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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. It may become necessary to make versions against -mm,
64 linux-next, or a subsystem tree, though, to facilitate wider testing and
65 review. Depending on the area of your patch and what is going on
66 elsewhere, basing a patch against these other trees can require a
67 significant amount of work resolving conflicts and dealing with API
68 changes.
70 Only the most simple changes should be formatted as a single patch;
71 everything else should be made as a logical series of changes. Splitting
72 up patches is a bit of an art; some developers spend a long time figuring
73 out how to do it in the way that the community expects. There are a few
74 rules of thumb, however, which can help considerably:
76 - The patch series you post will almost certainly not be the series of
77 changes found in your working revision control system. Instead, the
78 changes you have made need to be considered in their final form, then
79 split apart in ways which make sense. The developers are interested in
80 discrete, self-contained changes, not the path you took to get to those
81 changes.
83 - Each logically independent change should be formatted as a separate
84 patch. These changes can be small ("add a field to this structure") or
85 large (adding a significant new driver, for example), but they should be
86 conceptually small and amenable to a one-line description. Each patch
87 should make a specific change which can be reviewed on its own and
88 verified to do what it says it does.
90 - As a way of restating the guideline above: do not mix different types of
91 changes in the same patch. If a single patch fixes a critical security
92 bug, rearranges a few structures, and reformats the code, there is a
93 good chance that it will be passed over and the important fix will be
94 lost.
96 - Each patch should yield a kernel which builds and runs properly; if your
97 patch series is interrupted in the middle, the result should still be a
98 working kernel. Partial application of a patch series is a common
99 scenario when the "git bisect" tool is used to find regressions; if the
100 result is a broken kernel, you will make life harder for developers and
101 users who are engaging in the noble work of tracking down problems.
103 - Do not overdo it, though. One developer recently posted a set of edits
104 to a single file as 500 separate patches - an act which did not make him
105 the most popular person on the kernel mailing list. A single patch can
106 be reasonably large as long as it still contains a single *logical*
107 change.
109 - It can be tempting to add a whole new infrastructure with a series of
110 patches, but to leave that infrastructure unused until the final patch
111 in the series enables the whole thing. This temptation should be
112 avoided if possible; if that series adds regressions, bisection will
113 finger the last patch as the one which caused the problem, even though
114 the real bug is elsewhere. Whenever possible, a patch which adds new
115 code should make that code active immediately.
117 Working to create the perfect patch series can be a frustrating process
118 which takes quite a bit of time and thought after the "real work" has been
119 done. When done properly, though, it is time well spent.
122 5.4: PATCH FORMATTING AND CHANGELOGS
124 So now you have a perfect series of patches for posting, but the work is
125 not done quite yet. Each patch needs to be formatted into a message which
126 quickly and clearly communicates its purpose to the rest of the world. To
127 that end, each patch will be composed of the following:
129 - An optional "From" line naming the author of the patch. This line is
130 only necessary if you are passing on somebody else's patch via email,
131 but it never hurts to add it when in doubt.
133 - A one-line description of what the patch does. This message should be
134 enough for a reader who sees it with no other context to figure out the
135 scope of the patch; it is the line that will show up in the "short form"
136 changelogs. This message is usually formatted with the relevant
137 subsystem name first, followed by the purpose of the patch. For
138 example:
140 gpio: fix build on CONFIG_GPIO_SYSFS=n
142 - A blank line followed by a detailed description of the contents of the
143 patch. This description can be as long as is required; it should say
144 what the patch does and why it should be applied to the kernel.
146 - One or more tag lines, with, at a minimum, one Signed-off-by: line from
147 the author of the patch. Tags will be described in more detail below.
149 The items above, together, form the changelog for the patch. Writing good
150 changelogs is a crucial but often-neglected art; it's worth spending
151 another moment discussing this issue. When writing a changelog, you should
152 bear in mind that a number of different people will be reading your words.
153 These include subsystem maintainers and reviewers who need to decide
154 whether the patch should be included, distributors and other maintainers
155 trying to decide whether a patch should be backported to other kernels, bug
156 hunters wondering whether the patch is responsible for a problem they are
157 chasing, users who want to know how the kernel has changed, and more. A
158 good changelog conveys the needed information to all of these people in the
159 most direct and concise way possible.
161 To that end, the summary line should describe the effects of and motivation
162 for the change as well as possible given the one-line constraint. The
163 detailed description can then amplify on those topics and provide any
164 needed additional information. If the patch fixes a bug, cite the commit
165 which introduced the bug if possible. If a problem is associated with
166 specific log or compiler output, include that output to help others
167 searching for a solution to the same problem. If the change is meant to
168 support other changes coming in later patch, say so. If internal APIs are
169 changed, detail those changes and how other developers should respond. In
170 general, the more you can put yourself into the shoes of everybody who will
171 be reading your changelog, the better that changelog (and the kernel as a
172 whole) will be.
174 Needless to say, the changelog should be the text used when committing the
175 change to a revision control system. It will be followed by:
177 - The patch itself, in the unified ("-u") patch format. Using the "-p"
178 option to diff will associate function names with changes, making the
179 resulting patch easier for others to read.
