5: POSTING PATCHES
Sooner or later, the time comes when your work is ready to be presented to
the community for review and, eventually, inclusion into the mainline
kernel. Unsurprisingly, the kernel development community has evolved a set
of conventions and procedures which are used in the posting of patches;
following them will make life much easier for everybody involved. This
document will attempt to cover these expectations in reasonable detail;
more information can also be found in the files SubmittingPatches,
SubmittingDrivers, and SubmitChecklist in the kernel documentation
directory.
5.1: WHEN TO POST
There is a constant temptation to avoid posting patches before they are
completely "ready." For simple patches, that is not a problem. If the
work being done is complex, though, there is a lot to be gained by getting
feedback from the community before the work is complete. So you should
consider posting in-progress work, or even making a git tree available so
that interested developers can catch up with your work at any time.
When posting code which is not yet considered ready for inclusion, it is a
good idea to say so in the posting itself. Also mention any major work
which remains to be done and any known problems. Fewer people will look at
patches which are known to be half-baked, but those who do will come in
with the idea that they can help you drive the work in the right direction.
5.2: BEFORE CREATING PATCHES
There are a number of things which should be done before you consider
sending patches to the development community. These include:
- Test the code to the extent that you can. Make use of the kernel's
debugging tools, ensure that the kernel will build with all reasonable
combinations of configuration options, use cross-compilers to build for
different architectures, etc.
- Make sure your code is compliant with the kernel coding style
guidelines.
- Does your change have performance implications? If so, you should run
benchmarks showing what the impact (or benefit) of your change is; a
summary of the results should be included with the patch.
- Be sure that you have the right to post the code. If this work was done
for an employer, the employer likely has a right to the work and must be
agreeable with its release under the GPL.
As a general rule, putting in some extra thought before posting code almost
always pays back the effort in short order.
5.3: PATCH PREPARATION
The preparation of patches for posting can be a surprising amount of work,
but, once again, attempting to save time here is not generally advisable
even in the short term.
Patches must be prepared against a specific version of the kernel. As a
general rule, a patch should be based on the current mainline as found in
Linus's git tree. When basing on mainline, start with a well-known release
point - a stable or -rc release - rather than branching off the mainline at
an arbitrary spot.
It may become necessary to make versions against -mm, linux-next, or a
subsystem tree, though, to facilitate wider testing and review. Depending
on the area of your patch and what is going on elsewhere, basing a patch
against these other trees can require a significant amount of work
resolving conflicts and dealing with API changes.
Only the most simple changes should be formatted as a single patch;
everything else should be made as a logical series of changes. Splitting
up patches is a bit of an art; some developers spend a long time figuring
out how to do it in the way that the community expects. There are a few
rules of thumb, however, which can help considerably:
- The patch series you post will almost certainly not be the series of
changes found in your working revision control system. Instead, the
changes you have made need to be considered in their final form, then
split apart in ways which make sense. The developers are interested in
discrete, self-contained changes, not the path you took to get to those
changes.
- Each logically independent change should be formatted as a separate
patch. These changes can be small ("add a field to this structure") or
large (adding a significant new driver, for example), but they should be
conceptually small and amenable to a one-line description. Each patch
should make a specific change which can be reviewed on its own and
verified to do what it says it does.
- As a way of restating the guideline above: do not mix different types of
changes in the same patch. If a single patch fixes a critical security
bug, rearranges a few structures, and reformats the code, there is a
good chance that it will be passed over and the important fix will be
lost.
- Each patch should yield a kernel which builds and runs properly; if your
patch series is interrupted in the middle, the result should still be a
working kernel. Partial application of a patch series is a common
scenario when the "git bisect" tool is used to find regressions; if the
result is a broken kernel, you will make life harder for developers and
users who are engaging in the noble work of tracking down problems.
- Do not overdo it, though. One developer once posted a set of edits
to a single file as 500 separate patches - an act which did not make him
the most popular person on the kernel mailing list. A single patch can
be reasonably large as long as it still contains a single *logical*
change.
- It can be tempting to add a whole new infrastructure with a series of
patches, but to leave that infrastructure unused until the final patch
in the series enables the whole thing. This temptation should be
avoided if possible; if that series adds regressions, bisection will
finger the last patch as the one which caused the problem, even though
the real bug is elsewhere. Whenever possible, a patch which adds new
code should make that code active immediately.
Working to create the perfect patch series can be a frustrating process
which takes quite a bit of time and thought after the "real work" has been
done. When done properly, though, it is time well spent.
5.4: PATCH FORMATTING AND CHANGELOGS
So now you have a perfect series of patches for posting, but the work is
not done quite yet. Each patch needs to be formatted into a message which
quickly and clearly communicates its purpose to the rest of the world. To
that end, each patch will be composed of the following:
- An optional "From" line naming the author of the patch. This line is
only necessary if you are passing on somebody else's patch via email,
but it never hurts to add it when in doubt.
- A one-line description of what the patch does. This message should be
enough for a reader who sees it with no other context to figure out the
scope of the patch; it is the line that will show up in the "short form"
changelogs. This message is usually formatted with the relevant
subsystem name first, followed by the purpose of the patch. For
example:
gpio: fix build on CONFIG_GPIO_SYSFS=n
- A blank line followed by a detailed description of the contents of the
patch. This description can be as long as is required; it should say
what the patch does and why it should be applied to the kernel.
- One or more tag lines, with, at a minimum, one Signed-off-by: line from
the author of the patch. Tags will be described in more detail below.
The items above, together, form the changelog for the patch. Writing good
changelogs is a crucial but often-neglected art; it's worth spending
another moment discussing this issue. When writing a changelog, you should
bear in mind that a number of different people will be reading your words.
