[REVIEW] Submitting Media Patches

* [REVIEW] Submitting Media Patches
@ 2014-10-22 14:12 Hans Verkuil
  2014-10-22 19:56 ` Mauro Carvalho Chehab
                   ` (2 more replies)
  0 siblings, 3 replies; 4+ messages in thread
From: Hans Verkuil @ 2014-10-22 14:12 UTC (permalink / raw)
  To: linux-media; +Cc: Pawel Osciak

During the mini-summit it was decided that we should put up a document on how
to post/handle patches etc. for the media drivers in the wiki. I had such a
doc already which I'm posting now for review. Once it's all OK Pawel offered
to put it up in the wiki.

-----------------------------------------------------------------------------

For general information on how to submit patches see:

http://linuxtv.org/wiki/index.php/Developer_Section

In particular the section 'Submitting Your Work'.

This document goes into more detail regarding media specific requirements when
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
it.

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 all
that is required is that the driver compiles with the latest kernel, that an
entry is added to the MAINTAINERS file, and that a TODO file is added with a
list of action items that need to be taken before the driver can be moved to
drivers/media.

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 which
  there are already solutions in the kernel.
- Follow the CodingStyle guidelines, paying specific attention to the follow
  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.
- When adding/enhancing the API the documentation must be updated as well,
  otherwise the patch will not be accepted.

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 driver.
- 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.
- Must pass the v4l2-compliance tests.
- Hybrid tuners should be shared with DVB.
- When introducing new APIs:
	- update v4l2-ctl
	- updates to v4l2-compliance (if applicable) are highly appreciated
	- update valgrind: support for v4l2 and media ioctls was added for
	  valgrind 3.10 synced to the 3.18 kernel. This should be kept up to
	  date. Patches should be posted as a bug report. See:
	  http://valgrind.org/support/bug_reports.html

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 for
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 is
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 works.
By at least ensuring that the coding style is consistent with other code we can
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 same
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 recommends
does not actually result in the best readable code. This is particularly true
for the line length warnings. Use common sense there: if breaking up the line
can be done without reducing the code readability, then do so. Otherwise it is
better to keep the line as is.

As an example: typically function calls and function declarations can be split
up without reducing the readability, but splitting up literal strings just to
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 whether
or not to fix them.

Please do run checkpatch before posting any code to the mailinglist. Code that
clearly violates the kernel coding style will be rejected and you will be asked
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 for
that.

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

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 merge
all those patches for the next kernel.

Once that merge window is closed only regression fixes and serious bug fixes
will be accepted into the mainline kernel, everything else will have to stay
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 linux-next
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 if
you want your code to be merged for the next kernel you should have it posted
to the linux-media mailinglist no later than rc5 of the current kernel, or it
may be too late. In fact, the earlier the better since reviews will take time,
and if corrections need to be made you may have to do several review/submit
cycles.

Remember that the core media developers have a job as well, and so won't always
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 a
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 have
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 this
we are constrained by the time we have available. Just be aware of this, plan
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 of
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 or
in Europe, so take those timezones into account. If you ask something in the
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 conferences
relevant to us: the Linux Plumbers Conference, the Embedded Linux Conference
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 them
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, but
that need to be reviewed.

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

If you post a new version of a patch series, then add 'v1', 'v2', etc. to the
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 serious
bug fix), then add " FOR v3.x" after the PATCH or PULL keyword. For example:
"[REVIEW PATCH FOR v3.7 x/y]", or "[GIT PULL FOR v3.7]".

Git pull requests containing urgent fixes for the current rc kernel that
should be upstreamed quickly should use "[GIT FIXES for v3.x]".

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.

Reviewed-by/Acked-by
====================

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
- 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 video
  receivers and transmitters). In addition he'll be the main reviewer for V4L2
  core patches.

Finally there are maintainers for specific drivers. This is documented in the
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 go
through Hans' git tree just because he is the submaintainer for V4L2 drivers.

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 the
maintainer of that code. So CC that maintainer when posting patches. If said
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
review.

There are a few exceptions: code for certain platforms goes through git trees
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 submaintainer's
git tree.

The platform maintainers are:

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

How to submit patches for a stable kernel
=========================================

The standard method is to add this tag:

        Cc: stable@vger.kernel.org

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

        Cc: stable@vger.kernel.org      # for v3.5 and up

Then just send the patch/pull request to linux-media.

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
stable@vger.kernel.org, c/c the linux-media mailinglist, preferably pointing to
the upstream commit ID. The patch has to be merged upstream before it can be
merged at stable.

Patchwork
=========

Patchwork is an automated system that takes care of all posted patches. It can
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 correctly
(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 check
this yourself.

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

If you want to remove a patch or pull request from patchwork, then you can
reply to the patch with a 'Nacked-by:' line.

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