181 You should avoid including changes to irrelevant files (those generated by
182 the build process, for example, or editor backup files) in the patch. The
183 file "dontdiff" in the Documentation directory can help in this regard;
184 pass it to diff with the "-X" option.
186 The tags mentioned above are used to describe how various developers have
187 been associated with the development of this patch. They are described in
188 detail in the SubmittingPatches document; what follows here is a brief
189 summary. Each of these lines has the format:
191 tag: Full Name <email address> optional-other-stuff
193 The tags in common use are:
195 - Signed-off-by: this is a developer's certification that he or she has
196 the right to submit the patch for inclusion into the kernel. It is an
197 agreement to the Developer's Certificate of Origin, the full text of
198 which can be found in Documentation/SubmittingPatches. Code without a
199 proper signoff cannot be merged into the mainline.
201 - Acked-by: indicates an agreement by another developer (often a
202 maintainer of the relevant code) that the patch is appropriate for
203 inclusion into the kernel.
205 - Tested-by: states that the named person has tested the patch and found
206 it to work.
208 - Reviewed-by: the named developer has reviewed the patch for correctness;
209 see the reviewer's statement in Documentation/SubmittingPatches for more
210 detail.
212 - Reported-by: names a user who reported a problem which is fixed by this
213 patch; this tag is used to give credit to the (often underappreciated)
214 people who test our code and let us know when things do not work
215 correctly.
217 - Cc: the named person received a copy of the patch and had the
218 opportunity to comment on it.
220 Be careful in the addition of tags to your patches: only Cc: is appropriate
221 for addition without the explicit permission of the person named.
224 5.5: SENDING THE PATCH
226 Before you mail your patches, there are a couple of other things you should
227 take care of:
229 - Are you sure that your mailer will not corrupt the patches? Patches
230 which have had gratuitous white-space changes or line wrapping performed
231 by the mail client will not apply at the other end, and often will not
232 be examined in any detail. If there is any doubt at all, mail the patch
233 to yourself and convince yourself that it shows up intact.
235 Documentation/email-clients.txt has some helpful hints on making
236 specific mail clients work for sending patches.
238 - Are you sure your patch is free of silly mistakes? You should always
239 run patches through scripts/checkpatch.pl and address the complaints it
240 comes up with. Please bear in mind that checkpatch.pl, while being the
241 embodiment of a fair amount of thought about what kernel patches should
242 look like, is not smarter than you. If fixing a checkpatch.pl complaint
243 would make the code worse, don't do it.
245 Patches should always be sent as plain text. Please do not send them as
246 attachments; that makes it much harder for reviewers to quote sections of
247 the patch in their replies. Instead, just put the patch directly into your
248 message.
250 When mailing patches, it is important to send copies to anybody who might
251 be interested in it. Unlike some other projects, the kernel encourages
252 people to err on the side of sending too many copies; don't assume that the
253 relevant people will see your posting on the mailing lists. In particular,
254 copies should go to:
256 - The maintainer(s) of the affected subsystem(s). As described earlier,
257 the MAINTAINERS file is the first place to look for these people.
259 - Other developers who have been working in the same area - especially
260 those who might be working there now. Using git to see who else has
261 modified the files you are working on can be helpful.
263 - If you are responding to a bug report or a feature request, copy the
264 original poster as well.
266 - Send a copy to the relevant mailing list, or, if nothing else applies,
267 the linux-kernel list.
269 - If you are fixing a bug, think about whether the fix should go into the
270 next stable update. If so, stable@kernel.org should get a copy of the
271 patch. Also add a "Cc: stable@kernel.org" to the tags within the patch
272 itself; that will cause the stable team to get a notification when your
273 fix goes into the mainline.
275 When selecting recipients for a patch, it is good to have an idea of who
276 you think will eventually accept the patch and get it merged. While it
277 is possible to send patches directly to Linus Torvalds and have him merge
278 them, things are not normally done that way. Linus is busy, and there are
279 subsystem maintainers who watch over specific parts of the kernel. Usually
280 you will be wanting that maintainer to merge your patches. If there is no
281 obvious maintainer, Andrew Morton is often the patch target of last resort.
283 Patches need good subject lines. The canonical format for a patch line is
284 something like:
286 [PATCH nn/mm] subsys: one-line description of the patch
288 where "nn" is the ordinal number of the patch, "mm" is the total number of
289 patches in the series, and "subsys" is the name of the affected subsystem.
290 Clearly, nn/mm can be omitted for a single, standalone patch.
292 If you have a significant series of patches, it is customary to send an
293 introductory description as part zero. This convention is not universally
294 followed though; if you use it, remember that information in the
295 introduction does not make it into the kernel changelogs. So please ensure
296 that the patches, themselves, have complete changelog information.
298 In general, the second and following parts of a multi-part patch should be
299 sent as a reply to the first part so that they all thread together at the
300 receiving end. Tools like git and quilt have commands to mail out a set of
301 patches with the proper threading. If you have a long series, though, and
302 are using git, please provide the --no-chain-reply-to option to avoid
303 creating exceptionally deep nesting.