Documentation/linux_tv/media/v4l/vidioc-reqbufs.rst

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