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1 5: POSTING PATCHES
3 Sooner or later, the time comes when your work is ready to be presented to
4 the community for review and, eventually, inclusion into the mainline
5 kernel. Unsurprisingly, the kernel development community has evolved a set
6 of conventions and procedures which are used in the posting of patches;
7 following them will make life much easier for everybody involved. This
8 document will attempt to cover these expectations in reasonable detail;
9 more information can also be found in the files SubmittingPatches,
10 SubmittingDrivers, and SubmitChecklist in the kernel documentation
11 directory.
14 5.1: WHEN TO POST
16 There is a constant temptation to avoid posting patches before they are
17 completely "ready." For simple patches, that is not a problem. If the
18 work being done is complex, though, there is a lot to be gained by getting
19 feedback from the community before the work is complete. So you should
20 consider posting in-progress work, or even making a git tree available so
21 that interested developers can catch up with your work at any time.
23 When posting code which is not yet considered ready for inclusion, it is a
24 good idea to say so in the posting itself. Also mention any major work
25 which remains to be done and any known problems. Fewer people will look at
26 patches which are known to be half-baked, but those who do will come in
27 with the idea that they can help you drive the work in the right direction.
30 5.2: BEFORE CREATING PATCHES
32 There are a number of things which should be done before you consider
33 sending patches to the development community. These include:
35 - Test the code to the extent that you can. Make use of the kernel's
36 debugging tools, ensure that the kernel will build with all reasonable
37 combinations of configuration options, use cross-compilers to build for
38 different architectures, etc.
40 - Make sure your code is compliant with the kernel coding style
41 guidelines.
43 - Does your change have performance implications? If so, you should run
44 benchmarks showing what the impact (or benefit) of your change is; a
45 summary of the results should be included with the patch.
47 - Be sure that you have the right to post the code. If this work was done
48 for an employer, the employer likely has a right to the work and must be
49 agreeable with its release under the GPL.
51 As a general rule, putting in some extra thought before posting code almost
52 always pays back the effort in short order.
55 5.3: PATCH PREPARATION
57 The preparation of patches for posting can be a surprising amount of work,
58 but, once again, attempting to save time here is not generally advisable
59 even in the short term.
61 Patches must be prepared against a specific version of the kernel. As a
62 general rule, a patch should be based on the current mainline as found in
63 Linus's git tree. When basing on mainline, start with a well-known release
64 point - a stable or -rc release - rather than branching off the mainline at
65 an arbitrary spot.
67 It may become necessary to make versions against -mm, linux-next, or a
68 subsystem tree, though, to facilitate wider testing and review. Depending
69 on the area of your patch and what is going on elsewhere, basing a patch
70 against these other trees can require a significant amount of work
71 resolving conflicts and dealing with API changes.
73 Only the most simple changes should be formatted as a single patch;
74 everything else should be made as a logical series of changes. Splitting
75 up patches is a bit of an art; some developers spend a long time figuring
76 out how to do it in the way that the community expects. There are a few
77 rules of thumb, however, which can help considerably:
79 - The patch series you post will almost certainly not be the series of
80 changes found in your working revision control system. Instead, the
81 changes you have made need to be considered in their final form, then
82 split apart in ways which make sense. The developers are interested in
83 discrete, self-contained changes, not the path you took to get to those
84 changes.
86 - Each logically independent change should be formatted as a separate
87 patch. These changes can be small ("add a field to this structure") or
88 large (adding a significant new driver, for example), but they should be
89 conceptually small and amenable to a one-line description. Each patch
90 should make a specific change which can be reviewed on its own and
91 verified to do what it says it does.
93 - As a way of restating the guideline above: do not mix different types of
94 changes in the same patch. If a single patch fixes a critical security
95 bug, rearranges a few structures, and reformats the code, there is a
96 good chance that it will be passed over and the important fix will be
97 lost.
99 - Each patch should yield a kernel which builds and runs properly; if your
100 patch series is interrupted in the middle, the result should still be a
101 working kernel. Partial application of a patch series is a common
102 scenario when the "git bisect" tool is used to find regressions; if the
103 result is a broken kernel, you will make life harder for developers and
104 users who are engaging in the noble work of tracking down problems.
