Linux Kernel Multmedia Subsystem maintainer and Samsung OSG team member Mauro Chehab talks about how the multimedia subsystem is architected and contributed to in the Linux Kernel.

1. Linux Kernel and
Multimídia
Mauro Carvalho Chehab
Linux Kernel Maintainer of the Media Subsystem
Linux Kernel Expert
Samsung Open Source Group
Aug 17, 2013
Open Source Group – Silicon Valley
Not to be used for commercial purpose without getting permission
All information, opinions and ideas herein are exclusively the author's own opinion
© 2013 SAMSUNG Electronics Co.

2. Linux is Everywhere
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3. The Linux Kernel Devel Model
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Kernel is the core of the operational system that runs a Linux system
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Its development is dictated by the community, not by someone or some company
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If someone (and/or their company) wants a feature, submit a patch
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The community reviews the patch. If community is happy, it gets merged
The development community is huge and open
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–
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All discussions are via mailing lists (generally hosted at vger.kernel.org);
IRC (and physical meetings) help to get agreements on more polemic items
A new Linux Kernel version is released on every ~2 months
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–
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After a new release, 2 weeks are used to merge the features for the next one.
This period of time is called: “merge window”;
A variable period (typically 8 weeks) is used to review the code. After each
week, a new release candidate is released (so, about 7 rc's and a final version).
Contributions are open to everyone, and patches are publicly reviewed
Subsystem maintainers are responsible to collect the accepted patches, do a final
review and submit them to Linus Torvards (the creator of Linux).
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4. Development model of the Kernel
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5. Some devel stats on Kernel 3.10
Most active 3.10 developers
By changesets
By changed lines
H Hartley Sweeten
392
2.9% Joe Perches
34561
Jingoo Han
299
2.2% Hans Verkuil
18739
Hans Verkuil
293
2.2% Kent Overstreet
18690
Alex Elder
268
2.0% Larry Finger
17222
Al Viro
205
1.5% Greg Kroah-Hartman
16610
Felipe Balbi
202
1.5% Shawn Guo
12879
Sachin Kamat
192
1.4% Dave Chinner
12838
Laurent Pinchart
174
1.3% Paul Zimmerman
12637
Johan Hovold
159
1.2% H Hartley Sweeten
12518
Mauro Carvalho Chehab
158
1.2% Al Viro
11116
Wei Y
ongjun
139
1.0% Andrey Smirnov
11107
Arnd Bergmann
138
1.0% Mauro Carvalho Chehab
9726
Eduardo Valentin
138
1.0% Laurent Pinchart
9258
Axel Lin
112
0.8% Jussi Kivilinna
8960
Lee Jones
111
0.8% Lee Jones
8598
Lars-Peter Clausen
99
0.7% Sylwester Nawrocki
8305
Kuninori Morimoto
98
0.7% Artem Bityutskiy
8094
T
ejun Heo
97
0.7% Dave Airlie
7546
Mark Brown
97
0.7% Guenter Roeck
7510
Johannes Berg
96
0.7% Sanjay Lal
7428
Total of 1,374 developers and 13.500 changesets
In blue: media developers
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4.5%
2.4%
2.4%
2.2%
2.2%
1.7%
1.7%
1.6%
1.6%
1.4%
1.4%
1.3%
1.2%
1.2%
1.1%
1.1%
1.0%
1.0%
1.0%
1.0%
source: http://lwn.net/Articles/555867/
© 2013 SAMSUNG Electronics Co.

