1 .. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
12 VIDIOC_REQBUFS - Initiate Memory Mapping, User Pointer I/O or DMA buffer I/O
18 .. c:function:: int ioctl( int fd, VIDIOC_REQBUFS, struct v4l2_requestbuffers *argp )
26 File descriptor returned by :ref:`open() <func-open>`.
29 Pointer to struct :c:type:`v4l2_requestbuffers`.
34 This ioctl is used to initiate :ref:`memory mapped <mmap>`,
35 :ref:`user pointer <userp>` or :ref:`DMABUF <dmabuf>` based I/O.
36 Memory mapped buffers are located in device memory and must be allocated
37 with this ioctl before they can be mapped into the application's address
38 space. User buffers are allocated by applications themselves, and this
39 ioctl is merely used to switch the driver into user pointer I/O mode and
40 to setup some internal structures. Similarly, DMABUF buffers are
41 allocated by applications through a device driver, and this ioctl only
42 configures the driver into DMABUF I/O mode without performing any direct
45 To allocate device buffers applications initialize all fields of the
46 struct :c:type:`v4l2_requestbuffers` structure. They set the ``type``
47 field to the respective stream or buffer type, the ``count`` field to
48 the desired number of buffers, ``memory`` must be set to the requested
49 I/O method and the ``reserved`` array must be zeroed. When the ioctl is
50 called with a pointer to this structure the driver will attempt to
51 allocate the requested number of buffers and it stores the actual number
52 allocated in the ``count`` field. It can be smaller than the number
53 requested, even zero, when the driver runs out of free memory. A larger
54 number is also possible when the driver requires more buffers to
55 function correctly. For example video output requires at least two
56 buffers, one displayed and one filled by the application.
58 When the I/O method is not supported the ioctl returns an ``EINVAL`` error
61 Applications can call :ref:`VIDIOC_REQBUFS` again to change the number of
62 buffers. Note that if any buffers are still mapped or exported via DMABUF,
63 then :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.
66 If ``V4L2_BUF_CAP_SUPPORTS_ORPHANED_BUFS`` is set, then these buffers are
67 orphaned and will be freed when they are unmapped or when the exported DMABUF
68 fds are closed. A ``count`` value of zero frees or orphans all buffers, after
69 aborting or finishing any DMA in progress, an implicit
70 :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>`.
73 .. c:type:: v4l2_requestbuffers
75 .. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}|
77 .. flat-table:: struct v4l2_requestbuffers
84 - The number of buffers requested or granted.
87 - Type of the stream or buffers, this is the same as the struct
88 :c:type:`v4l2_format` ``type`` field. See
89 :c:type:`v4l2_buf_type` for valid values.
92 - Applications set this field to ``V4L2_MEMORY_MMAP``,
93 ``V4L2_MEMORY_DMABUF`` or ``V4L2_MEMORY_USERPTR``. See
94 :c:type:`v4l2_memory`.
97 - Set by the driver. If 0, then the driver doesn't support
98 capabilities. In that case all you know is that the driver is
99 guaranteed to support ``V4L2_MEMORY_MMAP`` and *might* support
100 other :c:type:`v4l2_memory` types. It will not support any other
103 If you want to query the capabilities with a minimum of side-effects,
104 then this can be called with ``count`` set to 0, ``memory`` set to
105 ``V4L2_MEMORY_MMAP`` and ``type`` set to the buffer type. This will
106 free any previously allocated buffers, so this is typically something
107 that will be done at the start of the application.
110 - A place holder for future extensions. Drivers and applications
111 must set the array to zero.
113 .. tabularcolumns:: |p{6.1cm}|p{2.2cm}|p{8.7cm}|
115 .. _v4l2-buf-capabilities:
116 .. _V4L2-BUF-CAP-SUPPORTS-MMAP:
117 .. _V4L2-BUF-CAP-SUPPORTS-USERPTR:
118 .. _V4L2-BUF-CAP-SUPPORTS-DMABUF:
119 .. _V4L2-BUF-CAP-SUPPORTS-REQUESTS:
120 .. _V4L2-BUF-CAP-SUPPORTS-ORPHANED-BUFS:
121 .. _V4L2-BUF-CAP-SUPPORTS-M2M-HOLD-CAPTURE-BUF:
122 .. _V4L2-BUF-CAP-SUPPORTS-MMAP-CACHE-HINTS:
124 .. cssclass:: longtable
126 .. flat-table:: V4L2 Buffer Capabilities Flags
131 * - ``V4L2_BUF_CAP_SUPPORTS_MMAP``
133 - This buffer type supports the ``V4L2_MEMORY_MMAP`` streaming mode.
134 * - ``V4L2_BUF_CAP_SUPPORTS_USERPTR``
136 - This buffer type supports the ``V4L2_MEMORY_USERPTR`` streaming mode.
137 * - ``V4L2_BUF_CAP_SUPPORTS_DMABUF``
139 - This buffer type supports the ``V4L2_MEMORY_DMABUF`` streaming mode.
140 * - ``V4L2_BUF_CAP_SUPPORTS_REQUESTS``
142 - This buffer type supports :ref:`requests <media-request-api>`.
143 * - ``V4L2_BUF_CAP_SUPPORTS_ORPHANED_BUFS``
145 - The kernel allows calling :ref:`VIDIOC_REQBUFS` while buffers are still
146 mapped or exported via DMABUF. These orphaned buffers will be freed
147 when they are unmapped or when the exported DMABUF fds are closed.
148 * - ``V4L2_BUF_CAP_SUPPORTS_M2M_HOLD_CAPTURE_BUF``
150 - Only valid for stateless decoders. If set, then userspace can set the
151 ``V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF`` flag to hold off on returning the
152 capture buffer until the OUTPUT timestamp changes.
153 * - ``V4L2_BUF_CAP_SUPPORTS_MMAP_CACHE_HINTS``
155 - This capability is set by the driver to indicate that the queue supports
156 cache and memory management hints. However, it's only valid when the
157 queue is used for :ref:`memory mapping <mmap>` streaming I/O. See
158 :ref:`V4L2_BUF_FLAG_NO_CACHE_INVALIDATE <V4L2-BUF-FLAG-NO-CACHE-INVALIDATE>` and
159 :ref:`V4L2_BUF_FLAG_NO_CACHE_CLEAN <V4L2-BUF-FLAG-NO-CACHE-CLEAN>`.
165 On success 0 is returned, on error -1 and the ``errno`` variable is set
166 appropriately. The generic error codes are described at the
167 :ref:`Generic Error Codes <gen-errors>` chapter.
170 The buffer type (``type`` field) or the requested I/O method
171 (``memory``) is not supported.