106 - Do not overdo it, though. One developer once posted a set of edits
107 to a single file as 500 separate patches - an act which did not make him
108 the most popular person on the kernel mailing list. A single patch can
109 be reasonably large as long as it still contains a single *logical*
110 change.
112 - It can be tempting to add a whole new infrastructure with a series of
113 patches, but to leave that infrastructure unused until the final patch
114 in the series enables the whole thing. This temptation should be
115 avoided if possible; if that series adds regressions, bisection will
116 finger the last patch as the one which caused the problem, even though
117 the real bug is elsewhere. Whenever possible, a patch which adds new
118 code should make that code active immediately.
120 Working to create the perfect patch series can be a frustrating process
121 which takes quite a bit of time and thought after the "real work" has been
122 done. When done properly, though, it is time well spent.
125 5.4: PATCH FORMATTING AND CHANGELOGS
127 So now you have a perfect series of patches for posting, but the work is
128 not done quite yet. Each patch needs to be formatted into a message which
129 quickly and clearly communicates its purpose to the rest of the world. To
130 that end, each patch will be composed of the following:
132 - An optional "From" line naming the author of the patch. This line is
133 only necessary if you are passing on somebody else's patch via email,
134 but it never hurts to add it when in doubt.
136 - A one-line description of what the patch does. This message should be
137 enough for a reader who sees it with no other context to figure out the
138 scope of the patch; it is the line that will show up in the "short form"
139 changelogs. This message is usually formatted with the relevant
140 subsystem name first, followed by the purpose of the patch. For
141 example:
143 gpio: fix build on CONFIG_GPIO_SYSFS=n
145 - A blank line followed by a detailed description of the contents of the
146 patch. This description can be as long as is required; it should say
147 what the patch does and why it should be applied to the kernel.
149 - One or more tag lines, with, at a minimum, one Signed-off-by: line from
150 the author of the patch. Tags will be described in more detail below.
152 The items above, together, form the changelog for the patch. Writing good
153 changelogs is a crucial but often-neglected art; it's worth spending
154 another moment discussing this issue. When writing a changelog, you should
155 bear in mind that a number of different people will be reading your words.
156 These include subsystem maintainers and reviewers who need to decide
157 whether the patch should be included, distributors and other maintainers
158 trying to decide whether a patch should be backported to other kernels, bug
159 hunters wondering whether the patch is responsible for a problem they are
160 chasing, users who want to know how the kernel has changed, and more. A
161 good changelog conveys the needed information to all of these people in the
162 most direct and concise way possible.
164 To that end, the summary line should describe the effects of and motivation
165 for the change as well as possible given the one-line constraint. The
166 detailed description can then amplify on those topics and provide any
167 needed additional information. If the patch fixes a bug, cite the commit
168 which introduced the bug if possible (and please provide both the commit ID
169 and the title when citing commits). If a problem is associated with
170 specific log or compiler output, include that output to help others
171 searching for a solution to the same problem. If the change is meant to
172 support other changes coming in later patch, say so. If internal APIs are
173 changed, detail those changes and how other developers should respond. In
174 general, the more you can put yourself into the shoes of everybody who will
175 be reading your changelog, the better that changelog (and the kernel as a
176 whole) will be.
178 Needless to say, the changelog should be the text used when committing the
179 change to a revision control system. It will be followed by:
181 - The patch itself, in the unified ("-u") patch format. Using the "-p"
182 option to diff will associate function names with changes, making the
183 resulting patch easier for others to read.
185 You should avoid including changes to irrelevant files (those generated by
186 the build process, for example, or editor backup files) in the patch. The
187 file "dontdiff" in the Documentation directory can help in this regard;
188 pass it to diff with the "-X" option.
190 The tags mentioned above are used to describe how various developers have
191 been associated with the development of this patch. They are described in
192 detail in the SubmittingPatches document; what follows here is a brief
193 summary. Each of these lines has the format:
195 tag: Full Name <email address> optional-other-stuff
197 The tags in common use are:
199 - Signed-off-by: this is a developer's certification that he or she has
200 the right to submit the patch for inclusion into the kernel. It is an
201 agreement to the Developer's Certificate of Origin, the full text of
202 which can be found in Documentation/SubmittingPatches. Code without a
203 proper signoff cannot be merged into the mainline.
