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1 .. _development_posting:
3 Posting patches
4 ===============
6 Sooner or later, the time comes when your work is ready to be presented to
7 the community for review and, eventually, inclusion into the mainline
8 kernel. Unsurprisingly, the kernel development community has evolved a set
9 of conventions and procedures which are used in the posting of patches;
10 following them will make life much easier for everybody involved. This
11 document will attempt to cover these expectations in reasonable detail;
12 more information can also be found in the files process/submitting-patches.rst,
13 process/submitting-drivers.rst, and process/submit-checklist.rst in the kernel documentation
14 directory.
17 When to post
18 ------------
20 There is a constant temptation to avoid posting patches before they are
21 completely "ready." For simple patches, that is not a problem. If the
22 work being done is complex, though, there is a lot to be gained by getting
23 feedback from the community before the work is complete. So you should
24 consider posting in-progress work, or even making a git tree available so
25 that interested developers can catch up with your work at any time.
27 When posting code which is not yet considered ready for inclusion, it is a
28 good idea to say so in the posting itself. Also mention any major work
29 which remains to be done and any known problems. Fewer people will look at
30 patches which are known to be half-baked, but those who do will come in
31 with the idea that they can help you drive the work in the right direction.
34 Before creating patches
35 -----------------------
37 There are a number of things which should be done before you consider
38 sending patches to the development community. These include:
40 - Test the code to the extent that you can. Make use of the kernel's
41 debugging tools, ensure that the kernel will build with all reasonable
42 combinations of configuration options, use cross-compilers to build for
43 different architectures, etc.
45 - Make sure your code is compliant with the kernel coding style
46 guidelines.
48 - Does your change have performance implications? If so, you should run
49 benchmarks showing what the impact (or benefit) of your change is; a
50 summary of the results should be included with the patch.
52 - Be sure that you have the right to post the code. If this work was done
53 for an employer, the employer likely has a right to the work and must be
54 agreeable with its release under the GPL.
56 As a general rule, putting in some extra thought before posting code almost
57 always pays back the effort in short order.
60 Patch preparation
61 -----------------
63 The preparation of patches for posting can be a surprising amount of work,
64 but, once again, attempting to save time here is not generally advisable
65 even in the short term.
67 Patches must be prepared against a specific version of the kernel. As a
68 general rule, a patch should be based on the current mainline as found in
69 Linus's git tree. When basing on mainline, start with a well-known release
70 point - a stable or -rc release - rather than branching off the mainline at
71 an arbitrary spot.
73 It may become necessary to make versions against -mm, linux-next, or a
74 subsystem tree, though, to facilitate wider testing and review. Depending
75 on the area of your patch and what is going on elsewhere, basing a patch
76 against these other trees can require a significant amount of work
77 resolving conflicts and dealing with API changes.
79 Only the most simple changes should be formatted as a single patch;
80 everything else should be made as a logical series of changes. Splitting
81 up patches is a bit of an art; some developers spend a long time figuring
82 out how to do it in the way that the community expects. There are a few
83 rules of thumb, however, which can help considerably:
85 - The patch series you post will almost certainly not be the series of
86 changes found in your working revision control system. Instead, the
87 changes you have made need to be considered in their final form, then
88 split apart in ways which make sense. The developers are interested in
89 discrete, self-contained changes, not the path you took to get to those
90 changes.
92 - Each logically independent change should be formatted as a separate
93 patch. These changes can be small ("add a field to this structure") or
94 large (adding a significant new driver, for example), but they should be
95 conceptually small and amenable to a one-line description. Each patch
96 should make a specific change which can be reviewed on its own and
97 verified to do what it says it does.
99 - As a way of restating the guideline above: do not mix different types of
100 changes in the same patch. If a single patch fixes a critical security
101 bug, rearranges a few structures, and reformats the code, there is a
102 good chance that it will be passed over and the important fix will be
103 lost.
105 - Each patch should yield a kernel which builds and runs properly; if your
106 patch series is interrupted in the middle, the result should still be a
107 working kernel. Partial application of a patch series is a common
108 scenario when the "git bisect" tool is used to find regressions; if the
109 result is a broken kernel, you will make life harder for developers and
110 users who are engaging in the noble work of tracking down problems.
112 - Do not overdo it, though. One developer once posted a set of edits
113 to a single file as 500 separate patches - an act which did not make him
114 the most popular person on the kernel mailing list. A single patch can
115 be reasonably large as long as it still contains a single *logical*
116 change.
