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1 | 5: POSTING PATCHES |
2 | ||
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. | |
12 | ||
13 | ||
14 | 5.1: WHEN TO POST | |
15 | ||
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. | |
22 | ||
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. | |
28 | ||
29 | ||
30 | 5.2: BEFORE CREATING PATCHES | |
31 | ||
32 | There are a number of things which should be done before you consider | |
33 | sending patches to the development community. These include: | |
34 | ||
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. | |
39 | ||
40 | - Make sure your code is compliant with the kernel coding style | |
41 | guidelines. | |
42 | ||
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. | |
46 | ||
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. | |
50 | ||
51 | As a general rule, putting in some extra thought before posting code almost | |
52 | always pays back the effort in short order. | |
53 | ||
54 | ||
55 | 5.3: PATCH PREPARATION | |
56 | ||
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. | |
60 | ||
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. | |
69 | ||
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: | |
75 | ||
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. | |
82 | ||
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. | |
89 | ||
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. | |
95 | ||
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. | |
102 | ||
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. | |
108 | ||
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. | |
116 | ||
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. | |
120 | ||
121 | ||
122 | 5.4: PATCH FORMATTING | |
123 | ||
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: | |
128 | ||
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. | |
132 | ||
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: | |
139 | ||
140 | gpio: fix build on CONFIG_GPIO_SYSFS=n | |
141 | ||
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. | |
145 | ||
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. | |
148 | ||
149 | The above three items should, normally, be the text used when committing | |
150 | the change to a revision control system. They are followed by: | |
151 | ||
152 | - The patch itself, in the unified ("-u") patch format. Using the "-p" | |
153 | option to diff will associate function names with changes, making the | |
154 | resulting patch easier for others to read. | |
155 | ||
156 | You should avoid including changes to irrelevant files (those generated by | |
157 | the build process, for example, or editor backup files) in the patch. The | |
158 | file "dontdiff" in the Documentation directory can help in this regard; | |
159 | pass it to diff with the "-X" option. | |
160 | ||
161 | The tags mentioned above are used to describe how various developers have | |
162 | been associated with the development of this patch. They are described in | |
163 | detail in the SubmittingPatches document; what follows here is a brief | |
164 | summary. Each of these lines has the format: | |
165 | ||
166 | tag: Full Name <email address> optional-other-stuff | |
167 | ||
168 | The tags in common use are: | |
169 | ||
170 | - Signed-off-by: this is a developer's certification that he or she has | |
171 | the right to submit the patch for inclusion into the kernel. It is an | |
172 | agreement to the Developer's Certificate of Origin, the full text of | |
173 | which can be found in Documentation/SubmittingPatches. Code without a | |
174 | proper signoff cannot be merged into the mainline. | |
175 | ||
176 | - Acked-by: indicates an agreement by another developer (often a | |
177 | maintainer of the relevant code) that the patch is appropriate for | |
178 | inclusion into the kernel. | |
179 | ||
180 | - Tested-by: states that the named person has tested the patch and found | |
181 | it to work. | |
182 | ||
183 | - Reviewed-by: the named developer has reviewed the patch for correctness; | |
184 | see the reviewer's statement in Documentation/SubmittingPatches for more | |
185 | detail. | |
186 | ||
187 | - Reported-by: names a user who reported a problem which is fixed by this | |
188 | patch; this tag is used to give credit to the (often underappreciated) | |
189 | people who test our code and let us know when things do not work | |
190 | correctly. | |
191 | ||
192 | - Cc: the named person received a copy of the patch and had the | |
193 | opportunity to comment on it. | |
194 | ||
195 | Be careful in the addition of tags to your patches: only Cc: is appropriate | |
196 | for addition without the explicit permission of the person named. | |
197 | ||
198 | ||
