1 .. _development_posting:
2
3 Posting patches
4 ===============
5
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
13 :ref:`Documentation/process/submitting-patches.rst <submittingpatches>`
14 and :ref:`Documentation/process/submit-checklist.rst <submitchecklist>`.
15
16
17 When to post
18 ------------
19
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.
26
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.
32
33
34 Before creating patches
35 -----------------------
36
37 There are a number of things which should be done before you consider
38 sending patches to the development community. These include:
39
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.
44
45 - Make sure your code is compliant with the kernel coding style
46 guidelines.
47
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.
51
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.
55
56 As a general rule, putting in some extra thought before posting code almost
57 always pays back the effort in short order.
58
59
60 Patch preparation
61 -----------------
62
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.
66
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.
72
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.
78
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:
84
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.
91
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.
98
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.
104
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.
111
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.
117
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.
125
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.
129
130
131 Patch formatting and changelogs
132 -------------------------------
133
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:
138
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.
142
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:
149
150 ::
151
152 gpio: fix build on CONFIG_GPIO_SYSFS=n
153
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.
157
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.
160
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.
172
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.
186
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:
189
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.
193
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.
198
199 The tags already briefly mentioned above are used to provide insights how
200 the patch came into being. They are described in detail in the
201 :ref:`Documentation/process/submitting-patches.rst <submittingpatches>`
202 document; what follows here is a brief summary.
203
204 One tag is used to refer to earlier commits which introduced problems fixed by
205 the patch::
206
207 Fixes: 1f2e3d4c5b6a ("The first line of the commit specified by the first 12 characters of its SHA-1 ID")
208
209 Another tag is used for linking web pages with additional backgrounds or
210 details, for example an earlier discussion which leads to the patch or a
211 document with a specification implemented by the patch::
212
213 Link: https://example.com/somewhere.html optional-other-stuff
214
215 Many maintainers when applying a patch also add this tag to link to the
216 latest public review posting of the patch; often this is automatically done
217 by tools like b4 or a git hook like the one described in
218 'Documentation/maintainer/configure-git.rst'.
219
220 If the URL points to a public bug report being fixed by the patch, use the
221 "Closes:" tag instead::
222
223 Closes: https://example.com/issues/1234 optional-other-stuff
224
225 Some bug trackers have the ability to close issues automatically when a
226 commit with such a tag is applied. Some bots monitoring mailing lists can
227 also track such tags and take certain actions. Private bug trackers and
228 invalid URLs are forbidden.
229
230 Another kind of tag is used to document who was involved in the development of
231 the patch. Each of these uses this format::
232
233 tag: Full Name <email address> optional-other-stuff
234
235 The tags in common use are:
236
237 - Signed-off-by: this is a developer's certification that he or she has
238 the right to submit the patch for inclusion into the kernel. It is an
239 agreement to the Developer's Certificate of Origin, the full text of
240 which can be found in :ref:`Documentation/process/submitting-patches.rst <submittingpatches>`
241 Code without a proper signoff cannot be merged into the mainline.
242
243 - Co-developed-by: states that the patch was co-created by several developers;
244 it is a used to give attribution to co-authors (in addition to the author
245 attributed by the From: tag) when multiple people work on a single patch.
246 Every Co-developed-by: must be immediately followed by a Signed-off-by: of
247 the associated co-author. Details and examples can be found in
248 :ref:`Documentation/process/submitting-patches.rst <submittingpatches>`.
249
250 - Acked-by: indicates an agreement by another developer (often a
251 maintainer of the relevant code) that the patch is appropriate for
252 inclusion into the kernel.
253
254 - Tested-by: states that the named person has tested the patch and found
255 it to work.
256
257 - Reviewed-by: the named developer has reviewed the patch for correctness;
258 see the reviewer's statement in :ref:`Documentation/process/submitting-patches.rst <submittingpatches>`
259 for more detail.
260
261 - Reported-by: names a user who reported a problem which is fixed by this
262 patch; this tag is used to give credit to the (often underappreciated)
263 people who test our code and let us know when things do not work
264 correctly. Note, this tag should be followed by a Closes: tag pointing to
265 the report, unless the report is not available on the web. The Link: tag
266 can be used instead of Closes: if the patch fixes a part of the issue(s)
267 being reported.