205 - Acked-by: indicates an agreement by another developer (often a
206 maintainer of the relevant code) that the patch is appropriate for
207 inclusion into the kernel.
209 - Tested-by: states that the named person has tested the patch and found
210 it to work.
212 - Reviewed-by: the named developer has reviewed the patch for correctness;
213 see the reviewer's statement in Documentation/SubmittingPatches for more
214 detail.
216 - Reported-by: names a user who reported a problem which is fixed by this
217 patch; this tag is used to give credit to the (often underappreciated)
218 people who test our code and let us know when things do not work
219 correctly.
221 - Cc: the named person received a copy of the patch and had the
222 opportunity to comment on it.
224 Be careful in the addition of tags to your patches: only Cc: is appropriate
225 for addition without the explicit permission of the person named.
228 5.5: SENDING THE PATCH
230 Before you mail your patches, there are a couple of other things you should
231 take care of:
233 - Are you sure that your mailer will not corrupt the patches? Patches
234 which have had gratuitous white-space changes or line wrapping performed
235 by the mail client will not apply at the other end, and often will not
236 be examined in any detail. If there is any doubt at all, mail the patch
237 to yourself and convince yourself that it shows up intact.
239 Documentation/email-clients.txt has some helpful hints on making
240 specific mail clients work for sending patches.
242 - Are you sure your patch is free of silly mistakes? You should always
243 run patches through scripts/checkpatch.pl and address the complaints it
244 comes up with. Please bear in mind that checkpatch.pl, while being the
245 embodiment of a fair amount of thought about what kernel patches should
246 look like, is not smarter than you. If fixing a checkpatch.pl complaint
247 would make the code worse, don't do it.
249 Patches should always be sent as plain text. Please do not send them as
250 attachments; that makes it much harder for reviewers to quote sections of
251 the patch in their replies. Instead, just put the patch directly into your
252 message.
254 When mailing patches, it is important to send copies to anybody who might
255 be interested in it. Unlike some other projects, the kernel encourages
256 people to err on the side of sending too many copies; don't assume that the
257 relevant people will see your posting on the mailing lists. In particular,
258 copies should go to:
260 - The maintainer(s) of the affected subsystem(s). As described earlier,
261 the MAINTAINERS file is the first place to look for these people.
263 - Other developers who have been working in the same area - especially
264 those who might be working there now. Using git to see who else has
265 modified the files you are working on can be helpful.
267 - If you are responding to a bug report or a feature request, copy the
268 original poster as well.
270 - Send a copy to the relevant mailing list, or, if nothing else applies,
271 the linux-kernel list.
273 - If you are fixing a bug, think about whether the fix should go into the
274 next stable update. If so, stable@kernel.org should get a copy of the
275 patch. Also add a "Cc: stable@kernel.org" to the tags within the patch
276 itself; that will cause the stable team to get a notification when your
277 fix goes into the mainline.
279 When selecting recipients for a patch, it is good to have an idea of who
280 you think will eventually accept the patch and get it merged. While it
281 is possible to send patches directly to Linus Torvalds and have him merge
282 them, things are not normally done that way. Linus is busy, and there are
283 subsystem maintainers who watch over specific parts of the kernel. Usually
284 you will be wanting that maintainer to merge your patches. If there is no
285 obvious maintainer, Andrew Morton is often the patch target of last resort.
287 Patches need good subject lines. The canonical format for a patch line is
288 something like:
290 [PATCH nn/mm] subsys: one-line description of the patch
292 where "nn" is the ordinal number of the patch, "mm" is the total number of
293 patches in the series, and "subsys" is the name of the affected subsystem.
294 Clearly, nn/mm can be omitted for a single, standalone patch.
296 If you have a significant series of patches, it is customary to send an
297 introductory description as part zero. This convention is not universally
298 followed though; if you use it, remember that information in the
299 introduction does not make it into the kernel changelogs. So please ensure
300 that the patches, themselves, have complete changelog information.
302 In general, the second and following parts of a multi-part patch should be
303 sent as a reply to the first part so that they all thread together at the
304 receiving end. Tools like git and quilt have commands to mail out a set of
305 patches with the proper threading. If you have a long series, though, and
306 are using git, please stay away from the --chain-reply-to option to avoid
307 creating exceptionally deep nesting.