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From: Hans Verkuil <hverkuil <at> xs4all.nl>
Subject: RFCv2: Second draft of guidelines for submitting patches to linux-media
Newsgroups: gmane.linux.drivers.video-input-infrastructure
Date: Friday 14th December 2012 14:28:43 UTC (over 5 years ago)
Hi all,

As discussed in Barcelona I would write a text describing requirements for
drivers and what to expect when submitting patches to linux-media.

This is the second rough draft and nothing is fixed yet.

I have incorporated all comments I received after I posted the first
although I've rewritten some of the suggestions.

I have a few open questions:

1) Where to put it? One thing I would propose that we improve is to move
dvb and video4linux directories in Documentation/ to Documentation/media to
correctly reflect the drivers/media structure. If we do that, then we can
this document in Documentation/media/SubmittingMediaPatches.

Alternatively, this is something we can document in our wiki.

It was also suggested to add it to media_build.git, but I am opposed to
media_build.git is only used to backport our code to older kernels, and
is rarely done when working with embedded drivers. So chances are
will not be aware of media_build.git as they may never need it.

2) The patchwork section is very short at the moment. It should be extended
when patchwork gets support to recognize the various tags.

3) Anything else that should be discussed here?

Again, remember that this is a rough draft only, so be gentle with me :-)

My vacation starts tomorrow, so I won't be very active on the mailinglist
for the next three weeks, but don't hesitate to give feedback as I will
go through it when I prepare v3 of this document.



--------------------------- cut here -------------------------------

For general information on how to submit patches see:


In particular the section 'Submitting Your Work'.

This document goes into more detail regarding media specific requirements
submitting patches and what the patch flow looks like in this subsystem.

Note 1: there are always exceptions to the rule, so if you believe certain
requirements do not apply to your code, then let us know and we can discuss

Note 2: this list is not exhaustive and will be updated over time, so make
sure you always use the latest version of this document. The latest version
will always be available here: TBD.

Note 3: when submitting a patch use ./scripts/get_maintainer.pl to figure
out who is maintaining the sources you touched.

Submitting New Media Drivers

When submitting new media drivers for inclusion in drivers/staging/media
that is required is that the driver compiles with the latest kernel, that
entry is added to the MAINTAINERS file, and that a TODO file is added with
list of action items that need to be taken before the driver can be moved

It should be noticed, however, that it is expected that the driver will be
fixed to fulfill the requirements for upstream addition. If a driver at
staging lacks relevant patches fixing it for more than a few kernel cycles,
it can be dropped from staging. We will contact you before doing that
provided that the email address of the maintainer is still valid.

For inclusion as a non-staging driver the requirements are more strict:

General requirements:

- It must pass checkpatch.pl, but see the note regarding interpreting the
  output from checkpatch below.
- An entry for the driver is added to the MAINTAINERS file.
- The kernel internal APIs are used properly.
- Don't reinvent the wheel by adding new defines, math logic, etc. for
  there are already solutions in the kernel.
- Follow the CodingStyle guidelines, paying specific attention to the
  frequently made mistakes:
	- Errors should be reported as negative numbers using the kernel
	  error codes. See also the CodingStyle document, chapter 16.
	- Don't use typedefs. See also the CodingStyle document, chapter 5.

V4L2 specific requirements:

- Use struct v4l2_device for bridge drivers, use struct v4l2_subdev for
  sub-device drivers.
- Each i2c/spi device should be implemented as a separate sub-device
- Use the control framework for control handling.
- Use struct v4l2_fh if the driver supports events (implied by the use of
  controls) or priority handling.
- Use videobuf2 for buffer handling. Mike Krufky will look into extending
  to support DVB buffers. Note: using vb2 for VBI devices has not been
  yet, but it should work. Please contact the mailinglist in case of
  with that.
- Must pass the v4l2-compliance tests.
- Hybrid tuners should be shared with DVB.

DVB specific requirements:

- Use the DVB core for both internal and external APIs.
- Each I2C-based chip should have its own driver.
- Tuners and frontends should be mapped as different drivers.
- dvb_frontend_ops should specify the delivery system instead of
  specifying the frontend type via the dvb_frontend_ops info.type field.
- DVB frontends should not implement dummy function handlers; if the
  function is not implemented, the DVB core should handle it properly.
- Hybrid tuners should be shared with V4L.

