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