
.. Version 1.1 or any later version published by the Free Software
|
||
|
|
.. Foundation, with no Invariant Sections, no Front-Cover Texts
|
||
|
|
.. and no Back-Cover Texts. A copy of the license is included at
|
||
|
|
.. Documentation/media/uapi/fdl-appendix.rst.
|
||
|
|
..
|
||
|
|
.. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections
|
||
|
|
|
||
|
|
.. _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:`VIVIOC_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 :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.
|