How to deal with checkpatch.pl?

First of all, the requirement to comply to the kernel coding style is there
a reason. Sometimes people feel that it is a pointless exercise: after all,
code is code, right? Why would just changing some spacing improve it?

But the coding style is not there to help you (at least, not directly), it
there to help those who have to review and/or maintain your code as it
takes a
lot of time to review code or try to figure out how someone else's code
By at least ensuring that the coding style is consistent with other code we
concentrate on what humans to best: pattern matching. Ever read a book or
article that did not use the correct spelling, grammar and/or punctuation
rules? Did you notice how your brain 'stumbles' whenever it encounters such
mistakes? It makes the text harder to understand and slower to read. The
happens with code that does not comply to the conventions of the project
and it
is the reason why most large projects, both open source and proprietary,
have a
coding style.

However, when interpreting the checkpatch output it is good to remember
that it
is just an automated tool and there are cases where what checkpatch
does not actually result in the best readable code. This is particularly
for the line length warnings. Use common sense there: if breaking up the
can be done without reducing the code readability, then do so. Otherwise it
better to keep the line as is.

As an example: typically function calls and function declarations can be
up without reducing the readability, but splitting up literal strings just
keep within the 80 character limit often leads to hard-to-read code.

So the guideline here is to check such warnings, but use common sense
or not to fix them.

Please do run checkpatch before posting any code to the mailinglist. Code
clearly violates the kernel coding style will be rejected and you will be
to repost after fixing the style. We are not going to waste time trying to
review code that uses a non-standard coding style, our time is too limited

The only exception are staging drivers as the only rule there is that it

Timeline for code submissions

After a new kernel is released the merge window will be open for about two
weeks for the maintainers to send Linus the patches they already received
during the last development cycle, and that went into the linux-next tree
in time for the other maintainers and reviewers to double-check the entire
set of changes for the next Linux version. During that time Linus will
all those patches for the next kernel.

Once that merge window is closed only regression fixes and serious bug
will be accepted into the mainline kernel, everything else will have to
in the maintainer's git tree until the next merge window opens.

In addition, before anything can be merged (regardless of whether this is
during the merge window or not) the new code should have been in the
tree for about a week at minimum to ensure there are no conflicts with work
being done in other kernel subsystems.

Furthermore, before code can be added to linux-next it has to be reviewed
first.  This will take time as well. Adding everything up this means that
you want your code to be merged for the next kernel you should have it
to the linux-media mailinglist no later than rc5 of the current kernel, or
may be too late. In fact, the earlier the better since reviews will take
and if corrections need to be made you may have to do several review/submit

Remember that the core media developers have a job as well, and so won't
have the time to review immediately. A general rule of thumb is to post a
reminder if a full week has passed without receiving any feedback. There is
fair amount of traffic on the mailinglist and it wouldn't be the first time
that a patch was missed by reviewers.

One consequence of this is that as submitter you can get into the situation
that you post something, two weeks later you get a review, you post the
corrected version, you get more reviews 10 days later, etc. So it can be a
drawn-out process. This can be frustrating, but please stick with it. We
seen cases where people seem to give up, but that is not our intention. We
welcome new code, but since none of the core developers work full time on
we are constrained by the time we have available. Just be aware of this,
accordingly and don't give up.

The reason for all these measures is simply to ensure to the best of our
abilities that no regressions are added into the kernel, the code remains
a high quality, and still be able to release a new kernel every 7-9 weeks.

Contacting developers

The linux-media mailinglist is the central place to get into contact with
developers. However, there are also two irc channels #linuxtv (mostly DVB
related) and #v4l (mostly V4L related). Most developers are based in the US
in Europe, so take those timezones into account. If you ask something in
irc channel, please wait for your answer as it may take some time for a
developer to be able to find a timeslot to answer you.

Finally, you can often find developers during the three main Linux
relevant to us: the Linux Plumbers Conference, the Embedded Linux
and the Embedded Linux Conference Europe. Check the mailinglist as well: we
often have a Media Summit during one of these conferences.

