linux/Documentation/DocBook/media/v4l/dev-codec.xml

  <title>Codec Interface</title>

  <para>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
<constant>V4L2_CAP_VIDEO_M2M</constant> or <constant>V4L2_CAP_VIDEO_M2M_MPLANE</constant>
capability set).
</para>

  <para>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 &VIDIOC-STREAMON; for both capture and output
to start the codec.</para>

  <para>Video compression codecs use the MPEG controls to setup their codec parameters
(note that the MPEG controls actually support many more codecs than just MPEG).
See <xref linkend="mpeg-controls"></xref>.</para>

  <para>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).</para>