155 строки
6.3 KiB
ReStructuredText
155 строки
6.3 KiB
ReStructuredText
.. -*- coding: utf-8; mode: rst -*-
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.. _VIDIOC_QBUF:
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*******************************
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ioctl VIDIOC_QBUF, VIDIOC_DQBUF
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*******************************
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Name
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====
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VIDIOC_QBUF - VIDIOC_DQBUF - Exchange a buffer with the driver
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Synopsis
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========
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.. c:function:: int ioctl( int fd, VIDIOC_QBUF, struct v4l2_buffer *argp )
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:name: VIDIOC_QBUF
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.. c:function:: int ioctl( int fd, VIDIOC_DQBUF, struct v4l2_buffer *argp )
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:name: VIDIOC_DQBUF
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Arguments
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=========
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``fd``
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File descriptor returned by :ref:`open() <func-open>`.
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``argp``
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Description
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===========
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Applications call the ``VIDIOC_QBUF`` ioctl to enqueue an empty
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(capturing) or filled (output) buffer in the driver's incoming queue.
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The semantics depend on the selected I/O method.
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To enqueue a buffer applications set the ``type`` field of a struct
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:c:type:`v4l2_buffer` to the same buffer type as was
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previously used with struct :c:type:`v4l2_format` ``type``
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and struct :c:type:`v4l2_requestbuffers` ``type``.
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Applications must also set the ``index`` field. Valid index numbers
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range from zero to the number of buffers allocated with
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:ref:`VIDIOC_REQBUFS` (struct
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:c:type:`v4l2_requestbuffers` ``count``) minus
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one. The contents of the struct :c:type:`v4l2_buffer` returned
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by a :ref:`VIDIOC_QUERYBUF` ioctl will do as well.
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When the buffer is intended for output (``type`` is
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``V4L2_BUF_TYPE_VIDEO_OUTPUT``, ``V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE``,
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or ``V4L2_BUF_TYPE_VBI_OUTPUT``) applications must also initialize the
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``bytesused``, ``field`` and ``timestamp`` fields, see :ref:`buffer`
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for details. Applications must also set ``flags`` to 0. The
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``reserved2`` and ``reserved`` fields must be set to 0. When using the
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:ref:`multi-planar API <planar-apis>`, the ``m.planes`` field must
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contain a userspace pointer to a filled-in array of struct
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:c:type:`v4l2_plane` and the ``length`` field must be set
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to the number of elements in that array.
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To enqueue a :ref:`memory mapped <mmap>` buffer applications set the
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``memory`` field to ``V4L2_MEMORY_MMAP``. When ``VIDIOC_QBUF`` is called
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with a pointer to this structure the driver sets the
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``V4L2_BUF_FLAG_MAPPED`` and ``V4L2_BUF_FLAG_QUEUED`` flags and clears
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the ``V4L2_BUF_FLAG_DONE`` flag in the ``flags`` field, or it returns an
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EINVAL error code.
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To enqueue a :ref:`user pointer <userp>` buffer applications set the
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``memory`` field to ``V4L2_MEMORY_USERPTR``, the ``m.userptr`` field to
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the address of the buffer and ``length`` to its size. When the
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multi-planar API is used, ``m.userptr`` and ``length`` members of the
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passed array of struct :c:type:`v4l2_plane` have to be used
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instead. When ``VIDIOC_QBUF`` is called with a pointer to this structure
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the driver sets the ``V4L2_BUF_FLAG_QUEUED`` flag and clears the
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``V4L2_BUF_FLAG_MAPPED`` and ``V4L2_BUF_FLAG_DONE`` flags in the
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``flags`` field, or it returns an error code. This ioctl locks the
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memory pages of the buffer in physical memory, they cannot be swapped
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out to disk. Buffers remain locked until dequeued, until the
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:ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` or
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:ref:`VIDIOC_REQBUFS` ioctl is called, or until the
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device is closed.
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To enqueue a :ref:`DMABUF <dmabuf>` buffer applications set the
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``memory`` field to ``V4L2_MEMORY_DMABUF`` and the ``m.fd`` field to a
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file descriptor associated with a DMABUF buffer. When the multi-planar
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API is used the ``m.fd`` fields of the passed array of struct
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:c:type:`v4l2_plane` have to be used instead. When
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``VIDIOC_QBUF`` is called with a pointer to this structure the driver
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sets the ``V4L2_BUF_FLAG_QUEUED`` flag and clears the
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``V4L2_BUF_FLAG_MAPPED`` and ``V4L2_BUF_FLAG_DONE`` flags in the
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``flags`` field, or it returns an error code. This ioctl locks the
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buffer. Locking a buffer means passing it to a driver for a hardware
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access (usually DMA). If an application accesses (reads/writes) a locked
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buffer then the result is undefined. Buffers remain locked until
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dequeued, until the :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` or
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:ref:`VIDIOC_REQBUFS` ioctl is called, or until the
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device is closed.
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Applications call the ``VIDIOC_DQBUF`` ioctl to dequeue a filled
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(capturing) or displayed (output) buffer from the driver's outgoing
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queue. They just set the ``type``, ``memory`` and ``reserved`` fields of
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a struct :c:type:`v4l2_buffer` as above, when
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``VIDIOC_DQBUF`` is called with a pointer to this structure the driver
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fills the remaining fields or returns an error code. The driver may also
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set ``V4L2_BUF_FLAG_ERROR`` in the ``flags`` field. It indicates a
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non-critical (recoverable) streaming error. In such case the application
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may continue as normal, but should be aware that data in the dequeued
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buffer might be corrupted. When using the multi-planar API, the planes
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array must be passed in as well.
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By default ``VIDIOC_DQBUF`` blocks when no buffer is in the outgoing
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queue. When the ``O_NONBLOCK`` flag was given to the
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:ref:`open() <func-open>` function, ``VIDIOC_DQBUF`` returns
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immediately with an ``EAGAIN`` error code when no buffer is available.
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The struct :c:type:`v4l2_buffer` structure is specified in
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:ref:`buffer`.
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Return Value
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============
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On success 0 is returned, on error -1 and the ``errno`` variable is set
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appropriately. The generic error codes are described at the
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:ref:`Generic Error Codes <gen-errors>` chapter.
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EAGAIN
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Non-blocking I/O has been selected using ``O_NONBLOCK`` and no
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buffer was in the outgoing queue.
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EINVAL
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The buffer ``type`` is not supported, or the ``index`` is out of
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bounds, or no buffers have been allocated yet, or the ``userptr`` or
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``length`` are invalid.
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EIO
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``VIDIOC_DQBUF`` failed due to an internal error. Can also indicate
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temporary problems like signal loss.
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.. note::
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The driver might dequeue an (empty) buffer despite returning
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an error, or even stop capturing. Reusing such buffer may be unsafe
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though and its details (e.g. ``index``) may not be returned either.
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It is recommended that drivers indicate recoverable errors by setting
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the ``V4L2_BUF_FLAG_ERROR`` and returning 0 instead. In that case the
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application should be able to safely reuse the buffer and continue
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streaming.
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EPIPE
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``VIDIOC_DQBUF`` returns this on an empty capture queue for mem2mem
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codecs if a buffer with the ``V4L2_BUF_FLAG_LAST`` was already
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dequeued and no new buffers are expected to become available.
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