tjh.dev
Linux kernel mirror (for testing)
git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
kernel
os
linux
1.. -*- coding: utf-8; mode: rst -*-
2
3.. _VIDIOC_REQBUFS:
4
5********************
6ioctl VIDIOC_REQBUFS
7********************
8
9Name
10====
11
12VIDIOC_REQBUFS - Initiate Memory Mapping, User Pointer I/O or DMA buffer I/O
13
14
15Synopsis
16========
17
18.. cpp:function:: int ioctl( int fd, int request, struct v4l2_requestbuffers *argp )
19
20
21Arguments
22=========
23
24``fd``
25 File descriptor returned by :ref:`open() <func-open>`.
26
27``request``
28 VIDIOC_REQBUFS
29
30``argp``
31
32
33Description
34===========
35
36This ioctl is used to initiate :ref:`memory mapped <mmap>`,
37:ref:`user pointer <userp>` or :ref:`DMABUF <dmabuf>` based I/O.
38Memory mapped buffers are located in device memory and must be allocated
39with this ioctl before they can be mapped into the application's address
40space. User buffers are allocated by applications themselves, and this
41ioctl is merely used to switch the driver into user pointer I/O mode and
42to setup some internal structures. Similarly, DMABUF buffers are
43allocated by applications through a device driver, and this ioctl only
44configures the driver into DMABUF I/O mode without performing any direct
45allocation.
46
47To allocate device buffers applications initialize all fields of the
48:ref:`struct v4l2_requestbuffers <v4l2-requestbuffers>` structure. They set the ``type``
49field to the respective stream or buffer type, the ``count`` field to
50the desired number of buffers, ``memory`` must be set to the requested
51I/O method and the ``reserved`` array must be zeroed. When the ioctl is
52called with a pointer to this structure the driver will attempt to
53allocate the requested number of buffers and it stores the actual number
54allocated in the ``count`` field. It can be smaller than the number
55requested, even zero, when the driver runs out of free memory. A larger
56number is also possible when the driver requires more buffers to
57function correctly. For example video output requires at least two
58buffers, one displayed and one filled by the application.
59
60When the I/O method is not supported the ioctl returns an ``EINVAL`` error
61code.
62
63Applications can call :ref:`VIDIOC_REQBUFS` again to change the number of
64buffers, however this cannot succeed when any buffers are still mapped.
65A ``count`` value of zero frees all buffers, after aborting or finishing
66any DMA in progress, an implicit
67:ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>`.
68
69
70.. _v4l2-requestbuffers:
71
72.. flat-table:: struct v4l2_requestbuffers
73 :header-rows: 0
74 :stub-columns: 0
75 :widths: 1 1 2
76
77
78 - .. row 1
79
80 - __u32
81
82 - ``count``
83
84 - The number of buffers requested or granted.
85
86 - .. row 2
87
88 - __u32
89
90 - ``type``
91
92 - Type of the stream or buffers, this is the same as the struct
93 :ref:`v4l2_format <v4l2-format>` ``type`` field. See
94 :ref:`v4l2-buf-type` for valid values.
95
96 - .. row 3
97
98 - __u32
99
100 - ``memory``
101
102 - Applications set this field to ``V4L2_MEMORY_MMAP``,
103 ``V4L2_MEMORY_DMABUF`` or ``V4L2_MEMORY_USERPTR``. See
104 :ref:`v4l2-memory`.
105
106 - .. row 4
107
108 - __u32
109
110 - ``reserved``\ [2]
111
112 - A place holder for future extensions. Drivers and applications
113 must set the array to zero.
114
115
116Return Value
117============
118
119On success 0 is returned, on error -1 and the ``errno`` variable is set
120appropriately. The generic error codes are described at the
121:ref:`Generic Error Codes <gen-errors>` chapter.
122
123EINVAL
124 The buffer type (``type`` field) or the requested I/O method
125 (``memory``) is not supported.