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| -rw-r--r-- | Documentation/userspace-api/media/v4l/vidioc-create-bufs.rst | 137 |
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diff --git a/Documentation/userspace-api/media/v4l/vidioc-create-bufs.rst b/Documentation/userspace-api/media/v4l/vidioc-create-bufs.rst new file mode 100644 index 000000000..a048a9f6b --- /dev/null +++ b/Documentation/userspace-api/media/v4l/vidioc-create-bufs.rst @@ -0,0 +1,137 @@ +.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later +.. c:namespace:: V4L + +.. _VIDIOC_CREATE_BUFS: + +************************ +ioctl VIDIOC_CREATE_BUFS +************************ + +Name +==== + +VIDIOC_CREATE_BUFS - Create buffers for Memory Mapped or User Pointer or DMA Buffer I/O + +Synopsis +======== + +.. c:macro:: VIDIOC_CREATE_BUFS + +``int ioctl(int fd, VIDIOC_CREATE_BUFS, struct v4l2_create_buffers *argp)`` + +Arguments +========= + +``fd`` + File descriptor returned by :c:func:`open()`. + +``argp`` + Pointer to struct :c:type:`v4l2_create_buffers`. + +Description +=========== + +This ioctl is used to create buffers for :ref:`memory mapped <mmap>` +or :ref:`user pointer <userp>` or :ref:`DMA buffer <dmabuf>` I/O. It +can be used as an alternative or in addition to the +:ref:`VIDIOC_REQBUFS` ioctl, when a tighter control +over buffers is required. This ioctl can be called multiple times to +create buffers of different sizes. + +To allocate the device buffers applications must initialize the relevant +fields of the struct :c:type:`v4l2_create_buffers` structure. The +``count`` field must be set to the number of requested buffers, the +``memory`` field specifies the requested I/O method and the ``reserved`` +array must be zeroed. + +The ``format`` field specifies the image format that the buffers must be +able to handle. The application has to fill in this struct +:c:type:`v4l2_format`. Usually this will be done using the +:ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` or +:ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` ioctls to ensure that the +requested format is supported by the driver. Based on the format's +``type`` field the requested buffer size (for single-planar) or plane +sizes (for multi-planar formats) will be used for the allocated buffers. +The driver may return an error if the size(s) are not supported by the +hardware (usually because they are too small). + +The buffers created by this ioctl will have as minimum size the size +defined by the ``format.pix.sizeimage`` field (or the corresponding +fields for other format types). Usually if the ``format.pix.sizeimage`` +field is less than the minimum required for the given format, then an +error will be returned since drivers will typically not allow this. If +it is larger, then the value will be used as-is. In other words, the +driver may reject the requested size, but if it is accepted the driver +will use it unchanged. + +When the ioctl is called with a pointer to this structure the driver +will attempt to allocate up to the requested number of buffers and store +the actual number allocated and the starting index in the ``count`` and +the ``index`` fields respectively. On return ``count`` can be smaller +than the number requested. + +.. c:type:: v4l2_create_buffers + +.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.5cm}| + +.. flat-table:: struct v4l2_create_buffers + :header-rows: 0 + :stub-columns: 0 + :widths: 1 1 2 + + * - __u32 + - ``index`` + - The starting buffer index, returned by the driver. + * - __u32 + - ``count`` + - The number of buffers requested or granted. If count == 0, then + :ref:`VIDIOC_CREATE_BUFS` will set ``index`` to the current number of + created buffers, and it will check the validity of ``memory`` and + ``format.type``. If those are invalid -1 is returned and errno is + set to ``EINVAL`` error code, otherwise :ref:`VIDIOC_CREATE_BUFS` returns + 0. It will never set errno to ``EBUSY`` error code in this particular + case. + * - __u32 + - ``memory`` + - Applications set this field to ``V4L2_MEMORY_MMAP``, + ``V4L2_MEMORY_DMABUF`` or ``V4L2_MEMORY_USERPTR``. See + :c:type:`v4l2_memory` + * - struct :c:type:`v4l2_format` + - ``format`` + - Filled in by the application, preserved by the driver. + * - __u32 + - ``capabilities`` + - Set by the driver. If 0, then the driver doesn't support + capabilities. In that case all you know is that the driver is + guaranteed to support ``V4L2_MEMORY_MMAP`` and *might* support + other :c:type:`v4l2_memory` types. It will not support any other + capabilities. See :ref:`here <v4l2-buf-capabilities>` for a list of the + capabilities. + + If you want to just query the capabilities without making any + other changes, then set ``count`` to 0, ``memory`` to + ``V4L2_MEMORY_MMAP`` and ``format.type`` to the buffer type. + + * - __u32 + - ``flags`` + - Specifies additional buffer management attributes. + See :ref:`memory-flags`. + + * - __u32 + - ``reserved``\ [6] + - A place holder for future extensions. Drivers and applications + must set the array to zero. + +Return Value +============ + +On success 0 is returned, on error -1 and the ``errno`` variable is set +appropriately. The generic error codes are described at the +:ref:`Generic Error Codes <gen-errors>` chapter. + +ENOMEM + No memory to allocate buffers for :ref:`memory mapped <mmap>` I/O. + +EINVAL + The buffer type (``format.type`` field), requested I/O method + (``memory``) or format (``format`` field) is not valid. |