Mauro Carvalho Chehab | 54f38fc | 2020-03-04 10:21:39 +0100 | [diff] [blame] | 1 | .. Permission is granted to copy, distribute and/or modify this |
| 2 | .. document under the terms of the GNU Free Documentation License, |
| 3 | .. Version 1.1 or any later version published by the Free Software |
| 4 | .. Foundation, with no Invariant Sections, no Front-Cover Texts |
| 5 | .. and no Back-Cover Texts. A copy of the license is included at |
| 6 | .. Documentation/userspace-api/media/fdl-appendix.rst. |
| 7 | .. |
| 8 | .. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections |
| 9 | |
| 10 | .. _VIDIOC_QBUF: |
| 11 | |
| 12 | ******************************* |
| 13 | ioctl VIDIOC_QBUF, VIDIOC_DQBUF |
| 14 | ******************************* |
| 15 | |
| 16 | Name |
| 17 | ==== |
| 18 | |
| 19 | VIDIOC_QBUF - VIDIOC_DQBUF - Exchange a buffer with the driver |
| 20 | |
| 21 | |
| 22 | Synopsis |
| 23 | ======== |
| 24 | |
| 25 | .. c:function:: int ioctl( int fd, VIDIOC_QBUF, struct v4l2_buffer *argp ) |
| 26 | :name: VIDIOC_QBUF |
| 27 | |
| 28 | .. c:function:: int ioctl( int fd, VIDIOC_DQBUF, struct v4l2_buffer *argp ) |
| 29 | :name: VIDIOC_DQBUF |
| 30 | |
| 31 | |
| 32 | Arguments |
| 33 | ========= |
| 34 | |
| 35 | ``fd`` |
| 36 | File descriptor returned by :ref:`open() <func-open>`. |
| 37 | |
| 38 | ``argp`` |
| 39 | Pointer to struct :c:type:`v4l2_buffer`. |
| 40 | |
| 41 | |
| 42 | Description |
| 43 | =========== |
| 44 | |
| 45 | Applications call the ``VIDIOC_QBUF`` ioctl to enqueue an empty |
| 46 | (capturing) or filled (output) buffer in the driver's incoming queue. |
| 47 | The semantics depend on the selected I/O method. |
| 48 | |
| 49 | To enqueue a buffer applications set the ``type`` field of a struct |
| 50 | :c:type:`v4l2_buffer` to the same buffer type as was |
| 51 | previously used with struct :c:type:`v4l2_format` ``type`` |
| 52 | and struct :c:type:`v4l2_requestbuffers` ``type``. |
| 53 | Applications must also set the ``index`` field. Valid index numbers |
| 54 | range from zero to the number of buffers allocated with |
| 55 | :ref:`VIDIOC_REQBUFS` (struct |
| 56 | :c:type:`v4l2_requestbuffers` ``count``) minus |
| 57 | one. The contents of the struct :c:type:`v4l2_buffer` returned |
| 58 | by a :ref:`VIDIOC_QUERYBUF` ioctl will do as well. |
| 59 | When the buffer is intended for output (``type`` is |
| 60 | ``V4L2_BUF_TYPE_VIDEO_OUTPUT``, ``V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE``, |
| 61 | or ``V4L2_BUF_TYPE_VBI_OUTPUT``) applications must also initialize the |
| 62 | ``bytesused``, ``field`` and ``timestamp`` fields, see :ref:`buffer` |
| 63 | for details. Applications must also set ``flags`` to 0. The |
| 64 | ``reserved2`` and ``reserved`` fields must be set to 0. When using the |
| 65 | :ref:`multi-planar API <planar-apis>`, the ``m.planes`` field must |
| 66 | contain a userspace pointer to a filled-in array of struct |
| 67 | :c:type:`v4l2_plane` and the ``length`` field must be set |
| 68 | to the number of elements in that array. |
| 69 | |
| 70 | To enqueue a :ref:`memory mapped <mmap>` buffer applications set the |
| 71 | ``memory`` field to ``V4L2_MEMORY_MMAP``. When ``VIDIOC_QBUF`` is called |
| 72 | with a pointer to this structure the driver sets the |
| 73 | ``V4L2_BUF_FLAG_MAPPED`` and ``V4L2_BUF_FLAG_QUEUED`` flags and clears |
| 74 | the ``V4L2_BUF_FLAG_DONE`` flag in the ``flags`` field, or it returns an |
| 75 | ``EINVAL`` error code. |
| 76 | |
| 77 | To enqueue a :ref:`user pointer <userp>` buffer applications set the |
| 78 | ``memory`` field to ``V4L2_MEMORY_USERPTR``, the ``m.userptr`` field to |
| 79 | the address of the buffer and ``length`` to its size. When the |
