Markus Heiser | 5377d91 | 2016-06-30 15:18:56 +0200 | [diff] [blame] | 1 | .. -*- coding: utf-8; mode: rst -*- |
| 2 | |
Mauro Carvalho Chehab | af4a4d0 | 2016-07-01 13:42:29 -0300 | [diff] [blame] | 3 | .. _VIDIOC_QBUF: |
Markus Heiser | 5377d91 | 2016-06-30 15:18:56 +0200 | [diff] [blame] | 4 | |
| 5 | ******************************* |
| 6 | ioctl VIDIOC_QBUF, VIDIOC_DQBUF |
| 7 | ******************************* |
| 8 | |
| 9 | *man VIDIOC_QBUF(2)* |
| 10 | |
| 11 | VIDIOC_DQBUF |
| 12 | Exchange a buffer with the driver |
| 13 | |
| 14 | |
| 15 | Synopsis |
| 16 | ======== |
| 17 | |
Mauro Carvalho Chehab | b7e67f6 | 2016-07-02 09:49:16 -0300 | [diff] [blame] | 18 | .. cpp:function:: int ioctl( int fd, int request, struct v4l2_buffer *argp ) |
Markus Heiser | 5377d91 | 2016-06-30 15:18:56 +0200 | [diff] [blame] | 19 | |
| 20 | Arguments |
| 21 | ========= |
| 22 | |
| 23 | ``fd`` |
| 24 | File descriptor returned by :ref:`open() <func-open>`. |
| 25 | |
| 26 | ``request`` |
| 27 | VIDIOC_QBUF, VIDIOC_DQBUF |
| 28 | |
| 29 | ``argp`` |
| 30 | |
| 31 | |
| 32 | Description |
| 33 | =========== |
| 34 | |
| 35 | Applications call the ``VIDIOC_QBUF`` ioctl to enqueue an empty |
| 36 | (capturing) or filled (output) buffer in the driver's incoming queue. |
| 37 | The semantics depend on the selected I/O method. |
| 38 | |
| 39 | To enqueue a buffer applications set the ``type`` field of a struct |
| 40 | :ref:`v4l2_buffer <v4l2-buffer>` to the same buffer type as was |
| 41 | previously used with struct :ref:`v4l2_format <v4l2-format>` ``type`` |
| 42 | and struct :ref:`v4l2_requestbuffers <v4l2-requestbuffers>` ``type``. |
| 43 | Applications must also set the ``index`` field. Valid index numbers |
| 44 | range from zero to the number of buffers allocated with |
Mauro Carvalho Chehab | 7347081 | 2016-07-01 13:58:44 -0300 | [diff] [blame] | 45 | :ref:`VIDIOC_REQBUFS` (struct |
Markus Heiser | 5377d91 | 2016-06-30 15:18:56 +0200 | [diff] [blame] | 46 | :ref:`v4l2_requestbuffers <v4l2-requestbuffers>` ``count``) minus |
Mauro Carvalho Chehab | acf309a | 2016-07-03 13:28:28 -0300 | [diff] [blame^] | 47 | one. The contents of the struct :ref:`struct v4l2_buffer <v4l2-buffer>` returned |
Mauro Carvalho Chehab | 7347081 | 2016-07-01 13:58:44 -0300 | [diff] [blame] | 48 | by a :ref:`VIDIOC_QUERYBUF` ioctl will do as well. |
Markus Heiser | 5377d91 | 2016-06-30 15:18:56 +0200 | [diff] [blame] | 49 | When the buffer is intended for output (``type`` is |
| 50 | ``V4L2_BUF_TYPE_VIDEO_OUTPUT``, ``V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE``, |
| 51 | or ``V4L2_BUF_TYPE_VBI_OUTPUT``) applications must also initialize the |
| 52 | ``bytesused``, ``field`` and ``timestamp`` fields, see :ref:`buffer` |
| 53 | for details. Applications must also set ``flags`` to 0. The |
| 54 | ``reserved2`` and ``reserved`` fields must be set to 0. When using the |
| 55 | :ref:`multi-planar API <planar-apis>`, the ``m.planes`` field must |
| 56 | contain a userspace pointer to a filled-in array of struct |
| 57 | :ref:`v4l2_plane <v4l2-plane>` and the ``length`` field must be set |
| 58 | to the number of elements in that array. |
| 59 | |
| 60 | To enqueue a :ref:`memory mapped <mmap>` buffer applications set the |
