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Embedded Linux Experts

.. _development_followthrough:


At this point, you have followed the guidelines given so far and, with the
addition of your own engineering skills, have posted a perfect series of
patches.  One of the biggest mistakes that even experienced kernel
developers can make is to conclude that their work is now done.  In truth,
posting patches indicates a transition into the next stage of the process,
with, possibly, quite a bit of work yet to be done.

It is a rare patch which is so good at its first posting that there is no
room for improvement.  The kernel development process recognizes this fact,
and, as a result, is heavily oriented toward the improvement of posted
code.  You, as the author of that code, will be expected to work with the
kernel community to ensure that your code is up to the kernel's quality
standards.  A failure to participate in this process is quite likely to
prevent the inclusion of your patches into the mainline.

Working with reviewers

A patch of any significance will result in a number of comments from other
developers as they review the code.  Working with reviewers can be, for
many developers, the most intimidating part of the kernel development
process.  Life can be made much easier, though, if you keep a few things in

 - If you have explained your patch well, reviewers will understand its
   value and why you went to the trouble of writing it.  But that value
   will not keep them from asking a fundamental question: what will it be
   like to maintain a kernel with this code in it five or ten years later?
   Many of the changes you may be asked to make - from coding style tweaks
   to substantial rewrites - come from the understanding that Linux will
   still be around and under development a decade from now.

 - Code review is hard work, and it is a relatively thankless occupation;
   people remember who wrote kernel code, but there is little lasting fame
   for those who reviewed it.  So reviewers can get grumpy, especially when
   they see the same mistakes being made over and over again.  If you get a
   review which seems angry, insulting, or outright offensive, resist the
   impulse to respond in kind.  Code review is about the code, not about
   the people, and code reviewers are not attacking you personally.

 - Similarly, code reviewers are not trying to promote their employers'
   agendas at the expense of your own.  Kernel developers often expect to
   be working on the kernel years from now, but they understand that their
   employer could change.  They truly are, almost without exception,
   working toward the creation of the best kernel they can; they are not
   trying to create discomfort for their employers' competitors.

What all of this comes down to is that, when reviewers send you comments,
you need to pay attention to the technical observations that they are
making.  Do not let their form of expression or your own pride keep that
from happening.  When you get review comments on a patch, take the time to
understand what the reviewer is trying to say.  If possible, fix the things
that the reviewer is asking you to fix.  And respond back to the reviewer:
thank them, and describe how you will answer their questions.

Note that you do not have to agree with every change suggested by
reviewers.  If you believe that the reviewer has misunderstood your code,
explain what is really going on.  If you have a technical objection to a
suggested change, describe it and justify your solution to the problem.  If
your explanations make sense, the reviewer will accept them.  Should your
explanation not prove persuasive, though, especially if others start to
agree with the reviewer, take some time to think things over again.  It can
be easy to become blinded by your own solution to a problem to the point
that you don't realize that something is fundamentally wrong or, perhaps,
you're not even solving the right problem.

Andrew Morton has suggested that every review comment which does not result
in a code change should result in an additional code comment instead; that
can help future reviewers avoid the questions which came up the first time

One fatal mistake is to ignore review comments in the hope that they will
go away.  They will not go away.  If you repost code without having
responded to the comments you got the time before, you're likely to find
that your patches go nowhere.

Speaking of reposting code: please bear in mind that reviewers are not
going to remember all the details of the code you posted the last time
around.  So it is always a good idea to remind reviewers of previously
raised issues and how you dealt with them; the patch changelog is a good
place for this kind of information.  Reviewers should not have to search
through list archives to familiarize themselves with what was said last
time; if you help them get a running start, they will be in a better mood
when they revisit your code.

What if you've tried to do everything right and things still aren't going
anywhere?  Most technical disagreements can be resolved through discussion,
but there are times when somebody simply has to make a decision.  If you
honestly believe that this decision is going against you wrongly, you can
always try appealing to a higher power.  As of this writing, that higher
power tends to be Andrew Morton.  Andrew has a great deal of respect in the
kernel development community; he can often unjam a situation which seems to
be hopelessly blocked.  Appealing to Andrew should not be done lightly,
though, and not before all other alternatives have been explored.  And bear
in mind, of course, that he may not agree with you either.

What happens next

If a patch is considered to be a good thing to add to the kernel, and once
most of the review issues have been resolved, the next step is usually
entry into a subsystem maintainer's tree.  How that works varies from one
subsystem to the next; each maintainer has his or her own way of doing
things.  In particular, there may be more than one tree - one, perhaps,
dedicated to patches planned for the next merge window, and another for
longer-term work.

