doc-rst: linux_tv DocBook to reST migration (docs-next)

[deliverable/linux.git] / Documentation / linux_tv / media / v4l / userp.rst

.. -*- 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 <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 <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 <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
``VIDIOC_QUERYBUF`` ioctl.


.. code-block:: c

    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
``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 <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 ``VIDIOC_DQBUF`` blocks when no
buffer is in the outgoing queue. When the ``O_NONBLOCK`` flag was given
to the :ref:`open() <func-open>` function, ``VIDIOC_DQBUF`` 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
``VIDIOC_STREAMOFF`` 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
``VIDIOC_REQBUFS``, ``VIDIOC_QBUF``, ``VIDIOC_DQBUF``,
``VIDIOC_STREAMON`` and ``VIDIOC_STREAMOFF`` ioctl, the
:c:func:`select()` and :c: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 :c:func:`select()` and :c:func:`poll()` are
   the same, and :c:func:`select()` is too important to be optional.
   The rest should be evident.


.. ------------------------------------------------------------------------------
.. This file was automatically converted from DocBook-XML with the dbxml
.. library (https://github.com/return42/sphkerneldoc). The origin XML comes
.. from the linux kernel, refer to:
..
.. * https://github.com/torvalds/linux/tree/master/Documentation/DocBook
.. ------------------------------------------------------------------------------