These include subsystem maintainers and reviewers who need to decide
whether the patch should be included, distributors and other maintainers
trying to decide whether a patch should be backported to other kernels, bug
hunters wondering whether the patch is responsible for a problem they are
chasing, users who want to know how the kernel has changed, and more. A
good changelog conveys the needed information to all of these people in the
most direct and concise way possible.
To that end, the summary line should describe the effects of and motivation
for the change as well as possible given the one-line constraint. The
detailed description can then amplify on those topics and provide any
needed additional information. If the patch fixes a bug, cite the commit
which introduced the bug if possible (and please provide both the commit ID
and the title when citing commits). If a problem is associated with
specific log or compiler output, include that output to help others
searching for a solution to the same problem. If the change is meant to
support other changes coming in later patch, say so. If internal APIs are
changed, detail those changes and how other developers should respond. In
general, the more you can put yourself into the shoes of everybody who will
be reading your changelog, the better that changelog (and the kernel as a
whole) will be.
Needless to say, the changelog should be the text used when committing the
change to a revision control system. It will be followed by:
- The patch itself, in the unified ("-u") patch format. Using the "-p"
option to diff will associate function names with changes, making the
resulting patch easier for others to read.
You should avoid including changes to irrelevant files (those generated by
the build process, for example, or editor backup files) in the patch. The
file "dontdiff" in the Documentation directory can help in this regard;
pass it to diff with the "-X" option.
The tags mentioned above are used to describe how various developers have
been associated with the development of this patch. They are described in
detail in the SubmittingPatches document; what follows here is a brief
summary. Each of these lines has the format:
tag: Full Name <email address> optional-other-stuff
The tags in common use are:
- Signed-off-by: this is a developer's certification that he or she has
the right to submit the patch for inclusion into the kernel. It is an
agreement to the Developer's Certificate of Origin, the full text of
which can be found in Documentation/SubmittingPatches. Code without a
proper signoff cannot be merged into the mainline.
- Acked-by: indicates an agreement by another developer (often a
maintainer of the relevant code) that the patch is appropriate for
inclusion into the kernel.
- Tested-by: states that the named person has tested the patch and found
it to work.
- Reviewed-by: the named developer has reviewed the patch for correctness;
see the reviewer's statement in Documentation/SubmittingPatches for more
detail.
- Reported-by: names a user who reported a problem which is fixed by this
patch; this tag is used to give credit to the (often underappreciated)
people who test our code and let us know when things do not work
correctly.
- Cc: the named person received a copy of the patch and had the
opportunity to comment on it.
Be careful in the addition of tags to your patches: only Cc: is appropriate
for addition without the explicit permission of the person named.
5.5: SENDING THE PATCH
Before you mail your patches, there are a couple of other things you should
take care of:
- Are you sure that your mailer will not corrupt the patches? Patches
which have had gratuitous white-space changes or line wrapping performed
by the mail client will not apply at the other end, and often will not
be examined in any detail. If there is any doubt at all, mail the patch
to yourself and convince yourself that it shows up intact.
Documentation/email-clients.txt has some helpful hints on making
specific mail clients work for sending patches.
- Are you sure your patch is free of silly mistakes? You should always
run patches through scripts/checkpatch.pl and address the complaints it
comes up with. Please bear in mind that checkpatch.pl, while being the
embodiment of a fair amount of thought about what kernel patches should
look like, is not smarter than you. If fixing a checkpatch.pl complaint
would make the code worse, don't do it.
Patches should always be sent as plain text. Please do not send them as
attachments; that makes it much harder for reviewers to quote sections of
the patch in their replies. Instead, just put the patch directly into your
message.
When mailing patches, it is important to send copies to anybody who might
be interested in it. Unlike some other projects, the kernel encourages
people to err on the side of sending too many copies; don't assume that the
relevant people will see your posting on the mailing lists. In particular,
copies should go to:
- The maintainer(s) of the affected subsystem(s). As described earlier,
the MAINTAINERS file is the first place to look for these people.
- Other developers who have been working in the same area - especially
those who might be working there now. Using git to see who else has
modified the files you are working on can be helpful.
- If you are responding to a bug report or a feature request, copy the
original poster as well.
- Send a copy to the relevant mailing list, or, if nothing else applies,
the linux-kernel list.
- If you are fixing a bug, think about whether the fix should go into the
next stable update. If so, stable@vger.kernel.org should get a copy of
the patch. Also add a "Cc: stable@vger.kernel.org" to the tags within
the patch itself; that will cause the stable team to get a notification
when your fix goes into the mainline.
When selecting recipients for a patch, it is good to have an idea of who
you think will eventually accept the patch and get it merged. While it
is possible to send patches directly to Linus Torvalds and have him merge
them, things are not normally done that way. Linus is busy, and there are
subsystem maintainers who watch over specific parts of the kernel. Usually
you will be wanting that maintainer to merge your patches. If there is no
obvious maintainer, Andrew Morton is often the patch target of last resort.
Patches need good subject lines. The canonical format for a patch line is
something like:
[PATCH nn/mm] subsys: one-line description of the patch
where "nn" is the ordinal number of the patch, "mm" is the total number of
patches in the series, and "subsys" is the name of the affected subsystem.
Clearly, nn/mm can be omitted for a single, standalone patch.
If you have a significant series of patches, it is customary to send an
introductory description as part zero. This convention is not universally
followed though; if you use it, remember that information in the
introduction does not make it into the kernel changelogs. So please ensure
that the patches, themselves, have complete changelog information.
In general, the second and following parts of a multi-part patch should be
sent as a reply to the first part so that they all thread together at the
receiving end. Tools like git and quilt have commands to mail out a set of
patches with the proper threading. If you have a long series, though, and
are using git, please stay away from the --chain-reply-to option to avoid
creating exceptionally deep nesting.