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author | Mauro Carvalho Chehab <mchehab@s-opensource.com> | 2016-07-08 11:40:06 -0300 |
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committer | Mauro Carvalho Chehab <mchehab@s-opensource.com> | 2016-07-08 11:59:40 -0300 |
commit | 60c2820d0f6d3497975b6488e2599f8f611d8b95 (patch) | |
tree | b3b03707c6438ea9b99cc57e847ebf517f968ab1 /Documentation/media/uapi/v4l/userp.rst | |
parent | a97369b5e21ea9b8b5fef7c0f4f48bbe60c07ca3 (diff) | |
download | linux-60c2820d0f6d3497975b6488e2599f8f611d8b95.tar.bz2 |
doc_rst: rename the media Sphinx suff to Documentation/media
The name of the subsystem is "media", and not "linux_tv". Also,
as we plan to add other stuff there in the future, let's
rename also the media uAPI book to media_uapi, to make it
clearer.
No functional changes.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Diffstat (limited to 'Documentation/media/uapi/v4l/userp.rst')
-rw-r--r-- | Documentation/media/uapi/v4l/userp.rst | 117 |
1 files changed, 117 insertions, 0 deletions
diff --git a/Documentation/media/uapi/v4l/userp.rst b/Documentation/media/uapi/v4l/userp.rst new file mode 100644 index 000000000000..0ecf7a13a7af --- /dev/null +++ b/Documentation/media/uapi/v4l/userp.rst @@ -0,0 +1,117 @@ +.. -*- coding: utf-8; mode: rst -*- + +.. _userp: + +***************************** +Streaming I/O (User Pointers) +***************************** + +Input and output devices support this I/O method when the +``V4L2_CAP_STREAMING`` flag in the ``capabilities`` field of struct +:ref:`v4l2_capability <v4l2-capability>` returned by the +:ref:`VIDIOC_QUERYCAP` ioctl is set. If the +particular user pointer method (not only memory mapping) is supported +must be determined by calling the +:ref:`VIDIOC_REQBUFS` ioctl. + +This I/O method combines advantages of the read/write and memory mapping +methods. Buffers (planes) are allocated by the application itself, and +can reside for example in virtual or shared memory. Only pointers to +data are exchanged, these pointers and meta-information are passed in +struct :ref:`v4l2_buffer <v4l2-buffer>` (or in struct +:ref:`v4l2_plane <v4l2-plane>` in the multi-planar API case). The +driver must be switched into user pointer I/O mode by calling the +:ref:`VIDIOC_REQBUFS` with the desired buffer type. +No buffers (planes) are allocated beforehand, consequently they are not +indexed and cannot be queried like mapped buffers with the +:ref:`VIDIOC_QUERYBUF <VIDIOC_QUERYBUF>` ioctl. + + +.. code-block:: c + :caption: Example 3.3. Initiating streaming I/O with user pointers + + struct v4l2_requestbuffers reqbuf; + + memset (&reqbuf, 0, sizeof (reqbuf)); + reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; + reqbuf.memory = V4L2_MEMORY_USERPTR; + + if (ioctl (fd, VIDIOC_REQBUFS, &reqbuf) == -1) { + if (errno == EINVAL) + printf ("Video capturing or user pointer streaming is not supported\\n"); + else + perror ("VIDIOC_REQBUFS"); + + exit (EXIT_FAILURE); + } + +Buffer (plane) addresses and sizes are passed on the fly with the +:ref:`VIDIOC_QBUF <VIDIOC_QBUF>` ioctl. Although buffers are commonly +cycled, applications can pass different addresses and sizes at each +:ref:`VIDIOC_QBUF <VIDIOC_QBUF>` call. If required by the hardware the +driver swaps memory pages within physical memory to create a continuous +area of memory. This happens transparently to the application in the +virtual memory subsystem of the kernel. When buffer pages have been +swapped out to disk they are brought back and finally locked in physical +memory for DMA. [1]_ + +Filled or displayed buffers are dequeued with the +:ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl. The driver can unlock the +memory pages at any time between the completion of the DMA and this +ioctl. The memory is also unlocked when +:ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` is called, +:ref:`VIDIOC_REQBUFS`, or when the device is closed. +Applications must take care not to free buffers without dequeuing. For +once, the buffers remain locked until further, wasting physical memory. +Second the driver will not be notified when the memory is returned to +the application's free list and subsequently reused for other purposes, +possibly completing the requested DMA and overwriting valuable data. + +For capturing applications it is customary to enqueue a number of empty +buffers, 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 output is +started. In the write loop, when the application runs out of free +buffers it must wait until an empty buffer can be dequeued and reused. +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>` function 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 and unlocks all buffers 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 :ref:`v4l2_buffer <v4l2-buffer>` ``timestamp`` of captured or +outputted buffers. + +Drivers implementing user pointer I/O must support the +:ref:`VIDIOC_REQBUFS <VIDIOC_REQBUFS>`, :ref:`VIDIOC_QBUF <VIDIOC_QBUF>`, +:ref:`VIDIOC_DQBUF <VIDIOC_QBUF>`, :ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>` +and :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` ioctls, the +:ref:`select() <func-select>` and :ref:`poll() <func-poll>` function. [2]_ + +.. [1] + We expect that frequently used buffers are typically not swapped out. + Anyway, the process of swapping, locking or generating scatter-gather + lists may be time consuming. The delay can be masked by the depth of + the incoming buffer queue, and perhaps by maintaining caches assuming + a buffer will be soon enqueued again. On the other hand, to optimize + memory usage drivers can limit the number of buffers locked in + advance and recycle the most recently used buffers first. Of course, + the pages of empty buffers in the incoming queue need not be saved to + disk. Output buffers must be saved on the incoming and outgoing queue + because an application may share them with other processes. + +.. [2] + 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. |