118 - It can be tempting to add a whole new infrastructure with a series of
119 patches, but to leave that infrastructure unused until the final patch
120 in the series enables the whole thing. This temptation should be
121 avoided if possible; if that series adds regressions, bisection will
122 finger the last patch as the one which caused the problem, even though
123 the real bug is elsewhere. Whenever possible, a patch which adds new
124 code should make that code active immediately.
126 Working to create the perfect patch series can be a frustrating process
127 which takes quite a bit of time and thought after the "real work" has been
128 done. When done properly, though, it is time well spent.
131 Patch formatting and changelogs
132 -------------------------------
134 So now you have a perfect series of patches for posting, but the work is
135 not done quite yet. Each patch needs to be formatted into a message which
136 quickly and clearly communicates its purpose to the rest of the world. To
137 that end, each patch will be composed of the following:
139 - An optional "From" line naming the author of the patch. This line is
140 only necessary if you are passing on somebody else's patch via email,
141 but it never hurts to add it when in doubt.
143 - A one-line description of what the patch does. This message should be
144 enough for a reader who sees it with no other context to figure out the
145 scope of the patch; it is the line that will show up in the "short form"
146 changelogs. This message is usually formatted with the relevant
147 subsystem name first, followed by the purpose of the patch. For
148 example:
150 ::
152 gpio: fix build on CONFIG_GPIO_SYSFS=n
154 - A blank line followed by a detailed description of the contents of the
155 patch. This description can be as long as is required; it should say
156 what the patch does and why it should be applied to the kernel.
158 - One or more tag lines, with, at a minimum, one Signed-off-by: line from
159 the author of the patch. Tags will be described in more detail below.
161 The items above, together, form the changelog for the patch. Writing good
162 changelogs is a crucial but often-neglected art; it's worth spending
163 another moment discussing this issue. When writing a changelog, you should
164 bear in mind that a number of different people will be reading your words.
165 These include subsystem maintainers and reviewers who need to decide
166 whether the patch should be included, distributors and other maintainers
167 trying to decide whether a patch should be backported to other kernels, bug
168 hunters wondering whether the patch is responsible for a problem they are
169 chasing, users who want to know how the kernel has changed, and more. A
170 good changelog conveys the needed information to all of these people in the
171 most direct and concise way possible.
173 To that end, the summary line should describe the effects of and motivation
174 for the change as well as possible given the one-line constraint. The
175 detailed description can then amplify on those topics and provide any
176 needed additional information. If the patch fixes a bug, cite the commit
177 which introduced the bug if possible (and please provide both the commit ID
178 and the title when citing commits). If a problem is associated with
179 specific log or compiler output, include that output to help others
180 searching for a solution to the same problem. If the change is meant to
181 support other changes coming in later patch, say so. If internal APIs are
182 changed, detail those changes and how other developers should respond. In
183 general, the more you can put yourself into the shoes of everybody who will
184 be reading your changelog, the better that changelog (and the kernel as a
185 whole) will be.
187 Needless to say, the changelog should be the text used when committing the
188 change to a revision control system. It will be followed by:
190 - The patch itself, in the unified ("-u") patch format. Using the "-p"
191 option to diff will associate function names with changes, making the
192 resulting patch easier for others to read.
194 You should avoid including changes to irrelevant files (those generated by
195 the build process, for example, or editor backup files) in the patch. The
196 file "dontdiff" in the Documentation directory can help in this regard;
197 pass it to diff with the "-X" option.
199 The tags mentioned above are used to describe how various developers have
200 been associated with the development of this patch. They are described in
201 detail in the process/submitting-patches.rst document; what follows here is a brief
202 summary. Each of these lines has the format:
204 ::
206 tag: Full Name <email address> optional-other-stuff
208 The tags in common use are:
210 - Signed-off-by: this is a developer's certification that he or she has
211 the right to submit the patch for inclusion into the kernel. It is an
212 agreement to the Developer's Certificate of Origin, the full text of
213 which can be found in Documentation/process/submitting-patches.rst. Code without a
214 proper signoff cannot be merged into the mainline.
