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diff --git a/Documentation/media/uapi/v4l/dev-mem2mem.rst b/Documentation/media/uapi/v4l/dev-mem2mem.rst new file mode 100644 index 000000000000..67a980818dc8 --- /dev/null +++ b/Documentation/media/uapi/v4l/dev-mem2mem.rst @@ -0,0 +1,42 @@ +.. Permission is granted to copy, distribute and/or modify this +.. document under the terms of the GNU Free Documentation License, +.. Version 1.1 or any later version published by the Free Software +.. Foundation, with no Invariant Sections, no Front-Cover Texts +.. and no Back-Cover Texts. A copy of the license is included at +.. Documentation/media/uapi/fdl-appendix.rst. +.. +.. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections + +.. _mem2mem: + +******************************** +Video Memory-To-Memory Interface +******************************** + +A V4L2 memory-to-memory device can compress, decompress, transform, or +otherwise convert video data from one format into another format, in memory. +Such memory-to-memory devices set the ``V4L2_CAP_VIDEO_M2M`` or +``V4L2_CAP_VIDEO_M2M_MPLANE`` capability. Examples of memory-to-memory +devices are codecs, scalers, deinterlacers or format converters (i.e. +converting from YUV to RGB). + +A memory-to-memory video node acts just like a normal video node, but it +supports both output (sending frames from memory to the hardware) +and capture (receiving the processed frames from the hardware into +memory) stream I/O. An application will have to setup the stream I/O for +both sides and finally call :ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>` +for both capture and output to start the hardware. + +Memory-to-memory devices function as a shared resource: you can +open the video node multiple times, each application setting up their +own properties that are local to the file handle, and each can use +it independently from the others. The driver will arbitrate access to +the hardware and reprogram it whenever another file handler gets access. +This is different from the usual video node behavior where the video +properties are global to the device (i.e. changing something through one +file handle is visible through another file handle). + +One of the most common memory-to-memory device is the codec. Codecs +are more complicated than most and require additional setup for +their codec parameters. This is done through codec controls. +See :ref:`mpeg-controls`. |