| 80 | multi-planar API is used, ``m.userptr`` and ``length`` members of the |
| 81 | passed array of struct :c:type:`v4l2_plane` have to be used |
| 82 | instead. When ``VIDIOC_QBUF`` is called with a pointer to this structure |
| 83 | the driver sets the ``V4L2_BUF_FLAG_QUEUED`` flag and clears the |
| 84 | ``V4L2_BUF_FLAG_MAPPED`` and ``V4L2_BUF_FLAG_DONE`` flags in the |
| 85 | ``flags`` field, or it returns an error code. This ioctl locks the |
| 86 | memory pages of the buffer in physical memory, they cannot be swapped |
| 87 | out to disk. Buffers remain locked until dequeued, until the |
| 88 | :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` or |
| 89 | :ref:`VIDIOC_REQBUFS` ioctl is called, or until the |
| 90 | device is closed. |
| 91 | |
| 92 | To enqueue a :ref:`DMABUF <dmabuf>` buffer applications set the |
| 93 | ``memory`` field to ``V4L2_MEMORY_DMABUF`` and the ``m.fd`` field to a |
| 94 | file descriptor associated with a DMABUF buffer. When the multi-planar |
| 95 | API is used the ``m.fd`` fields of the passed array of struct |
| 96 | :c:type:`v4l2_plane` have to be used instead. When |
| 97 | ``VIDIOC_QBUF`` is called with a pointer to this structure the driver |
| 98 | sets the ``V4L2_BUF_FLAG_QUEUED`` flag and clears the |
| 99 | ``V4L2_BUF_FLAG_MAPPED`` and ``V4L2_BUF_FLAG_DONE`` flags in the |
| 100 | ``flags`` field, or it returns an error code. This ioctl locks the |
| 101 | buffer. Locking a buffer means passing it to a driver for a hardware |
| 102 | access (usually DMA). If an application accesses (reads/writes) a locked |
| 103 | buffer then the result is undefined. Buffers remain locked until |
| 104 | dequeued, until the :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` or |
| 105 | :ref:`VIDIOC_REQBUFS` ioctl is called, or until the |
| 106 | device is closed. |
| 107 | |
| 108 | The ``request_fd`` field can be used with the ``VIDIOC_QBUF`` ioctl to specify |
| 109 | the file descriptor of a :ref:`request <media-request-api>`, if requests are |
| 110 | in use. Setting it means that the buffer will not be passed to the driver |
| 111 | until the request itself is queued. Also, the driver will apply any |
| 112 | settings associated with the request for this buffer. This field will |
| 113 | be ignored unless the ``V4L2_BUF_FLAG_REQUEST_FD`` flag is set. |
| 114 | If the device does not support requests, then ``EBADR`` will be returned. |
| 115 | If requests are supported but an invalid request file descriptor is given, |
| 116 | then ``EINVAL`` will be returned. |
| 117 | |
| 118 | .. caution:: |
| 119 | It is not allowed to mix queuing requests with queuing buffers directly. |
| 120 | ``EBUSY`` will be returned if the first buffer was queued directly and |
| 121 | then the application tries to queue a request, or vice versa. After |
| 122 | closing the file descriptor, calling |
| 123 | :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` or calling :ref:`VIDIOC_REQBUFS` |
| 124 | the check for this will be reset. |
| 125 | |
| 126 | For :ref:`memory-to-memory devices <mem2mem>` you can specify the |
| 127 | ``request_fd`` only for output buffers, not for capture buffers. Attempting |
| 128 | to specify this for a capture buffer will result in an ``EBADR`` error. |
| 129 | |
| 130 | Applications call the ``VIDIOC_DQBUF`` ioctl to dequeue a filled |
| 131 | (capturing) or displayed (output) buffer from the driver's outgoing |
| 132 | queue. They just set the ``type``, ``memory`` and ``reserved`` fields of |
| 133 | a struct :c:type:`v4l2_buffer` as above, when |
| 134 | ``VIDIOC_DQBUF`` is called with a pointer to this structure the driver |