Patch tags

When posting patches it is recommended to tag them to help us sort through
quickly and efficiently.

The tags are:

[RFC PATCH x/y]: use this for preliminary patches for which you want to get
some early feedback.

[REVIEW PATCH x/y]: use this for patches that you consider OK for merging,
that need to be reviewed.

Once your patches have been reviewed/acked you can post either a pull
("[GIT PULL]") or use the "[FINAL PATCH x/y]" tag if you don't have a
git tree.

If you post a new version of a patch series, then add 'v1', 'v2', etc. to
RFC or REVIEW word, e.g.: "[RFCv2 PATCH x/y]".

If your patch is for the current rc kernel (so it is a regression or
bug fix), then add " FOR v3.x" after the PATCH or PULL keyword. For
"[REVIEW PATCH FOR v3.7 x/y]", or "[GIT PULL FOR v3.7]".

You can use the option --subject-prefix="REVIEW PATCHv1" with the 'git
send-email' to specify the prefix.

Patches without the appropriate tags will be processed manually, which will
take more time and may actually cause them to be dropped altogether.


Within the media subsystem there are three levels of maintainership: Mauro
Carvalho Chehab is the maintainer of the whole subsystem and the
DVB/V4L/IR/Media Controller core code in particular, then there are a
number of
submaintainers for specific areas of the subsystem:

- Kamil Debski: codec (aka memory-to-memory) drivers
- Hans de Goede: non-UVC USB webcam drivers
- Mike Krufky: frontends/tuners/demodulators In addition he'll be the main
  reviewer for DVB core patches.
- Guennadi Liakhovetski: soc-camera drivers
- Laurent Pinchart: sensor subdev drivers.  In addition he'll be the main
  reviewer for Media Controller core patches.
- Hans Verkuil: V4L2 drivers and video A/D and D/A subdev drivers (aka
  receivers and transmitters). In addition he'll be the main reviewer for
  core patches.

Finally there are maintainers for specific drivers. This is documented in
MAINTAINERS file. Note: if a submaintainer also maintains specific drivers,
then they should also go through his own git tree. E.g. Laurent maintains
the UVC driver, but it would be silly if UVC driver patches would have to
through Hans' git tree just because he is the submaintainer for V4L2

BTW, just for the record: everyone is invited to review code posted to the
mailinglist, especially core patches. It can be a good way to learn how the
media drivers work.

When modifying existing code you need to get the Reviewed-by/Acked-by of
maintainer of that code. So CC that maintainer when posting patches. If
maintainer is unavailable then the submaintainer or even Mauro can accept
it as
well, but that should be the exception, not the rule.

Once patches are accepted they will flow through the git tree of the
submaintainer to the git tree of the maintainer (Mauro) who will do a final

There are a few exceptions: code for certain platforms goes through git
specific to that platform. The submaintainer will still review it and add a
acked-by or reviewed-by line, but it will not go through the
git tree.

The platform maintainers are:

- Prabhakar Lad for all DaVinci drivers (drivers/media/platform/davinci)
- Sylwester Nawrocki for all s5p/exynos drivers
  and drivers/media/platform/exynos*)

In case patches touch on areas that are the responsibility of multiple
submaintainers, then they will decide among one another who will merge the

How to submit patches for a stable kernel

The standard method is to add this tag:

        Cc: [email protected]

possibly with a comment saying to which versions it should be applied,

        Cc: [email protected]      # for v3.5 and up

If it is only noticed later that a patch should be added to stable, or if a
backport is needed, then the patch author should send the patch to
[email protected], c/c the linux-media mailinglist, preferably
pointing to
the upstream commit ID. The patch has to be merged upstream before it can
merged at stable.


Patchwork is an automated system that takes care of all posted patches. It
be found here: http://patchwork.linuxtv.org/project/linux-media/list/

If your patch does not appear in patchwork after a couple of minutes, then
check if you used the right patch tags and if your patch is formatted
(no HTML, no mangled lines). Unfortunately, patchwork currently doesn't
send you
any email when a patch successfully arrives there, so you will have to
this yourself.

Whenever you patch changes state you'll get an email informing you about
Note that you can change the mail settings in order to opt-out of these
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