| 61 | ``memory`` field to ``V4L2_MEMORY_MMAP``. When ``VIDIOC_QBUF`` is called |
| 62 | with a pointer to this structure the driver sets the |
| 63 | ``V4L2_BUF_FLAG_MAPPED`` and ``V4L2_BUF_FLAG_QUEUED`` flags and clears |
| 64 | the ``V4L2_BUF_FLAG_DONE`` flag in the ``flags`` field, or it returns an |
| 65 | EINVAL error code. |
| 66 | |
| 67 | To enqueue a :ref:`user pointer <userp>` buffer applications set the |
| 68 | ``memory`` field to ``V4L2_MEMORY_USERPTR``, the ``m.userptr`` field to |
| 69 | the address of the buffer and ``length`` to its size. When the |
| 70 | multi-planar API is used, ``m.userptr`` and ``length`` members of the |
| 71 | passed array of struct :ref:`v4l2_plane <v4l2-plane>` have to be used |
| 72 | instead. When ``VIDIOC_QBUF`` is called with a pointer to this structure |
| 73 | the driver sets the ``V4L2_BUF_FLAG_QUEUED`` flag and clears the |
| 74 | ``V4L2_BUF_FLAG_MAPPED`` and ``V4L2_BUF_FLAG_DONE`` flags in the |
| 75 | ``flags`` field, or it returns an error code. This ioctl locks the |
| 76 | memory pages of the buffer in physical memory, they cannot be swapped |
| 77 | out to disk. Buffers remain locked until dequeued, until the |
Mauro Carvalho Chehab | af4a4d0 | 2016-07-01 13:42:29 -0300 | [diff] [blame] | 78 | :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` or |
Mauro Carvalho Chehab | 7347081 | 2016-07-01 13:58:44 -0300 | [diff] [blame] | 79 | :ref:`VIDIOC_REQBUFS` ioctl is called, or until the |
Markus Heiser | 5377d91 | 2016-06-30 15:18:56 +0200 | [diff] [blame] | 80 | device is closed. |
| 81 | |
| 82 | To enqueue a :ref:`DMABUF <dmabuf>` buffer applications set the |
| 83 | ``memory`` field to ``V4L2_MEMORY_DMABUF`` and the ``m.fd`` field to a |
| 84 | file descriptor associated with a DMABUF buffer. When the multi-planar |
| 85 | API is used the ``m.fd`` fields of the passed array of struct |
| 86 | :ref:`v4l2_plane <v4l2-plane>` have to be used instead. When |
| 87 | ``VIDIOC_QBUF`` is called with a pointer to this structure the driver |
| 88 | sets the ``V4L2_BUF_FLAG_QUEUED`` flag and clears the |
| 89 | ``V4L2_BUF_FLAG_MAPPED`` and ``V4L2_BUF_FLAG_DONE`` flags in the |
| 90 | ``flags`` field, or it returns an error code. This ioctl locks the |
| 91 | buffer. Locking a buffer means passing it to a driver for a hardware |
| 92 | access (usually DMA). If an application accesses (reads/writes) a locked |
| 93 | buffer then the result is undefined. Buffers remain locked until |
Mauro Carvalho Chehab | af4a4d0 | 2016-07-01 13:42:29 -0300 | [diff] [blame] | 94 | dequeued, until the :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` or |
Mauro Carvalho Chehab | 7347081 | 2016-07-01 13:58:44 -0300 | [diff] [blame] | 95 | :ref:`VIDIOC_REQBUFS` ioctl is called, or until the |
Markus Heiser | 5377d91 | 2016-06-30 15:18:56 +0200 | [diff] [blame] | 96 | device is closed. |
| 97 | |
| 98 | Applications call the ``VIDIOC_DQBUF`` ioctl to dequeue a filled |
| 99 | (capturing) or displayed (output) buffer from the driver's outgoing |
| 100 | queue. They just set the ``type``, ``memory`` and ``reserved`` fields of |
| 101 | a struct :ref:`v4l2_buffer <v4l2-buffer>` as above, when |
| 102 | ``VIDIOC_DQBUF`` is called with a pointer to this structure the driver |