199 | 5.5: SENDING THE PATCH | |
200 | ||
201 | Before you mail your patches, there are a couple of other things you should | |
202 | take care of: | |
203 | ||
204 | - Are you sure that your mailer will not corrupt the patches? Patches | |
205 | which have had gratuitous white-space changes or line wrapping performed | |
206 | by the mail client will not apply at the other end, and often will not | |
207 | be examined in any detail. If there is any doubt at all, mail the patch | |
208 | to yourself and convince yourself that it shows up intact. | |
209 | ||
210 | Documentation/email-clients.txt has some helpful hints on making | |
211 | specific mail clients work for sending patches. | |
212 | ||
213 | - Are you sure your patch is free of silly mistakes? You should always | |
214 | run patches through scripts/checkpatch.pl and address the complaints it | |
215 | comes up with. Please bear in mind that checkpatch.pl, while being the | |
216 | embodiment of a fair amount of thought about what kernel patches should | |
217 | look like, is not smarter than you. If fixing a checkpatch.pl complaint | |
218 | would make the code worse, don't do it. | |
219 | ||
220 | Patches should always be sent as plain text. Please do not send them as | |
221 | attachments; that makes it much harder for reviewers to quote sections of | |
222 | the patch in their replies. Instead, just put the patch directly into your | |
223 | message. | |
224 | ||
225 | When mailing patches, it is important to send copies to anybody who might | |
226 | be interested in it. Unlike some other projects, the kernel encourages | |
227 | people to err on the side of sending too many copies; don't assume that the | |
228 | relevant people will see your posting on the mailing lists. In particular, | |
229 | copies should go to: | |
230 | ||
231 | - The maintainer(s) of the affected subsystem(s). As described earlier, | |
232 | the MAINTAINERS file is the first place to look for these people. | |
233 | ||
234 | - Other developers who have been working in the same area - especially | |
235 | those who might be working there now. Using git to see who else has | |
236 | modified the files you are working on can be helpful. | |
237 | ||
238 | - If you are responding to a bug report or a feature request, copy the | |
239 | original poster as well. | |
240 | ||
241 | - Send a copy to the relevant mailing list, or, if nothing else applies, | |
242 | the linux-kernel list. | |
243 | ||
244 | - If you are fixing a bug, think about whether the fix should go into the | |
245 | next stable update. If so, stable@kernel.org should get a copy of the | |
246 | patch. Also add a "Cc: stable@kernel.org" to the tags within the patch | |
247 | itself; that will cause the stable team to get a notification when your | |
248 | fix goes into the mainline. | |
249 | ||
250 | When selecting recipients for a patch, it is good to have an idea of who | |
251 | you think will eventually accept the patch and get it merged. While it | |
252 | is possible to send patches directly to Linus Torvalds and have him merge | |
253 | them, things are not normally done that way. Linus is busy, and there are | |
254 | subsystem maintainers who watch over specific parts of the kernel. Usually | |
255 | you will be wanting that maintainer to merge your patches. If there is no | |
256 | obvious maintainer, Andrew Morton is often the patch target of last resort. | |
257 | ||
258 | Patches need good subject lines. The canonical format for a patch line is | |
259 | something like: | |
260 | ||
261 | [PATCH nn/mm] subsys: one-line description of the patch | |
262 | ||
263 | where "nn" is the ordinal number of the patch, "mm" is the total number of | |
264 | patches in the series, and "subsys" is the name of the affected subsystem. | |
265 | Clearly, nn/mm can be omitted for a single, standalone patch. | |
266 | ||
267 | If you have a significant series of patches, it is customary to send an | |
268 | introductory description as part zero. This convention is not universally | |
269 | followed though; if you use it, remember that information in the | |
270 | introduction does not make it into the kernel changelogs. So please ensure | |
271 | that the patches, themselves, have complete changelog information. | |
272 | ||
273 | In general, the second and following parts of a multi-part patch should be | |
274 | sent as a reply to the first part so that they all thread together at the | |
275 | receiving end. Tools like git and quilt have commands to mail out a set of | |
276 | patches with the proper threading. If you have a long series, though, and | |
277 | are using git, please provide the --no-chain-reply-to option to avoid | |
278 | creating exceptionally deep nesting. |