Add the development process document

This is an extended document intended to help interested developers, their
managers, and their employers work with the kernel development process.

This work was supported by the Linux Foundation.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 278 insertions, 0 deletions

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+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.  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 recently 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
+
+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 above three items should, normally, be the text used when committing
+the change to a revision control system.  They are 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@kernel.org should get a copy of the
+   patch.  Also add a "Cc: stable@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 provide the --no-chain-reply-to option to avoid
+creating exceptionally deep nesting.