| 103 | fills the remaining fields or returns an error code. The driver may also |
| 104 | set ``V4L2_BUF_FLAG_ERROR`` in the ``flags`` field. It indicates a |
| 105 | non-critical (recoverable) streaming error. In such case the application |
| 106 | may continue as normal, but should be aware that data in the dequeued |
| 107 | buffer might be corrupted. When using the multi-planar API, the planes |
| 108 | array must be passed in as well. |
| 109 | |
| 110 | By default ``VIDIOC_DQBUF`` blocks when no buffer is in the outgoing |
| 111 | queue. When the ``O_NONBLOCK`` flag was given to the |
| 112 | :ref:`open() <func-open>` function, ``VIDIOC_DQBUF`` returns |
Mauro Carvalho Chehab | cdb4af0 | 2016-07-03 11:53:09 -0300 | [diff] [blame] | 113 | immediately with an ``EAGAIN`` error code when no buffer is available. |
Markus Heiser | 5377d91 | 2016-06-30 15:18:56 +0200 | [diff] [blame] | 114 | |
Mauro Carvalho Chehab | acf309a | 2016-07-03 13:28:28 -0300 | [diff] [blame^] | 115 | The :ref:`struct v4l2_buffer <v4l2-buffer>` structure is specified in |
Markus Heiser | 5377d91 | 2016-06-30 15:18:56 +0200 | [diff] [blame] | 116 | :ref:`buffer`. |
| 117 | |
| 118 | |
| 119 | Return Value |
| 120 | ============ |
| 121 | |
| 122 | On success 0 is returned, on error -1 and the ``errno`` variable is set |
| 123 | appropriately. The generic error codes are described at the |
| 124 | :ref:`Generic Error Codes <gen-errors>` chapter. |
| 125 | |
| 126 | EAGAIN |
| 127 | Non-blocking I/O has been selected using ``O_NONBLOCK`` and no |
| 128 | buffer was in the outgoing queue. |
| 129 | |
| 130 | EINVAL |
| 131 | The buffer ``type`` is not supported, or the ``index`` is out of |
| 132 | bounds, or no buffers have been allocated yet, or the ``userptr`` or |
| 133 | ``length`` are invalid. |
| 134 | |
| 135 | EIO |
| 136 | ``VIDIOC_DQBUF`` failed due to an internal error. Can also indicate |
| 137 | temporary problems like signal loss. Note the driver might dequeue |
| 138 | an (empty) buffer despite returning an error, or even stop |
| 139 | capturing. Reusing such buffer may be unsafe though and its details |
| 140 | (e.g. ``index``) may not be returned either. It is recommended that |
| 141 | drivers indicate recoverable errors by setting the |
| 142 | ``V4L2_BUF_FLAG_ERROR`` and returning 0 instead. In that case the |
| 143 | application should be able to safely reuse the buffer and continue |
| 144 | streaming. |
| 145 | |
| 146 | EPIPE |
| 147 | ``VIDIOC_DQBUF`` returns this on an empty capture queue for mem2mem |
| 148 | codecs if a buffer with the ``V4L2_BUF_FLAG_LAST`` was already |
| 149 | dequeued and no new buffers are expected to become available. |
| 150 | |
| 151 | |
| 152 | .. ------------------------------------------------------------------------------ |
| 153 | .. This file was automatically converted from DocBook-XML with the dbxml |
| 154 | .. library (https://github.com/return42/sphkerneldoc). The origin XML comes |
| 155 | .. from the linux kernel, refer to: |
| 156 | .. |
| 157 | .. * https://github.com/torvalds/linux/tree/master/Documentation/DocBook |
| 158 | .. ------------------------------------------------------------------------------ |