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diff --git a/Documentation/media/uapi/v4l/dev-codec.rst b/Documentation/media/uapi/v4l/dev-codec.rst new file mode 100644 index 000000000000..dfb20328e34d --- /dev/null +++ b/Documentation/media/uapi/v4l/dev-codec.rst @@ -0,0 +1,34 @@ +.. -*- coding: utf-8; mode: rst -*- + +.. _codec: + +*************** +Codec Interface +*************** + +A V4L2 codec can compress, decompress, transform, or otherwise convert +video data from one format into another format, in memory. Typically +such devices are memory-to-memory devices (i.e. devices with the +``V4L2_CAP_VIDEO_M2M`` or ``V4L2_CAP_VIDEO_M2M_MPLANE`` capability set). + +A memory-to-memory video node acts just like a normal video node, but it +supports both output (sending frames from memory to the codec hardware) +and capture (receiving the processed frames from the codec 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 codec. + +Video compression codecs use the MPEG controls to setup their codec +parameters + +.. note:: The MPEG controls actually support many more codecs than + just MPEG. See :ref:`mpeg-controls`. + +Memory-to-memory devices can often be used as a shared resource: you can +open the video node multiple times, each application setting up their +own codec properties that are local to the file handle, and each can use +it independently from the others. The driver will arbitrate access to +the codec 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). |