6. How to start contributing
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RTFM: there are several documents inside the Kernel source explaining the proccess:
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/Documentation/HOWTO
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/Documentation/CodingStyle
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Main view for new developers
Describes the coding style used by the Linux Kernel
/Documentation/SubmittingPathes e SubmittingDrivers
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General procedures for submittins patches/drivers
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It should be noticed that each subsystem may have additional rules
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–
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/MAINTAINERS:
Show the subsystem maintainers, driver maintainer and mailing lists that should
be used to send patches
There are two scripts that help to:
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Check the coding style (checkpatch.pl)
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Check to whom a patch should be sent (get_maintainer.pl)
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7. Kernel API's for media devices
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Video4Linux API (version 2)
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For video inputs/outputs and radio, video and analog TV device control
Extending it for Software Digital Radio is currently being discussed
DVB API (version 5)
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Remote Controller / Input evdev APIs
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For Remote Controllers and webcam/grabber buttons
The Advanced Linux Sound Architecture (ALSA)
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For digital TV inputs
For audio inputs, outputs, mixers (and sequencers)
Media Controller API
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To control complex devices with configurable pipelines
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Used only on embeeded devices and on UVC (USB Video Class)
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Subdev API
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To control each single logical element on complex devices
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8. Example of a new patch
Patch summary:
Subject: uvc: more buffers
Date: Fri, 09 Aug 2013 12:11:36 -0000
main author
From: Oliver Neukum <oneukum@suse.de>
To: laurent.pinchart@ideasonboard.com , linux-media@vger.kernel.org
Driver maintainer
This is necessary to let the new generation of cameras
from LiteOn used in Haswell ULT notebook operate. Otherwise
the images will be truncated.
Signed-off-by: Oliver Neukum <oneukum@suse.de>
what it does
Mailing List
of the subsystem
Patch description:
why it is needed and
how was done
diff --git a/drivers/media/usb/uvc/uvcvideo.h b/drivers/media/usb/uvc/uvcvideo.h
index 9e35982..9f1930b 100644
--- a/drivers/media/usb/uvc/uvcvideo.h
Patch custodity chain
+++ b/drivers/media/usb/uvc/uvcvideo.h
@@ -114,9 +114,9 @@
confirming that the pach
/* Number of isochronous URBs. */
is GPL plus acks, reviews,
#define UVC_URBS
5
Tests, etc.
/* Maximum number of packets per URB. */
-#define UVC_MAX_PACKETS
32
+#define UVC_MAX_PACKETS
128
/* Maximum number of video buffers. */
-#define UVC_MAX_VIDEO_BUFFERS 32
Unified diff of
+#define UVC_MAX_VIDEO_BUFFERS 128
the changes
/* Maximum status buffer size in bytes of interrupt URB. */
#define UVC_MAX_STATUS_SIZE
16
Source:https://patchwork.linuxtv.org/patch/19656/
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9. Steps for writing/submitting (1)
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1) Get a copy of the latest development tree:
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http://git.linuxtv.org/media_tree.git (for a media patch)
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2) Write and test the changes
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3) generate the patch
$ git commit -a
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write a proper summary, descriptions and add your SOB
$ git show > my_changes.patch
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3) Check if CodingStyle was not violated or if it doesn't have some common
mistakes:
$ ./script/checkpatch.pl my_changes.patch
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Fix patch until checkpatch.pl is happy and test it again
4) Send it to the maintainers
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Use get_maintainers.pl to know to whom address the email
$ ./script/get_maintainers.pl my_changes.patch
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10. Steps for writing/submitting (2)
$ ./scripts/get_maintainer.pl /tmp/uvc-more-buffers.patch
Laurent Pinchart <laurent.pinchart@ideasonboard.com>
(maintainer:USB VIDEO CLASS)
Mauro Carvalho Chehab <m.chehab@samsung.com> (maintainer:MEDIA
INPUT INFRA...)
linux-media@vger.kernel.org (open list:USB VIDEO CLASS)
linux-kernel@vger.kernel.org (open list)
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In the specific case of media drivers, don't c/c the maintainer. If everything is ok in
the patch, it will appear in a few at patchwork:
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https://patchwork.linuxtv.org/project/linux-media/list/
Also, in general, don't copy LKML (linux-kernel). That's the last resort when
there's no specific subsystem mailing list.
Stuff not maintained and without any subsystem maintainer should be submitted
to Andrew Morton <akpm@linux-foundation.org>.
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11. Now, what happens?
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Wait for people to review it. It may take some time, as Kernel developers are
generally busy with other stuff
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Everybody can review a patch, including you.
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If you find a problem, write an email describing what you noticed.
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If you tested the patch, reply to it with
Tested-by: you <your.email>
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If you reviewed the patch but didn't test, reply to it with
Reviewed-by: you <your.email>
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Driver/system maintainers can also reply with Acked-by if they're ok if
someone else would review and apply that patch.
A patch with reviews/tests will likely flow quicker than one that would
depend solely on the driver maintainer to test
The driver maintainer will get your patch, test or review and either apply or ask
someone else to apply it;
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The patch will be applied by a sub-maintainer or by a subsystem maintainer;
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The subsystem maintainer will send it to Linus on the next merge window.
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12. Some literature
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You should read:
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The pertinent docs at Kernel source tree, under Documentation/ directory
Some books for starting working on Kernel:
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Corbet, J; Rubini, A; Kroah-Hartman, Greg, Linux Device Drivers, 3rd edition,
O'Reilly, 2005
There's an online version: http://lwn.net/Kernel/LDD3/
Love, Robert, Linux Kernel Development, 3rd edition, Addison-Wesley, 2010
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Daniel P. Bovet, Marco Cesati, Understanding the Linux Kernel, 3rd edition,
O'Reilly, 2005
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Venkateswaran, Sreekrishnan, Essential Linux Device Drivers, Prentice Hall,
2008
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Web useful resources:
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http://kernelnewbies.org/
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http://lwn.net
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V4L2 articles:
● http://lwn.net/Articles/203924/
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13. References on media drivers
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How to get and test the latest drivers:
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See http://git.linuxtv.org/media_build.git
Userspace tools:
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Remote controller/V4L/DVD: http://git.linuxtv.org/v4l-utils.git
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Wiki pages: http://linuxtv.org/wiki/
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Mailing list: linux-media@vger.kernel.org
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IRC channel: irc.freenode.net
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channels #v4l and #linuxtv
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Can be assessed via http://webchat.freenode.net/
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14. Thank you.
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