268
269 - Cc: the named person received a copy of the patch and had the
270 opportunity to comment on it.
271
272 Be careful in the addition of tags to your patches, as only Cc: is appropriate
273 for addition without the explicit permission of the person named; using
274 Reported-by: is fine most of the time as well, but ask for permission if
275 the bug was reported in private.
276
277
278 Sending the patch
279 -----------------
280
281 Before you mail your patches, there are a couple of other things you should
282 take care of:
283
284 - Are you sure that your mailer will not corrupt the patches? Patches
285 which have had gratuitous white-space changes or line wrapping performed
286 by the mail client will not apply at the other end, and often will not
287 be examined in any detail. If there is any doubt at all, mail the patch
288 to yourself and convince yourself that it shows up intact.
289
290 :ref:`Documentation/process/email-clients.rst <email_clients>` has some
291 helpful hints on making specific mail clients work for sending patches.
292
293 - Are you sure your patch is free of silly mistakes? You should always
294 run patches through scripts/checkpatch.pl and address the complaints it
295 comes up with. Please bear in mind that checkpatch.pl, while being the
296 embodiment of a fair amount of thought about what kernel patches should
297 look like, is not smarter than you. If fixing a checkpatch.pl complaint
298 would make the code worse, don't do it.
299
300 Patches should always be sent as plain text. Please do not send them as
301 attachments; that makes it much harder for reviewers to quote sections of
302 the patch in their replies. Instead, just put the patch directly into your
303 message.
304
305 When mailing patches, it is important to send copies to anybody who might
306 be interested in it. Unlike some other projects, the kernel encourages
307 people to err on the side of sending too many copies; don't assume that the
308 relevant people will see your posting on the mailing lists. In particular,
309 copies should go to:
310
311 - The maintainer(s) of the affected subsystem(s). As described earlier,
312 the MAINTAINERS file is the first place to look for these people.
313
314 - Other developers who have been working in the same area - especially
315 those who might be working there now. Using git to see who else has
316 modified the files you are working on can be helpful.
317
318 - If you are responding to a bug report or a feature request, copy the
319 original poster as well.
320
321 - Send a copy to the relevant mailing list, or, if nothing else applies,
322 the linux-kernel list.
323
324 - If you are fixing a bug, think about whether the fix should go into the
325 next stable update. If so, stable@vger.kernel.org should get a copy of
326 the patch. Also add a "Cc: stable@vger.kernel.org" to the tags within
327 the patch itself; that will cause the stable team to get a notification
328 when your fix goes into the mainline.
329
330 When selecting recipients for a patch, it is good to have an idea of who
331 you think will eventually accept the patch and get it merged. While it
332 is possible to send patches directly to Linus Torvalds and have him merge
333 them, things are not normally done that way. Linus is busy, and there are
334 subsystem maintainers who watch over specific parts of the kernel. Usually
335 you will be wanting that maintainer to merge your patches. If there is no
336 obvious maintainer, Andrew Morton is often the patch target of last resort.
337
338 Patches need good subject lines. The canonical format for a patch line is
339 something like:
340
341 ::
342
343 [PATCH nn/mm] subsys: one-line description of the patch
344
345 where "nn" is the ordinal number of the patch, "mm" is the total number of
346 patches in the series, and "subsys" is the name of the affected subsystem.
347 Clearly, nn/mm can be omitted for a single, standalone patch.
348
349 If you have a significant series of patches, it is customary to send an
350 introductory description as part zero. This convention is not universally
351 followed though; if you use it, remember that information in the
352 introduction does not make it into the kernel changelogs. So please ensure
353 that the patches, themselves, have complete changelog information.
354
355 In general, the second and following parts of a multi-part patch should be
356 sent as a reply to the first part so that they all thread together at the
357 receiving end. Tools like git and quilt have commands to mail out a set of
358 patches with the proper threading. If you have a long series, though, and
359 are using git, please stay away from the --chain-reply-to option to avoid
360 creating exceptionally deep nesting.
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