Documentation/linux_tv/media/v4l/vidioc-create-bufs.rst

   1 .. -*- coding: utf-8; mode: rst -*-
   2
   3 .. _VIDIOC_CREATE_BUFS:
   4
   5 ************************
   6 ioctl VIDIOC_CREATE_BUFS
   7 ************************
   8
   9 *man VIDIOC_CREATE_BUFS(2)*
  10
  11 Create buffers for Memory Mapped or User Pointer or DMA Buffer I/O
  12
  13
  14 Synopsis
  15 ========
  16
  17 .. c:function:: int ioctl( int fd, int request, struct v4l2_create_buffers *argp )
  18
  19 Arguments
  20 =========
  21
  22 ``fd``
  23     File descriptor returned by :ref:`open() <func-open>`.
  24
  25 ``request``
  26     VIDIOC_CREATE_BUFS
  27
  28 ``argp``
  29
  30
  31 Description
  32 ===========
  33
  34 This ioctl is used to create buffers for :ref:`memory mapped <mmap>`
  35 or :ref:`user pointer <userp>` or :ref:`DMA buffer <dmabuf>` I/O. It
  36 can be used as an alternative or in addition to the
  37 :ref:`VIDIOC_REQBUFS` ioctl, when a tighter control
  38 over buffers is required. This ioctl can be called multiple times to
  39 create buffers of different sizes.
  40
  41 To allocate the device buffers applications must initialize the relevant
  42 fields of the :c:type:`struct v4l2_create_buffers` structure. The
  43 ``count`` field must be set to the number of requested buffers, the
  44 ``memory`` field specifies the requested I/O method and the ``reserved``
  45 array must be zeroed.
  46
  47 The ``format`` field specifies the image format that the buffers must be
  48 able to handle. The application has to fill in this struct
  49 :ref:`v4l2_format <v4l2-format>`. Usually this will be done using the
  50 :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` or
  51 :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` ioctls to ensure that the
  52 requested format is supported by the driver. Based on the format's
  53 ``type`` field the requested buffer size (for single-planar) or plane
  54 sizes (for multi-planar formats) will be used for the allocated buffers.
  55 The driver may return an error if the size(s) are not supported by the
  56 hardware (usually because they are too small).
  57
  58 The buffers created by this ioctl will have as minimum size the size
  59 defined by the ``format.pix.sizeimage`` field (or the corresponding
  60 fields for other format types). Usually if the ``format.pix.sizeimage``
  61 field is less than the minimum required for the given format, then an
  62 error will be returned since drivers will typically not allow this. If
  63 it is larger, then the value will be used as-is. In other words, the
  64 driver may reject the requested size, but if it is accepted the driver
  65 will use it unchanged.
  66
  67 When the ioctl is called with a pointer to this structure the driver
  68 will attempt to allocate up to the requested number of buffers and store
  69 the actual number allocated and the starting index in the ``count`` and
  70 the ``index`` fields respectively. On return ``count`` can be smaller
  71 than the number requested.
  72
  73
  74 .. _v4l2-create-buffers:
  75
  76 .. flat-table:: struct v4l2_create_buffers
  77     :header-rows:  0
  78     :stub-columns: 0
  79     :widths:       1 1 2
  80
  81
  82     -  .. row 1
  83
  84        -  __u32
  85
  86        -  ``index``
  87
  88        -  The starting buffer index, returned by the driver.
  89
  90     -  .. row 2
  91
  92        -  __u32
  93
  94        -  ``count``
  95
  96        -  The number of buffers requested or granted. If count == 0, then
  97           :ref:`VIDIOC_CREATE_BUFS` will set ``index`` to the current number of
  98           created buffers, and it will check the validity of ``memory`` and
  99           ``format.type``. If those are invalid -1 is returned and errno is
 100           set to EINVAL error code, otherwise :ref:`VIDIOC_CREATE_BUFS` returns
 101           0. It will never set errno to EBUSY error code in this particular
 102           case.
 103
 104     -  .. row 3
 105
 106        -  __u32
 107
 108        -  ``memory``
 109
 110        -  Applications set this field to ``V4L2_MEMORY_MMAP``,
 111           ``V4L2_MEMORY_DMABUF`` or ``V4L2_MEMORY_USERPTR``. See
 112           :ref:`v4l2-memory`
 113
 114     -  .. row 4
 115
 116        -  struct :ref:`v4l2_format <v4l2-format>`
 117
 118        -  ``format``
 119
 120        -  Filled in by the application, preserved by the driver.
 121
 122     -  .. row 5
 123
 124        -  __u32
 125
 126        -  ``reserved``\ [8]
 127
 128        -  A place holder for future extensions. Drivers and applications
 129           must set the array to zero.
 130
 131
 132
 133 Return Value
 134 ============
 135
 136 On success 0 is returned, on error -1 and the ``errno`` variable is set
 137 appropriately. The generic error codes are described at the
 138 :ref:`Generic Error Codes <gen-errors>` chapter.
 139
 140 ENOMEM
 141     No memory to allocate buffers for :ref:`memory mapped <mmap>` I/O.
 142
 143 EINVAL
 144     The buffer type (``format.type`` field), requested I/O method
 145     (``memory``) or format (``format`` field) is not valid.
 146
 147
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