vidioc-reqbufs.rst

  1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
  2.. c:namespace:: V4L
  3
  4.. _VIDIOC_REQBUFS:
  5
  6********************
  7ioctl VIDIOC_REQBUFS
  8********************
  9
 10Name
 11====
 12
 13VIDIOC_REQBUFS - Initiate Memory Mapping, User Pointer I/O or DMA buffer I/O
 14
 15Synopsis
 16========
 17
 18.. c:macro:: VIDIOC_REQBUFS
 19
 20``int ioctl(int fd, VIDIOC_REQBUFS, struct v4l2_requestbuffers *argp)``
 21
 22Arguments
 23=========
 24
 25``fd``
 26    File descriptor returned by :c:func:`open()`.
 27
 28``argp``
 29    Pointer to struct :c:type:`v4l2_requestbuffers`.
 30
 31Description
 32===========
 33
 34This ioctl is used to initiate :ref:`memory mapped <mmap>`,
 35:ref:`user pointer <userp>` or :ref:`DMABUF <dmabuf>` based I/O.
 36Memory mapped buffers are located in device memory and must be allocated
 37with this ioctl before they can be mapped into the application's address
 38space. User buffers are allocated by applications themselves, and this
 39ioctl is merely used to switch the driver into user pointer I/O mode and
 40to setup some internal structures. Similarly, DMABUF buffers are
 41allocated by applications through a device driver, and this ioctl only
 42configures the driver into DMABUF I/O mode without performing any direct
 43allocation.
 44
 45To allocate device buffers applications initialize all fields of the
 46struct :c:type:`v4l2_requestbuffers` structure. They set the ``type``
 47field to the respective stream or buffer type, the ``count`` field to
 48the desired number of buffers, ``memory`` must be set to the requested
 49I/O method and the ``reserved`` array must be zeroed. When the ioctl is
 50called with a pointer to this structure the driver will attempt to
 51allocate the requested number of buffers and it stores the actual number
 52allocated in the ``count`` field. It can be smaller than the number
 53requested, even zero, when the driver runs out of free memory. A larger
 54number is also possible when the driver requires more buffers to
 55function correctly. For example video output requires at least two
 56buffers, one displayed and one filled by the application.
 57
 58When the I/O method is not supported the ioctl returns an ``EINVAL`` error
 59code.
 60
 61Applications can call :ref:`VIDIOC_REQBUFS` again to change the number of
 62buffers. Note that if any buffers are still mapped or exported via DMABUF,
 63then :ref:`VIDIOC_REQBUFS` can only succeed if the
 64``V4L2_BUF_CAP_SUPPORTS_ORPHANED_BUFS`` capability is set. Otherwise
 65:ref:`VIDIOC_REQBUFS` will return the ``EBUSY`` error code.
 66If ``V4L2_BUF_CAP_SUPPORTS_ORPHANED_BUFS`` is set, then these buffers are
 67orphaned and will be freed when they are unmapped or when the exported DMABUF
 68fds are closed. A ``count`` value of zero frees or orphans all buffers, after
 69aborting or finishing any DMA in progress, an implicit
 70:ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>`.
 71
 72.. c:type:: v4l2_requestbuffers
 73
 74.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.5cm}|
 75
 76.. cssclass:: longtable
 77
 78.. flat-table:: struct v4l2_requestbuffers
 79    :header-rows:  0
 80    :stub-columns: 0
 81    :widths:       1 1 2
 82
 83    * - __u32
 84      - ``count``
 85      - The number of buffers requested or granted.
 86    * - __u32
 87      - ``type``
 88      - Type of the stream or buffers, this is the same as the struct
 89	:c:type:`v4l2_format` ``type`` field. See
 90	:c:type:`v4l2_buf_type` for valid values.
 91    * - __u32
 92      - ``memory``
 93      - Applications set this field to ``V4L2_MEMORY_MMAP``,
 94	``V4L2_MEMORY_DMABUF`` or ``V4L2_MEMORY_USERPTR``. See
 95	:c:type:`v4l2_memory`.
 96    * - __u32
 97      - ``capabilities``
 98      - Set by the driver. If 0, then the driver doesn't support
 99        capabilities. In that case all you know is that the driver is
100	guaranteed to support ``V4L2_MEMORY_MMAP`` and *might* support
101	other :c:type:`v4l2_memory` types. It will not support any other
102	capabilities.
103
104	If you want to query the capabilities with a minimum of side-effects,
105	then this can be called with ``count`` set to 0, ``memory`` set to
106	``V4L2_MEMORY_MMAP`` and ``type`` set to the buffer type. This will
107	free any previously allocated buffers, so this is typically something
108	that will be done at the start of the application.
109    * - __u8
110      - ``flags``
111      - Specifies additional buffer management attributes.
112	See :ref:`memory-flags`.
113    * - __u8
114      - ``reserved``\ [3]
115      - Reserved for future extensions.
