path: root/Documentation
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authorHans Verkuil <>2013-05-07 08:06:11 -0300
committerMauro Carvalho Chehab <>2013-05-21 07:59:11 -0300
commit6e7df1cd373d688486b0b0b6626cb32f59ebdcfa (patch)
treefacc98c76ad71d4b076a34bcbd47d114bfb06010 /Documentation
parentea457ad9dbf32c8d22cc1cc39b4c1a2e06bf9b89 (diff)
[media] DocBook: media: update codec section, drop obsolete 'suspended' state
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 <> Acked-by: Kamil Debski <> Signed-off-by: Mauro Carvalho Chehab <>
Diffstat (limited to 'Documentation')
1 files changed, 22 insertions, 13 deletions
diff --git a/Documentation/DocBook/media/v4l/dev-codec.xml b/Documentation/DocBook/media/v4l/dev-codec.xml
index dca0ecd54dc6..ff44c16fc080 100644
--- a/Documentation/DocBook/media/v4l/dev-codec.xml
+++ b/Documentation/DocBook/media/v4l/dev-codec.xml
@@ -1,18 +1,27 @@
<title>Codec Interface</title>
- <note>
- <title>Suspended</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>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 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>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
+ <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>[to do]</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>