Based on kernel version 4.9. Page generated on 2016-12-21 14:28 EST.
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 SubmittingPatches, 13 SubmittingDrivers, and SubmitChecklist in the kernel documentation 14 directory. 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 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 SubmittingPatches document; what follows here is a brief 202 summary. Each of these lines has the format: 203 204 :: 205 206 tag: Full Name <email address> optional-other-stuff 207 208 The tags in common use are: 209 210 - 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/SubmittingPatches. Code without a 214 proper signoff cannot be merged into the mainline. 215 216 - Acked-by: indicates an agreement by another developer (often a 217 maintainer of the relevant code) that the patch is appropriate for 218 inclusion into the kernel. 219 220 - Tested-by: states that the named person has tested the patch and found 221 it to work. 222 223 - Reviewed-by: the named developer has reviewed the patch for correctness; 224 see the reviewer's statement in Documentation/SubmittingPatches for more 225 detail. 226 227 - Reported-by: names a user who reported a problem which is fixed by this 228 patch; this tag is used to give credit to the (often underappreciated) 229 people who test our code and let us know when things do not work 230 correctly. 231 232 - Cc: the named person received a copy of the patch and had the 233 opportunity to comment on it. 234 235 Be careful in the addition of tags to your patches: only Cc: is appropriate 236 for addition without the explicit permission of the person named. 237 238 239 Sending the patch 240 ----------------- 241 242 Before you mail your patches, there are a couple of other things you should 243 take care of: 244 245 - Are you sure that your mailer will not corrupt the patches? Patches 246 which have had gratuitous white-space changes or line wrapping performed 247 by the mail client will not apply at the other end, and often will not 248 be examined in any detail. If there is any doubt at all, mail the patch 249 to yourself and convince yourself that it shows up intact. 250 251 Documentation/email-clients.txt has some helpful hints on making 252 specific mail clients work for sending patches. 253 254 - Are you sure your patch is free of silly mistakes? You should always 255 run patches through scripts/checkpatch.pl and address the complaints it 256 comes up with. Please bear in mind that checkpatch.pl, while being the 257 embodiment of a fair amount of thought about what kernel patches should 258 look like, is not smarter than you. If fixing a checkpatch.pl complaint 259 would make the code worse, don't do it. 260 261 Patches should always be sent as plain text. Please do not send them as 262 attachments; that makes it much harder for reviewers to quote sections of 263 the patch in their replies. Instead, just put the patch directly into your 264 message. 265 266 When mailing patches, it is important to send copies to anybody who might 267 be interested in it. Unlike some other projects, the kernel encourages 268 people to err on the side of sending too many copies; don't assume that the 269 relevant people will see your posting on the mailing lists. In particular, 270 copies should go to: 271 272 - The maintainer(s) of the affected subsystem(s). As described earlier, 273 the MAINTAINERS file is the first place to look for these people. 274 275 - Other developers who have been working in the same area - especially 276 those who might be working there now. Using git to see who else has 277 modified the files you are working on can be helpful. 278 279 - If you are responding to a bug report or a feature request, copy the 280 original poster as well. 281 282 - Send a copy to the relevant mailing list, or, if nothing else applies, 283 the linux-kernel list. 284 285 - If you are fixing a bug, think about whether the fix should go into the 286 next stable update. If so, stable@vger.kernel.org should get a copy of 287 the patch. Also add a "Cc: stable@vger.kernel.org" to the tags within 288 the patch itself; that will cause the stable team to get a notification 289 when your fix goes into the mainline. 290 291 When selecting recipients for a patch, it is good to have an idea of who 292 you think will eventually accept the patch and get it merged. While it 293 is possible to send patches directly to Linus Torvalds and have him merge 294 them, things are not normally done that way. Linus is busy, and there are 295 subsystem maintainers who watch over specific parts of the kernel. Usually 296 you will be wanting that maintainer to merge your patches. If there is no 297 obvious maintainer, Andrew Morton is often the patch target of last resort. 298 299 Patches need good subject lines. The canonical format for a patch line is 300 something like: 301 302 :: 303 304 [PATCH nn/mm] subsys: one-line description of the patch 305 306 where "nn" is the ordinal number of the patch, "mm" is the total number of 307 patches in the series, and "subsys" is the name of the affected subsystem. 308 Clearly, nn/mm can be omitted for a single, standalone patch. 309 310 If you have a significant series of patches, it is customary to send an 311 introductory description as part zero. This convention is not universally 312 followed though; if you use it, remember that information in the 313 introduction does not make it into the kernel changelogs. So please ensure 314 that the patches, themselves, have complete changelog information. 315 316 In general, the second and following parts of a multi-part patch should be 317 sent as a reply to the first part so that they all thread together at the 318 receiving end. Tools like git and quilt have commands to mail out a set of 319 patches with the proper threading. If you have a long series, though, and 320 are using git, please stay away from the --chain-reply-to option to avoid 321 creating exceptionally deep nesting.