116
117.. _v4l2-buf-capabilities:
118.. _V4L2-BUF-CAP-SUPPORTS-MMAP:
119.. _V4L2-BUF-CAP-SUPPORTS-USERPTR:
120.. _V4L2-BUF-CAP-SUPPORTS-DMABUF:
121.. _V4L2-BUF-CAP-SUPPORTS-REQUESTS:
122.. _V4L2-BUF-CAP-SUPPORTS-ORPHANED-BUFS:
123.. _V4L2-BUF-CAP-SUPPORTS-M2M-HOLD-CAPTURE-BUF:
124.. _V4L2-BUF-CAP-SUPPORTS-MMAP-CACHE-HINTS:
125.. _V4L2-BUF-CAP-SUPPORTS-MAX-NUM-BUFFERS:
126.. _V4L2-BUF-CAP-SUPPORTS-REMOVE-BUFS:
127
128.. flat-table:: V4L2 Buffer Capabilities Flags
129    :header-rows:  0
130    :stub-columns: 0
131    :widths:       3 1 4
132
133    * - ``V4L2_BUF_CAP_SUPPORTS_MMAP``
134      - 0x00000001
135      - This buffer type supports the ``V4L2_MEMORY_MMAP`` streaming mode.
136    * - ``V4L2_BUF_CAP_SUPPORTS_USERPTR``
137      - 0x00000002
138      - This buffer type supports the ``V4L2_MEMORY_USERPTR`` streaming mode.
139    * - ``V4L2_BUF_CAP_SUPPORTS_DMABUF``
140      - 0x00000004
141      - This buffer type supports the ``V4L2_MEMORY_DMABUF`` streaming mode.
142    * - ``V4L2_BUF_CAP_SUPPORTS_REQUESTS``
143      - 0x00000008
144      - This buffer type supports :ref:`requests <media-request-api>`.
145    * - ``V4L2_BUF_CAP_SUPPORTS_ORPHANED_BUFS``
146      - 0x00000010
147      - The kernel allows calling :ref:`VIDIOC_REQBUFS` while buffers are still
148        mapped or exported via DMABUF. These orphaned buffers will be freed
149        when they are unmapped or when the exported DMABUF fds are closed.
150    * - ``V4L2_BUF_CAP_SUPPORTS_M2M_HOLD_CAPTURE_BUF``
151      - 0x00000020
152      - Only valid for stateless decoders. If set, then userspace can set the
153        ``V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF`` flag to hold off on returning the
154	capture buffer until the OUTPUT timestamp changes.
155    * - ``V4L2_BUF_CAP_SUPPORTS_MMAP_CACHE_HINTS``
156      - 0x00000040
157      - This capability is set by the driver to indicate that the queue supports
158        cache and memory management hints. However, it's only valid when the
159        queue is used for :ref:`memory mapping <mmap>` streaming I/O. See
160        :ref:`V4L2_BUF_FLAG_NO_CACHE_INVALIDATE <V4L2-BUF-FLAG-NO-CACHE-INVALIDATE>`,
161        :ref:`V4L2_BUF_FLAG_NO_CACHE_CLEAN <V4L2-BUF-FLAG-NO-CACHE-CLEAN>` and
162        :ref:`V4L2_MEMORY_FLAG_NON_COHERENT <V4L2-MEMORY-FLAG-NON-COHERENT>`.
163    * - ``V4L2_BUF_CAP_SUPPORTS_MAX_NUM_BUFFERS``
164      - 0x00000080
165      - If set, then the ``max_num_buffers`` field in ``struct v4l2_create_buffers``
166        is valid. If not set, then the maximum is ``VIDEO_MAX_FRAME`` buffers.
167    * - ``V4L2_BUF_CAP_SUPPORTS_REMOVE_BUFS``
168      - 0x00000100
169      - If set, then ``VIDIOC_REMOVE_BUFS`` is supported.
170
171.. _memory-flags:
172.. _V4L2-MEMORY-FLAG-NON-COHERENT:
173
174.. flat-table:: Memory Consistency Flags
175    :header-rows:  0
176    :stub-columns: 0
177    :widths:       3 1 4
178
179    * - ``V4L2_MEMORY_FLAG_NON_COHERENT``
180      - 0x00000001
181      - A buffer is allocated either in coherent (it will be automatically
182	coherent between the CPU and the bus) or non-coherent memory. The
183	latter can provide performance gains, for instance the CPU cache
184	sync/flush operations can be avoided if the buffer is accessed by the
185	corresponding device only and the CPU does not read/write to/from that
186	buffer. However, this requires extra care from the driver -- it must
187	guarantee memory consistency by issuing a cache flush/sync when
188	consistency is needed. If this flag is set V4L2 will attempt to
189	allocate the buffer in non-coherent memory. The flag takes effect
190	only if the buffer is used for :ref:`memory mapping <mmap>` I/O and the
191	queue reports the :ref:`V4L2_BUF_CAP_SUPPORTS_MMAP_CACHE_HINTS
192	<V4L2-BUF-CAP-SUPPORTS-MMAP-CACHE-HINTS>` capability.
193
194.. raw:: latex
195
196   \normalsize
197
198Return Value
199============
200
201On success 0 is returned, on error -1 and the ``errno`` variable is set
202appropriately. The generic error codes are described at the
203:ref:`Generic Error Codes <gen-errors>` chapter.
204
205EINVAL
206    The buffer type (``type`` field) or the requested I/O method
207    (``memory``) is not supported.