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diff --git a/Documentation/userspace-api/media/v4l/vidioc-qbuf.rst b/Documentation/userspace-api/media/v4l/vidioc-qbuf.rst new file mode 100644 index 000000000000..ec0a54fbeb43 --- /dev/null +++ b/Documentation/userspace-api/media/v4l/vidioc-qbuf.rst @@ -0,0 +1,205 @@ +.. 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/userspace-api/media/fdl-appendix.rst. +.. +.. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections + +.. _VIDIOC_QBUF: + +******************************* +ioctl VIDIOC_QBUF, VIDIOC_DQBUF +******************************* + +Name +==== + +VIDIOC_QBUF - VIDIOC_DQBUF - Exchange a buffer with the driver + + +Synopsis +======== + +.. c:function:: int ioctl( int fd, VIDIOC_QBUF, struct v4l2_buffer *argp ) + :name: VIDIOC_QBUF + +.. c:function:: int ioctl( int fd, VIDIOC_DQBUF, struct v4l2_buffer *argp ) + :name: VIDIOC_DQBUF + + +Arguments +========= + +``fd`` + File descriptor returned by :ref:`open() <func-open>`. + +``argp`` + Pointer to struct :c:type:`v4l2_buffer`. + + +Description +=========== + +Applications call the ``VIDIOC_QBUF`` ioctl to enqueue an empty +(capturing) or filled (output) buffer in the driver's incoming queue. +The semantics depend on the selected I/O method. + +To enqueue a buffer applications set the ``type`` field of a struct +:c:type:`v4l2_buffer` to the same buffer type as was +previously used with struct :c:type:`v4l2_format` ``type`` +and struct :c:type:`v4l2_requestbuffers` ``type``. +Applications must also set the ``index`` field. Valid index numbers +range from zero to the number of buffers allocated with +:ref:`VIDIOC_REQBUFS` (struct +:c:type:`v4l2_requestbuffers` ``count``) minus +one. The contents of the struct :c:type:`v4l2_buffer` returned +by a :ref:`VIDIOC_QUERYBUF` ioctl will do as well. +When the buffer is intended for output (``type`` is +``V4L2_BUF_TYPE_VIDEO_OUTPUT``, ``V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE``, +or ``V4L2_BUF_TYPE_VBI_OUTPUT``) applications must also initialize the +``bytesused``, ``field`` and ``timestamp`` fields, see :ref:`buffer` +for details. Applications must also set ``flags`` to 0. The +``reserved2`` and ``reserved`` fields must be set to 0. When using the +:ref:`multi-planar API <planar-apis>`, the ``m.planes`` field must +contain a userspace pointer to a filled-in array of struct +:c:type:`v4l2_plane` and the ``length`` field must be set +to the number of elements in that array. + +To enqueue a :ref:`memory mapped <mmap>` buffer applications set the +``memory`` field to ``V4L2_MEMORY_MMAP``. When ``VIDIOC_QBUF`` is called +with a pointer to this structure the driver sets the +``V4L2_BUF_FLAG_MAPPED`` and ``V4L2_BUF_FLAG_QUEUED`` flags and clears +the ``V4L2_BUF_FLAG_DONE`` flag in the ``flags`` field, or it returns an +``EINVAL`` error code. + +To enqueue a :ref:`user pointer <userp>` buffer applications set the +``memory`` field to ``V4L2_MEMORY_USERPTR``, the ``m.userptr`` field to +the address of the buffer and ``length`` to its size. When the +multi-planar API is used, ``m.userptr`` and ``length`` members of the +passed array of struct :c:type:`v4l2_plane` have to be used +instead. When ``VIDIOC_QBUF`` is called with a pointer to this structure +the driver sets the ``V4L2_BUF_FLAG_QUEUED`` flag and clears the +``V4L2_BUF_FLAG_MAPPED`` and ``V4L2_BUF_FLAG_DONE`` flags in the +``flags`` field, or it returns an error code. This ioctl locks the +memory pages of the buffer in physical memory, they cannot be swapped +out to disk. Buffers remain locked until dequeued, until the +:ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` or +:ref:`VIDIOC_REQBUFS` ioctl is called, or until the +device is closed. + +To enqueue a :ref:`DMABUF <dmabuf>` buffer applications set the +``memory`` field to ``V4L2_MEMORY_DMABUF`` and the ``m.fd`` field to a +file descriptor associated with a DMABUF buffer. When the multi-planar +API is used the ``m.fd`` fields of the passed array of struct +:c:type:`v4l2_plane` have to be used instead. When +``VIDIOC_QBUF`` is called with a pointer to this structure the driver +sets the ``V4L2_BUF_FLAG_QUEUED`` flag and clears the +``V4L2_BUF_FLAG_MAPPED`` and ``V4L2_BUF_FLAG_DONE`` flags in the +``flags`` field, or it returns an error code. This ioctl locks the +buffer. Locking a buffer means passing it to a driver for a hardware +access (usually DMA). If an application accesses (reads/writes) a locked +buffer then the result is undefined. Buffers remain locked until +dequeued, until the :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` or +:ref:`VIDIOC_REQBUFS` ioctl is called, or until the +device is closed. + +The ``request_fd`` field can be used with the ``VIDIOC_QBUF`` ioctl to specify +the file descriptor of a :ref:`request <media-request-api>`, if requests are +in use. Setting it means that the buffer will not be passed to the driver +until the request itself is queued. Also, the driver will apply any +settings associated with the request for this buffer. This field will +be ignored unless the ``V4L2_BUF_FLAG_REQUEST_FD`` flag is set. +If the device does not support requests, then ``EBADR`` will be returned. +If requests are supported but an invalid request file descriptor is given, +then ``EINVAL`` will be returned. + +.. caution:: + It is not allowed to mix queuing requests with queuing buffers directly. + ``EBUSY`` will be returned if the first buffer was queued directly and + then the application tries to queue a request, or vice versa. After + closing the file descriptor, calling + :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` or calling :ref:`VIDIOC_REQBUFS` + the check for this will be reset. + + For :ref:`memory-to-memory devices <mem2mem>` you can specify the + ``request_fd`` only for output buffers, not for capture buffers. Attempting + to specify this for a capture buffer will result in an ``EBADR`` error. + +Applications call the ``VIDIOC_DQBUF`` ioctl to dequeue a filled +(capturing) or displayed (output) buffer from the driver's outgoing +queue. They just set the ``type``, ``memory`` and ``reserved`` fields of +a struct :c:type:`v4l2_buffer` as above, when +``VIDIOC_DQBUF`` is called with a pointer to this structure the driver +fills the remaining fields or returns an error code. The driver may also +set ``V4L2_BUF_FLAG_ERROR`` in the ``flags`` field. It indicates a +non-critical (recoverable) streaming error. In such case the application +may continue as normal, but should be aware that data in the dequeued +buffer might be corrupted. When using the multi-planar API, the planes +array must be passed in as well. + +If the application sets the ``memory`` field to ``V4L2_MEMORY_DMABUF`` to +dequeue a :ref:`DMABUF <dmabuf>` buffer, the driver fills the ``m.fd`` field +with a file descriptor numerically the same as the one given to ``VIDIOC_QBUF`` +when the buffer was enqueued. No new file descriptor is created at dequeue time +and the value is only for the application convenience. When the multi-planar +API is used the ``m.fd`` fields of the passed array of struct +:c:type:`v4l2_plane` are filled instead. + +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 struct :c:type:`v4l2_buffer` structure is specified in +:ref:`buffer`. + + +Return Value +============ + +On success 0 is returned, on error -1 and the ``errno`` variable is set +appropriately. The generic error codes are described at the +:ref:`Generic Error Codes <gen-errors>` chapter. + +EAGAIN + Non-blocking I/O has been selected using ``O_NONBLOCK`` and no + buffer was in the outgoing queue. + +EINVAL + The buffer ``type`` is not supported, or the ``index`` is out of + bounds, or no buffers have been allocated yet, or the ``userptr`` or + ``length`` are invalid, or the ``V4L2_BUF_FLAG_REQUEST_FD`` flag was + set but the the given ``request_fd`` was invalid, or ``m.fd`` was + an invalid DMABUF file descriptor. + +EIO + ``VIDIOC_DQBUF`` failed due to an internal error. Can also indicate + temporary problems like signal loss. + + .. note:: + + The driver might dequeue an (empty) buffer despite returning + an error, or even stop capturing. Reusing such buffer may be unsafe + though and its details (e.g. ``index``) may not be returned either. + It is recommended that drivers indicate recoverable errors by setting + the ``V4L2_BUF_FLAG_ERROR`` and returning 0 instead. In that case the + application should be able to safely reuse the buffer and continue + streaming. + +EPIPE + ``VIDIOC_DQBUF`` returns this on an empty capture queue for mem2mem + codecs if a buffer with the ``V4L2_BUF_FLAG_LAST`` was already + dequeued and no new buffers are expected to become available. + +EBADR + The ``V4L2_BUF_FLAG_REQUEST_FD`` flag was set but the device does not + support requests for the given buffer type, or + the ``V4L2_BUF_FLAG_REQUEST_FD`` flag was not set but the device requires + that the buffer is part of a request. + +EBUSY + The first buffer was queued via a request, but the application now tries + to queue it directly, or vice versa (it is not permitted to mix the two + APIs). |