| 135 | fills the remaining fields or returns an error code. The driver may also |
| 136 | set ``V4L2_BUF_FLAG_ERROR`` in the ``flags`` field. It indicates a |
| 137 | non-critical (recoverable) streaming error. In such case the application |
| 138 | may continue as normal, but should be aware that data in the dequeued |
| 139 | buffer might be corrupted. When using the multi-planar API, the planes |
| 140 | array must be passed in as well. |
| 141 | |
| 142 | If the application sets the ``memory`` field to ``V4L2_MEMORY_DMABUF`` to |
| 143 | dequeue a :ref:`DMABUF <dmabuf>` buffer, the driver fills the ``m.fd`` field |
| 144 | with a file descriptor numerically the same as the one given to ``VIDIOC_QBUF`` |
| 145 | when the buffer was enqueued. No new file descriptor is created at dequeue time |
| 146 | and the value is only for the application convenience. When the multi-planar |
| 147 | API is used the ``m.fd`` fields of the passed array of struct |
| 148 | :c:type:`v4l2_plane` are filled instead. |
| 149 | |
| 150 | By default ``VIDIOC_DQBUF`` blocks when no buffer is in the outgoing |
| 151 | queue. When the ``O_NONBLOCK`` flag was given to the |
| 152 | :ref:`open() <func-open>` function, ``VIDIOC_DQBUF`` returns |
| 153 | immediately with an ``EAGAIN`` error code when no buffer is available. |
| 154 | |
| 155 | The struct :c:type:`v4l2_buffer` structure is specified in |
| 156 | :ref:`buffer`. |
| 157 | |
| 158 | |
| 159 | Return Value |
| 160 | ============ |
| 161 | |
| 162 | On success 0 is returned, on error -1 and the ``errno`` variable is set |
| 163 | appropriately. The generic error codes are described at the |
| 164 | :ref:`Generic Error Codes <gen-errors>` chapter. |
| 165 | |
| 166 | EAGAIN |
| 167 | Non-blocking I/O has been selected using ``O_NONBLOCK`` and no |
| 168 | buffer was in the outgoing queue. |
| 169 | |
| 170 | EINVAL |
| 171 | The buffer ``type`` is not supported, or the ``index`` is out of |
| 172 | bounds, or no buffers have been allocated yet, or the ``userptr`` or |
| 173 | ``length`` are invalid, or the ``V4L2_BUF_FLAG_REQUEST_FD`` flag was |
| 174 | set but the the given ``request_fd`` was invalid, or ``m.fd`` was |
| 175 | an invalid DMABUF file descriptor. |
| 176 | |
| 177 | EIO |
| 178 | ``VIDIOC_DQBUF`` failed due to an internal error. Can also indicate |
| 179 | temporary problems like signal loss. |
| 180 | |
| 181 | .. note:: |
| 182 | |
| 183 | The driver might dequeue an (empty) buffer despite returning |
| 184 | an error, or even stop capturing. Reusing such buffer may be unsafe |
| 185 | though and its details (e.g. ``index``) may not be returned either. |
| 186 | It is recommended that drivers indicate recoverable errors by setting |
| 187 | the ``V4L2_BUF_FLAG_ERROR`` and returning 0 instead. In that case the |
| 188 | application should be able to safely reuse the buffer and continue |
| 189 | streaming. |
| 190 | |
| 191 | EPIPE |
| 192 | ``VIDIOC_DQBUF`` returns this on an empty capture queue for mem2mem |
| 193 | codecs if a buffer with the ``V4L2_BUF_FLAG_LAST`` was already |
| 194 | dequeued and no new buffers are expected to become available. |
| 195 | |
| 196 | EBADR |
| 197 | The ``V4L2_BUF_FLAG_REQUEST_FD`` flag was set but the device does not |
| 198 | support requests for the given buffer type, or |
| 199 | the ``V4L2_BUF_FLAG_REQUEST_FD`` flag was not set but the device requires |
| 200 | that the buffer is part of a request. |
| 201 | |
| 202 | EBUSY |
| 203 | The first buffer was queued via a request, but the application now tries |
| 204 | to queue it directly, or vice versa (it is not permitted to mix the two |
| 205 | APIs). |