17611d3fb4
The V4L2_CAP_ASYNCIO capability was never implemented (and in fact it isn't clear what it was supposed to do in the first place). Drop it from the capabilities list. Keep it in videodev2.h with the other defines under ifndef __KERNEL__ for backwards compatibility. This will free up a capability bit for other future uses. And having an unused and undefined I/O method is just plain confusing. Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl> Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com> Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
50 lines
1.7 KiB
ReStructuredText
50 lines
1.7 KiB
ReStructuredText
.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
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.. c:namespace:: V4L
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.. _io:
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############
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Input/Output
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############
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The V4L2 API defines several different methods to read from or write to
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a device. All drivers exchanging data with applications must support at
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least one of them.
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The classic I/O method using the :c:func:`read()` and
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:c:func:`write()` function is automatically selected after opening a
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V4L2 device. When the driver does not support this method attempts to
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read or write will fail at any time.
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Other methods must be negotiated. To select the streaming I/O method
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with memory mapped or user buffers applications call the
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:ref:`VIDIOC_REQBUFS` ioctl.
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Video overlay can be considered another I/O method, although the
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application does not directly receive the image data. It is selected by
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initiating video overlay with the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`
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ioctl. For more information see :ref:`overlay`.
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Generally exactly one I/O method, including overlay, is associated with
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each file descriptor. The only exceptions are applications not
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exchanging data with a driver ("panel applications", see :ref:`open`)
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and drivers permitting simultaneous video capturing and overlay using
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the same file descriptor, for compatibility with V4L and earlier
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versions of V4L2.
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:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` and :ref:`VIDIOC_REQBUFS` would permit this to some
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degree, but for simplicity drivers need not support switching the I/O
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method (after first switching away from read/write) other than by
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closing and reopening the device.
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The following sections describe the various I/O methods in more detail.
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.. toctree::
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:maxdepth: 1
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rw
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mmap
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userp
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dmabuf
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buffer
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field-order
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