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[media] DocBook: media: update codec section, drop obsolete 'suspended' state

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The Codec section in the V4L2 specification was marked as 'suspended', even
though codec support has been around for quite some time. Update this
section, explaining a bit about memory-to-memory devices and pointing to
the MPEG controls section.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
This commit is contained in:
Hans Verkuil 2013-05-07 08:06:11 -03:00 committed by Mauro Carvalho Chehab
parent ea457ad9db
commit 6e7df1cd37
1 changed files with 23 additions and 14 deletions
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37
Documentation/DocBook/media/v4l/dev-codec.xml
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@ -1,18 +1,27 @@
<title>Codec Interface</title>
<note>
<title>Suspended</title>
<para>This interface has been be suspended from the V4L2 API
implemented in Linux 2.6 until we have more experience with codec
device interfaces.</para>
</note>
<para>A V4L2 codec can compress, decompress, transform, or otherwise
convert video data from one format into another format, in memory.
Applications send data to be converted to the driver through a
&func-write; call, and receive the converted data through a
&func-read; call. For efficiency a driver may also support streaming
I/O.</para>
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>[to do]</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>