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{8.5cm}| 75 76.. cssclass:: longtable 77 78.. flat-table:: struct v4l2_requestbuffers 79 :header-rows: 0 80 :stub-columns: 0 81 :widths: 1 1 2 82 83 * - __u32 84 - ``count`` 85 - The number of buffers requested or granted. 86 * - __u32 87 - ``type`` 88 - Type of the stream or buffers, this is the same as the struct 89 :c:type:`v4l2_format` ``type`` field. See 90 :c:type:`v4l2_buf_type` for valid values. 91 * - __u32 92 - ``memory`` 93 - Applications set this field to ``V4L2_MEMORY_MMAP``, 94 ``V4L2_MEMORY_DMABUF`` or ``V4L2_MEMORY_USERPTR``. See 95 :c:type:`v4l2_memory`. 96 * - __u32 97 - ``capabilities`` 98 - Set by the driver. If 0, then the driver doesn't support 99 capabilities. In that case all you know is that the driver is 100 guaranteed to support ``V4L2_MEMORY_MMAP`` and *might* support 101 other :c:type:`v4l2_memory` types. It will not support any other 102 capabilities. 103 104 If you want to query the capabilities with a minimum of side-effects, 105 then this can be called with ``count`` set to 0, ``memory`` set to 106 ``V4L2_MEMORY_MMAP`` and ``type`` set to the buffer type. This will 107 free any previously allocated buffers, so this is typically something 108 that will be done at the start of the application. 109 * - __u8 110 - ``flags`` 111 - Specifies additional buffer management attributes. 112 See :ref:`memory-flags`. 113 * - __u8 114 - ``reserved``\ [3] 115 - Reserved for future extensions. 116 117.. _v4l2-buf-capabilities: 118.. _V4L2-BUF-CAP-SUPPORTS-MMAP: 119.. _V4L2-BUF-CAP-SUPPORTS-USERPTR: 120.. _V4L2-BUF-CAP-SUPPORTS-DMABUF: 121.. _V4L2-BUF-CAP-SUPPORTS-REQUESTS: 122.. _V4L2-BUF-CAP-SUPPORTS-ORPHANED-BUFS: 123.. _V4L2-BUF-CAP-SUPPORTS-M2M-HOLD-CAPTURE-BUF: 124.. _V4L2-BUF-CAP-SUPPORTS-MMAP-CACHE-HINTS: 125.. _V4L2-BUF-CAP-SUPPORTS-MAX-NUM-BUFFERS: 126.. _V4L2-BUF-CAP-SUPPORTS-REMOVE-BUFS: 127 128.. flat-table:: V4L2 Buffer Capabilities Flags 129 :header-rows: 0 130 :stub-columns: 0 131 :widths: 3 1 4 132 133 * - ``V4L2_BUF_CAP_SUPPORTS_MMAP`` 134 - 0x00000001 135 - This buffer type supports the ``V4L2_MEMORY_MMAP`` streaming mode. 136 * - ``V4L2_BUF_CAP_SUPPORTS_USERPTR`` 137 - 0x00000002 138 - This buffer type supports the ``V4L2_MEMORY_USERPTR`` streaming mode. 139 * - ``V4L2_BUF_CAP_SUPPORTS_DMABUF`` 140 - 0x00000004 141 - This buffer type supports the ``V4L2_MEMORY_DMABUF`` streaming mode. 142 * - ``V4L2_BUF_CAP_SUPPORTS_REQUESTS`` 143 - 0x00000008 144 - This buffer type supports :ref:`requests <media-request-api>`. 145 * - ``V4L2_BUF_CAP_SUPPORTS_ORPHANED_BUFS`` 146 - 0x00000010 147 - The kernel allows calling :ref:`VIDIOC_REQBUFS` while buffers are still 148 mapped or exported via DMABUF. These orphaned buffers will be freed 149 when they are unmapped or when the exported DMABUF fds are closed. 150 * - ``V4L2_BUF_CAP_SUPPORTS_M2M_HOLD_CAPTURE_BUF`` 151 - 0x00000020 152 - Only valid for stateless decoders. If set, then userspace can set the 153 ``V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF`` flag to hold off on returning the 154 capture buffer until the OUTPUT timestamp changes. 155 * - ``V4L2_BUF_CAP_SUPPORTS_MMAP_CACHE_HINTS`` 156 - 0x00000040 157 - This capability is set by the driver to indicate that the queue supports 158 cache and memory management hints. However, it's only valid when the 159 queue is used for :ref:`memory mapping <mmap>` streaming I/O. See 160 :ref:`V4L2_BUF_FLAG_NO_CACHE_INVALIDATE <V4L2-BUF-FLAG-NO-CACHE-INVALIDATE>`, 161 :ref:`V4L2_BUF_FLAG_NO_CACHE_CLEAN <V4L2-BUF-FLAG-NO-CACHE-CLEAN>` and 162 :ref:`V4L2_MEMORY_FLAG_NON_COHERENT <V4L2-MEMORY-FLAG-NON-COHERENT>`. 163 * - ``V4L2_BUF_CAP_SUPPORTS_MAX_NUM_BUFFERS`` 164 - 0x00000080 165 - If set, then the ``max_num_buffers`` field in ``struct v4l2_create_buffers`` 166 is valid. If not set, then the maximum is ``VIDEO_MAX_FRAME`` buffers. 167 * - ``V4L2_BUF_CAP_SUPPORTS_REMOVE_BUFS`` 168 - 0x00000100 169 - If set, then ``VIDIOC_REMOVE_BUFS`` is supported. 170 171.. _memory-flags: 172.. _V4L2-MEMORY-FLAG-NON-COHERENT: 173 174.. flat-table:: Memory Consistency Flags 175 :header-rows: 0 176 :stub-columns: 0 177 :widths: 3 1 4 178 179 * - ``V4L2_MEMORY_FLAG_NON_COHERENT`` 180 - 0x00000001 181 - A buffer is allocated either in coherent (it will be automatically 182 coherent between the CPU and the bus) or non-coherent memory. The 183 latter can provide performance gains, for instance the CPU cache 184 sync/flush operations can be avoided if the buffer is accessed by the 185 corresponding device only and the CPU does not read/write to/from that 186 buffer. However, this requires extra care from the driver -- it must 187 guarantee memory consistency by issuing a cache flush/sync when 188 consistency is needed. If this flag is set V4L2 will attempt to 189 allocate the buffer in non-coherent memory. The flag takes effect 190 only if the buffer is used for :ref:`memory mapping <mmap>` I/O and the 191 queue reports the :ref:`V4L2_BUF_CAP_SUPPORTS_MMAP_CACHE_HINTS 192 <V4L2-BUF-CAP-SUPPORTS-MMAP-CACHE-HINTS>` capability. 193 194.. raw:: latex 195 196 \normalsize 197 198Return Value 199============ 200 201On success 0 is returned, on error -1 and the ``errno`` variable is set 202appropriately. The generic error codes are described at the 203:ref:`Generic Error Codes <gen-errors>` chapter. 204 205EINVAL 206 The buffer type (``type`` field) or the requested I/O method 207 (``memory``) is not supported. 208