106 lines
3.4 KiB
ReStructuredText
106 lines
3.4 KiB
ReStructuredText
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.. -*- coding: utf-8; mode: rst -*-
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.. _VIDIOC_REQBUFS:
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********************
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ioctl VIDIOC_REQBUFS
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********************
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Name
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====
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VIDIOC_REQBUFS - Initiate Memory Mapping, User Pointer I/O or DMA buffer I/O
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Synopsis
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========
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.. c:function:: int ioctl( int fd, VIDIOC_REQBUFS, struct v4l2_requestbuffers *argp )
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:name: VIDIOC_REQBUFS
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Arguments
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=========
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``fd``
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File descriptor returned by :ref:`open() <func-open>`.
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``argp``
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Pointer to struct :c:type:`v4l2_requestbuffers`.
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Description
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===========
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This ioctl is used to initiate :ref:`memory mapped <mmap>`,
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:ref:`user pointer <userp>` or :ref:`DMABUF <dmabuf>` based I/O.
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Memory mapped buffers are located in device memory and must be allocated
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with this ioctl before they can be mapped into the application's address
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space. User buffers are allocated by applications themselves, and this
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ioctl is merely used to switch the driver into user pointer I/O mode and
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to setup some internal structures. Similarly, DMABUF buffers are
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allocated by applications through a device driver, and this ioctl only
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configures the driver into DMABUF I/O mode without performing any direct
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allocation.
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To allocate device buffers applications initialize all fields of the
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struct :c:type:`v4l2_requestbuffers` structure. They set the ``type``
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field to the respective stream or buffer type, the ``count`` field to
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the desired number of buffers, ``memory`` must be set to the requested
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I/O method and the ``reserved`` array must be zeroed. When the ioctl is
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called with a pointer to this structure the driver will attempt to
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allocate the requested number of buffers and it stores the actual number
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allocated in the ``count`` field. It can be smaller than the number
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requested, even zero, when the driver runs out of free memory. A larger
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number is also possible when the driver requires more buffers to
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function correctly. For example video output requires at least two
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buffers, one displayed and one filled by the application.
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When the I/O method is not supported the ioctl returns an ``EINVAL`` error
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code.
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Applications can call :ref:`VIDIOC_REQBUFS` again to change the number of
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buffers, however this cannot succeed when any buffers are still mapped.
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A ``count`` value of zero frees all buffers, after aborting or finishing
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any DMA in progress, an implicit
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:ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>`.
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.. c:type:: v4l2_requestbuffers
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.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}|
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.. flat-table:: struct v4l2_requestbuffers
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:header-rows: 0
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:stub-columns: 0
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:widths: 1 1 2
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* - __u32
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- ``count``
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- The number of buffers requested or granted.
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* - __u32
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- ``type``
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- Type of the stream or buffers, this is the same as the struct
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:c:type:`v4l2_format` ``type`` field. See
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:c:type:`v4l2_buf_type` for valid values.
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* - __u32
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- ``memory``
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- Applications set this field to ``V4L2_MEMORY_MMAP``,
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``V4L2_MEMORY_DMABUF`` or ``V4L2_MEMORY_USERPTR``. See
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:c:type:`v4l2_memory`.
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* - __u32
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- ``reserved``\ [2]
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- A place holder for future extensions. Drivers and applications
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must set the array to zero.
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Return Value
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============
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On success 0 is returned, on error -1 and the ``errno`` variable is set
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appropriately. The generic error codes are described at the
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:ref:`Generic Error Codes <gen-errors>` chapter.
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EINVAL
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The buffer type (``type`` field) or the requested I/O method
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(``memory``) is not supported.
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