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54f38fcae5
Since 2017, there is an space reserved for userspace API,
created by changeset 1d596dee38 ("docs: Create a user-space API guide").
As the media subsystem was one of the first subsystems to use
Sphinx, until this patch, we were keeping things on a separate
place.
Let's just use the new location, as having all uAPI altogether
will likely make things easier for developers.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
50 lines
2.2 KiB
ReStructuredText
50 lines
2.2 KiB
ReStructuredText
.. Permission is granted to copy, distribute and/or modify this
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.. document under the terms of the GNU Free Documentation License,
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.. Version 1.1 or any later version published by the Free Software
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.. Foundation, with no Invariant Sections, no Front-Cover Texts
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.. and no Back-Cover Texts. A copy of the license is included at
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.. Documentation/userspace-api/media/fdl-appendix.rst.
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..
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.. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections
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.. _mem2mem:
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********************************
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Video Memory-To-Memory Interface
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********************************
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A V4L2 memory-to-memory device can compress, decompress, transform, or
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otherwise convert video data from one format into another format, in memory.
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Such memory-to-memory devices set the ``V4L2_CAP_VIDEO_M2M`` or
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``V4L2_CAP_VIDEO_M2M_MPLANE`` capability. Examples of memory-to-memory
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devices are codecs, scalers, deinterlacers or format converters (i.e.
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converting from YUV to RGB).
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A memory-to-memory video node acts just like a normal video node, but it
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supports both output (sending frames from memory to the hardware)
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and capture (receiving the processed frames from the hardware into
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memory) stream I/O. An application will have to setup the stream I/O for
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both sides and finally call :ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>`
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for both capture and output to start the hardware.
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Memory-to-memory devices function as a shared resource: you can
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open the video node multiple times, each application setting up their
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own properties that are local to the file handle, and each can use
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it independently from the others. The driver will arbitrate access to
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the hardware and reprogram it whenever another file handler gets access.
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This is different from the usual video node behavior where the video
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properties are global to the device (i.e. changing something through one
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file handle is visible through another file handle).
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One of the most common memory-to-memory device is the codec. Codecs
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are more complicated than most and require additional setup for
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their codec parameters. This is done through codec controls.
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See :ref:`mpeg-controls`. More details on how to use codec memory-to-memory
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devices are given in the following sections.
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.. toctree::
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:maxdepth: 1
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dev-decoder
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dev-stateless-decoder
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