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.. c:function:: int ioctl( int fd, VIDIOC_REQBUFS, struct v4l2_requestbuffers *argp ) 19 :name: VIDIOC_REQBUFS 20 21 22Arguments 23========= 24 25``fd`` 26 File descriptor returned by :ref:`open() <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, however this cannot succeed when any buffers are still mapped. 63A ``count`` value of zero frees all buffers, after aborting or finishing 64any DMA in progress, an implicit 65:ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>`. 66 67 68.. c:type:: v4l2_requestbuffers 69 70.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}| 71 72.. flat-table:: struct v4l2_requestbuffers 73 :header-rows: 0 74 :stub-columns: 0 75 :widths: 1 1 2 76 77 * - __u32 78 - ``count`` 79 - The number of buffers requested or granted. 80 * - __u32 81 - ``type`` 82 - Type of the stream or buffers, this is the same as the struct 83 :c:type:`v4l2_format` ``type`` field. See 84 :c:type:`v4l2_buf_type` for valid values. 85 * - __u32 86 - ``memory`` 87 - Applications set this field to ``V4L2_MEMORY_MMAP``, 88 ``V4L2_MEMORY_DMABUF`` or ``V4L2_MEMORY_USERPTR``. See 89 :c:type:`v4l2_memory`. 90 * - __u32 91 - ``reserved``\ [2] 92 - A place holder for future extensions. Drivers and applications 93 must set the array to zero. 94 95 96Return Value 97============ 98 99On success 0 is returned, on error -1 and the ``errno`` variable is set 100appropriately. The generic error codes are described at the 101:ref:`Generic Error Codes <gen-errors>` chapter. 102 103EINVAL 104 The buffer type (``type`` field) or the requested I/O method 105 (``memory``) is not supported. 106