1 .. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
2 .. c:namespace:: V4L
3
4 .. _VIDIOC_REQBUFS:
5
6 ********************
7 ioctl VIDIOC_REQBUFS
8 ********************
9
10 Name
11 ====
12
13 VIDIOC_REQBUFS - Initiate Memory Mapping, User Pointer I/O or DMA buffer I/O
14
15 Synopsis
16 ========
17
18 .. c:macro:: VIDIOC_REQBUFS
19
20 ``int ioctl(int fd, VIDIOC_REQBUFS, struct v4l2_requestbuffers *argp)``
21
22 Arguments
23 =========
24
25 ``fd``
26 File descriptor returned by :c:func:`open()`.
27
28 ``argp``
29 Pointer to struct :c:type:`v4l2_requestbuffers`.
30
31 Description
32 ===========
33
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
43 allocation.
44
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.
57
58 When the I/O method is not supported the ioctl returns an ``EINVAL`` error
59 code.
60
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>`.
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
198 Return Value
199 ============
200
201 On success 0 is returned, on error -1 and the ``errno`` variable is set
202 appropriately. The generic error codes are described at the
203 :ref:`Generic Error Codes <gen-errors>` chapter.
204
205 EINVAL
206 The buffer type (``type`` field) or the requested I/O method
207 (``memory``) is not supported.
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