For patches applying to areas for which there is no obvious subsystem tree
(memory management patches, for example), the default tree often ends up
being -mm.  Patches which affect multiple subsystems can also end up going
through the -mm tree.

Inclusion into a subsystem tree can bring a higher level of visibility to a
patch.  Now other developers working with that tree will get the patch by
default.  Subsystem trees typically feed linux-next as well, making their
contents visible to the development community as a whole.  At this point,
there's a good chance that you will get more comments from a new set of
reviewers; these comments need to be answered as in the previous round.

What may also happen at this point, depending on the nature of your patch,
is that conflicts with work being done by others turn up.  In the worst
case, heavy patch conflicts can result in some work being put on the back
burner so that the remaining patches can be worked into shape and merged.
Other times, conflict resolution will involve working with the other
developers and, possibly, moving some patches between trees to ensure that
everything applies cleanly.  This work can be a pain, but count your
blessings: before the advent of the linux-next tree, these conflicts often
only turned up during the merge window and had to be addressed in a hurry.
Now they can be resolved at leisure, before the merge window opens.

Some day, if all goes well, you'll log on and see that your patch has been
merged into the mainline kernel.  Congratulations!  Once the celebration is
complete (and you have added yourself to the MAINTAINERS file), though, it
is worth remembering an important little fact: the job still is not done.
Merging into the mainline brings its own challenges.

To begin with, the visibility of your patch has increased yet again.  There
may be a new round of comments from developers who had not been aware of
the patch before.  It may be tempting to ignore them, since there is no
longer any question of your code being merged.  Resist that temptation,
though; you still need to be responsive to developers who have questions or

More importantly, though: inclusion into the mainline puts your code into
the hands of a much larger group of testers.  Even if you have contributed
a driver for hardware which is not yet available, you will be surprised by
how many people will build your code into their kernels.  And, of course,
where there are testers, there will be bug reports.

The worst sort of bug reports are regressions.  If your patch causes a
regression, you'll find an uncomfortable number of eyes upon you;
regressions need to be fixed as soon as possible.  If you are unwilling or
unable to fix the regression (and nobody else does it for you), your patch
will almost certainly be removed during the stabilization period.  Beyond
negating all of the work you have done to get your patch into the mainline,
having a patch pulled as the result of a failure to fix a regression could
well make it harder for you to get work merged in the future.

After any regressions have been dealt with, there may be other, ordinary
bugs to deal with.  The stabilization period is your best opportunity to
fix these bugs and ensure that your code's debut in a mainline kernel
release is as solid as possible.  So, please, answer bug reports, and fix
the problems if at all possible.  That's what the stabilization period is
for; you can start creating cool new patches once any problems with the old
ones have been taken care of.

And don't forget that there are other milestones which may also create bug
reports: the next mainline stable release, when prominent distributors pick
up a version of the kernel containing your patch, etc.  Continuing to
respond to these reports is a matter of basic pride in your work.  If that
is insufficient motivation, though, it's also worth considering that the
development community remembers developers who lose interest in their code
after it's merged.  The next time you post a patch, they will be evaluating
it with the assumption that you will not be around to maintain it

Other things that can happen

One day, you may open your mail client and see that somebody has mailed you
a patch to your code.  That is one of the advantages of having your code
out there in the open, after all.  If you agree with the patch, you can
either forward it on to the subsystem maintainer (be sure to include a
proper From: line so that the attribution is correct, and add a signoff of
your own), or send an Acked-by: response back and let the original poster
send it upward.

If you disagree with the patch, send a polite response explaining why.  If
possible, tell the author what changes need to be made to make the patch
acceptable to you.  There is a certain resistance to merging patches which
are opposed by the author and maintainer of the code, but it only goes so
far.  If you are seen as needlessly blocking good work, those patches will
eventually flow around you and get into the mainline anyway.  In the Linux
kernel, nobody has absolute veto power over any code.  Except maybe Linus.

On very rare occasion, you may see something completely different: another
developer posts a different solution to your problem.  At that point,
chances are that one of the two patches will not be merged, and "mine was
here first" is not considered to be a compelling technical argument.  If
somebody else's patch displaces yours and gets into the mainline, there is
really only one way to respond: be pleased that your problem got solved and
get on with your work.  Having one's work shoved aside in this manner can
be hurtful and discouraging, but the community will remember your reaction
long after they have forgotten whose patch actually got merged.