216 - Co-Developed-by: states that the patch was also created by another developer
217 along with the original author. This is useful at times when multiple
218 people work on a single patch. Note, this person also needs to have a
219 Signed-off-by: line in the patch as well.
221 - Acked-by: indicates an agreement by another developer (often a
222 maintainer of the relevant code) that the patch is appropriate for
223 inclusion into the kernel.
225 - Tested-by: states that the named person has tested the patch and found
226 it to work.
228 - Reviewed-by: the named developer has reviewed the patch for correctness;
229 see the reviewer's statement in Documentation/process/submitting-patches.rst for more
230 detail.
232 - Reported-by: names a user who reported a problem which is fixed by this
233 patch; this tag is used to give credit to the (often underappreciated)
234 people who test our code and let us know when things do not work
235 correctly.
237 - Cc: the named person received a copy of the patch and had the
238 opportunity to comment on it.
240 Be careful in the addition of tags to your patches: only Cc: is appropriate
241 for addition without the explicit permission of the person named.
244 Sending the patch
245 -----------------
247 Before you mail your patches, there are a couple of other things you should
248 take care of:
250 - Are you sure that your mailer will not corrupt the patches? Patches
251 which have had gratuitous white-space changes or line wrapping performed
252 by the mail client will not apply at the other end, and often will not
253 be examined in any detail. If there is any doubt at all, mail the patch
254 to yourself and convince yourself that it shows up intact.
256 Documentation/process/email-clients.rst has some helpful hints on making
257 specific mail clients work for sending patches.
259 - Are you sure your patch is free of silly mistakes? You should always
260 run patches through scripts/checkpatch.pl and address the complaints it
261 comes up with. Please bear in mind that checkpatch.pl, while being the
262 embodiment of a fair amount of thought about what kernel patches should
263 look like, is not smarter than you. If fixing a checkpatch.pl complaint
264 would make the code worse, don't do it.
266 Patches should always be sent as plain text. Please do not send them as
267 attachments; that makes it much harder for reviewers to quote sections of
268 the patch in their replies. Instead, just put the patch directly into your
269 message.
271 When mailing patches, it is important to send copies to anybody who might
272 be interested in it. Unlike some other projects, the kernel encourages
273 people to err on the side of sending too many copies; don't assume that the
274 relevant people will see your posting on the mailing lists. In particular,
275 copies should go to:
277 - The maintainer(s) of the affected subsystem(s). As described earlier,
278 the MAINTAINERS file is the first place to look for these people.
280 - Other developers who have been working in the same area - especially
281 those who might be working there now. Using git to see who else has
282 modified the files you are working on can be helpful.
284 - If you are responding to a bug report or a feature request, copy the
285 original poster as well.
287 - Send a copy to the relevant mailing list, or, if nothing else applies,
288 the linux-kernel list.
290 - If you are fixing a bug, think about whether the fix should go into the
291 next stable update. If so, stable@vger.kernel.org should get a copy of
292 the patch. Also add a "Cc: stable@vger.kernel.org" to the tags within
293 the patch itself; that will cause the stable team to get a notification
294 when your fix goes into the mainline.
296 When selecting recipients for a patch, it is good to have an idea of who
297 you think will eventually accept the patch and get it merged. While it
298 is possible to send patches directly to Linus Torvalds and have him merge
299 them, things are not normally done that way. Linus is busy, and there are
300 subsystem maintainers who watch over specific parts of the kernel. Usually
301 you will be wanting that maintainer to merge your patches. If there is no
302 obvious maintainer, Andrew Morton is often the patch target of last resort.
304 Patches need good subject lines. The canonical format for a patch line is
305 something like:
307 ::
309 [PATCH nn/mm] subsys: one-line description of the patch
311 where "nn" is the ordinal number of the patch, "mm" is the total number of
312 patches in the series, and "subsys" is the name of the affected subsystem.
313 Clearly, nn/mm can be omitted for a single, standalone patch.
315 If you have a significant series of patches, it is customary to send an
316 introductory description as part zero. This convention is not universally
317 followed though; if you use it, remember that information in the
318 introduction does not make it into the kernel changelogs. So please ensure
319 that the patches, themselves, have complete changelog information.
321 In general, the second and following parts of a multi-part patch should be
322 sent as a reply to the first part so that they all thread together at the
323 receiving end. Tools like git and quilt have commands to mail out a set of
324 patches with the proper threading. If you have a long series, though, and
325 are using git, please stay away from the --chain-reply-to option to avoid
326 creating exceptionally deep nesting.