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{8.7cm}|
75
76.. flat-table:: struct v4l2_requestbuffers
77    :header-rows:  0
78    :stub-columns: 0
79    :widths:       1 1 2
80
81    * - __u32
82      - ``count``
83      - The number of buffers requested or granted.
84    * - __u32
85      - ``type``
86      - Type of the stream or buffers, this is the same as the struct
87	:c:type:`v4l2_format` ``type`` field. See
88	:c:type:`v4l2_buf_type` for valid values.
89    * - __u32
90      - ``memory``
91      - Applications set this field to ``V4L2_MEMORY_MMAP``,
92	``V4L2_MEMORY_DMABUF`` or ``V4L2_MEMORY_USERPTR``. See
93	:c:type:`v4l2_memory`.
94    * - __u32
95      - ``capabilities``
96      - Set by the driver. If 0, then the driver doesn't support
97        capabilities. In that case all you know is that the driver is
98	guaranteed to support ``V4L2_MEMORY_MMAP`` and *might* support
99	other :c:type:`v4l2_memory` types. It will not support any other
100	capabilities.
101
102	If you want to query the capabilities with a minimum of side-effects,
103	then this can be called with ``count`` set to 0, ``memory`` set to
104	``V4L2_MEMORY_MMAP`` and ``type`` set to the buffer type. This will
105	free any previously allocated buffers, so this is typically something
106	that will be done at the start of the application.
107    * - __u32
108      - ``reserved``\ [1]
109      - A place holder for future extensions. Drivers and applications
110	must set the array to zero.
111
112.. tabularcolumns:: |p{6.1cm}|p{2.2cm}|p{8.7cm}|
113
114.. _v4l2-buf-capabilities:
115.. _V4L2-BUF-CAP-SUPPORTS-MMAP:
116.. _V4L2-BUF-CAP-SUPPORTS-USERPTR:
117.. _V4L2-BUF-CAP-SUPPORTS-DMABUF:
118.. _V4L2-BUF-CAP-SUPPORTS-REQUESTS:
119.. _V4L2-BUF-CAP-SUPPORTS-ORPHANED-BUFS:
120.. _V4L2-BUF-CAP-SUPPORTS-M2M-HOLD-CAPTURE-BUF:
121.. _V4L2-BUF-CAP-SUPPORTS-MMAP-CACHE-HINTS:
122
123.. cssclass:: longtable
124
125.. flat-table:: V4L2 Buffer Capabilities Flags
126    :header-rows:  0
127    :stub-columns: 0
128    :widths:       3 1 4
129
130    * - ``V4L2_BUF_CAP_SUPPORTS_MMAP``
131      - 0x00000001
132      - This buffer type supports the ``V4L2_MEMORY_MMAP`` streaming mode.
133    * - ``V4L2_BUF_CAP_SUPPORTS_USERPTR``
134      - 0x00000002
135      - This buffer type supports the ``V4L2_MEMORY_USERPTR`` streaming mode.
136    * - ``V4L2_BUF_CAP_SUPPORTS_DMABUF``
137      - 0x00000004
138      - This buffer type supports the ``V4L2_MEMORY_DMABUF`` streaming mode.
139    * - ``V4L2_BUF_CAP_SUPPORTS_REQUESTS``
140      - 0x00000008
141      - This buffer type supports :ref:`requests <media-request-api>`.
142    * - ``V4L2_BUF_CAP_SUPPORTS_ORPHANED_BUFS``
143      - 0x00000010
144      - The kernel allows calling :ref:`VIDIOC_REQBUFS` while buffers are still
145        mapped or exported via DMABUF. These orphaned buffers will be freed
146        when they are unmapped or when the exported DMABUF fds are closed.
147    * - ``V4L2_BUF_CAP_SUPPORTS_M2M_HOLD_CAPTURE_BUF``
148      - 0x00000020
149      - Only valid for stateless decoders. If set, then userspace can set the
150        ``V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF`` flag to hold off on returning the
151	capture buffer until the OUTPUT timestamp changes.
152    * - ``V4L2_BUF_CAP_SUPPORTS_MMAP_CACHE_HINTS``
153      - 0x00000040
154      - This capability is set by the driver to indicate that the queue supports
155        cache and memory management hints. However, it's only valid when the
156        queue is used for :ref:`memory mapping <mmap>` streaming I/O. See
157        :ref:`V4L2_BUF_FLAG_NO_CACHE_INVALIDATE <V4L2-BUF-FLAG-NO-CACHE-INVALIDATE>` and
158        :ref:`V4L2_BUF_FLAG_NO_CACHE_CLEAN <V4L2-BUF-FLAG-NO-CACHE-CLEAN>`.
159
160Return Value
161============
162
163On success 0 is returned, on error -1 and the ``errno`` variable is set
164appropriately. The generic error codes are described at the
165:ref:`Generic Error Codes <gen-errors>` chapter.
166
167EINVAL
168    The buffer type (``type`` field) or the requested I/O method
169    (``memory``) is not supported.
170