1.. _development_posting: 2 3Posting patches 4=============== 5 6Sooner or later, the time comes when your work is ready to be presented to 7the community for review and, eventually, inclusion into the mainline 8kernel. Unsurprisingly, the kernel development community has evolved a set 9of conventions and procedures which are used in the posting of patches; 10following them will make life much easier for everybody involved. This 11document will attempt to cover these expectations in reasonable detail; 12more information can also be found in the files 13:ref:`Documentation/process/submitting-patches.rst <submittingpatches>` 14and :ref:`Documentation/process/submit-checklist.rst <submitchecklist>`. 15 16 17When to post 18------------ 19 20There is a constant temptation to avoid posting patches before they are 21completely "ready." For simple patches, that is not a problem. If the 22work being done is complex, though, there is a lot to be gained by getting 23feedback from the community before the work is complete. So you should 24consider posting in-progress work, or even making a git tree available so 25that interested developers can catch up with your work at any time. 26 27When posting code which is not yet considered ready for inclusion, it is a 28good idea to say so in the posting itself. Also mention any major work 29which remains to be done and any known problems. Fewer people will look at 30patches which are known to be half-baked, but those who do will come in 31with the idea that they can help you drive the work in the right direction. 32 33 34Before creating patches 35----------------------- 36 37There are a number of things which should be done before you consider 38sending 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 56As a general rule, putting in some extra thought before posting code almost 57always pays back the effort in short order. 58 59 60Patch preparation 61----------------- 62 63The preparation of patches for posting can be a surprising amount of work, 64but, once again, attempting to save time here is not generally advisable 65even in the short term. 66 67Patches must be prepared against a specific version of the kernel. As a 68general rule, a patch should be based on the current mainline as found in 69Linus's git tree. When basing on mainline, start with a well-known release 70point - a stable or -rc release - rather than branching off the mainline at 71an arbitrary spot. 72 73It may become necessary to make versions against -mm, linux-next, or a 74subsystem tree, though, to facilitate wider testing and review. Depending 75on the area of your patch and what is going on elsewhere, basing a patch 76against these other trees can require a significant amount of work 77resolving conflicts and dealing with API changes. 78 79Only the most simple changes should be formatted as a single patch; 80everything else should be made as a logical series of changes. Splitting 81up patches is a bit of an art; some developers spend a long time figuring 82out how to do it in the way that the community expects. There are a few 83rules 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 126Working to create the perfect patch series can be a frustrating process 127which takes quite a bit of time and thought after the "real work" has been 128done. When done properly, though, it is time well spent. 129 130 131Patch formatting and changelogs 132------------------------------- 133 134So now you have a perfect series of patches for posting, but the work is 135not done quite yet. Each patch needs to be formatted into a message which 136quickly and clearly communicates its purpose to the rest of the world. To 137that 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 161The items above, together, form the changelog for the patch. Writing good 162changelogs is a crucial but often-neglected art; it's worth spending 163another moment discussing this issue. When writing a changelog, you should 164bear in mind that a number of different people will be reading your words. 165These include subsystem maintainers and reviewers who need to decide 166whether the patch should be included, distributors and other maintainers 167trying to decide whether a patch should be backported to other kernels, bug 168hunters wondering whether the patch is responsible for a problem they are 169chasing, users who want to know how the kernel has changed, and more. A 170good changelog conveys the needed information to all of these people in the 171most direct and concise way possible. 172 173To that end, the summary line should describe the effects of and motivation 174for the change as well as possible given the one-line constraint. The 175detailed description can then amplify on those topics and provide any 176needed additional information. If the patch fixes a bug, cite the commit 177which introduced the bug if possible (and please provide both the commit ID 178and the title when citing commits). If a problem is associated with 179specific log or compiler output, include that output to help others 180searching for a solution to the same problem. If the change is meant to 181support other changes coming in later patch, say so. If internal APIs are 182changed, detail those changes and how other developers should respond. In 183general, the more you can put yourself into the shoes of everybody who will 184be reading your changelog, the better that changelog (and the kernel as a 185whole) will be. 186 187Needless to say, the changelog should be the text used when committing the 188change 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 194You should avoid including changes to irrelevant files (those generated by 195the build process, for example, or editor backup files) in the patch. The 196file "dontdiff" in the Documentation directory can help in this regard; 197pass it to diff with the "-X" option. 198 199The tags already briefly mentioned above are used to provide insights how 200the patch came into being. They are described in detail in the 201:ref:`Documentation/process/submitting-patches.rst <submittingpatches>` 202document; what follows here is a brief summary. 203 204One tag is used to refer to earlier commits which introduced problems fixed by 205the patch:: 206 207 Fixes: 1f2e3d4c5b6a ("The first line of the commit specified by the first 12 characters of its SHA-1 ID") 208 209Another tag is used for linking web pages with additional backgrounds or 210details, for example an earlier discussion which leads to the patch or a 211document with a specification implemented by the patch:: 212 213 Link: https://example.com/somewhere.html optional-other-stuff 214 215Many maintainers when applying a patch also add this tag to link to the 216latest public review posting of the patch; often this is automatically done 217by tools like b4 or a git hook like the one described in 218'Documentation/maintainer/configure-git.rst'. 219 220If 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 225Some bug trackers have the ability to close issues automatically when a 226commit with such a tag is applied. Some bots monitoring mailing lists can 227also track such tags and take certain actions. Private bug trackers and 228invalid URLs are forbidden. 229 230Another kind of tag is used to document who was involved in the development of 231the patch. Each of these uses this format:: 232 233 tag: Full Name <email address> optional-other-stuff 234 235The 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 272Be careful in the addition of tags to your patches, as only Cc: is appropriate 273for addition without the explicit permission of the person named; using 274Reported-by: is fine most of the time as well, but ask for permission if 275the bug was reported in private. 276 277 278Sending the patch 279----------------- 280 281Before you mail your patches, there are a couple of other things you should 282take 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 300Patches should always be sent as plain text. Please do not send them as 301attachments; that makes it much harder for reviewers to quote sections of 302the patch in their replies. Instead, just put the patch directly into your 303message. 304 305When mailing patches, it is important to send copies to anybody who might 306be interested in it. Unlike some other projects, the kernel encourages 307people to err on the side of sending too many copies; don't assume that the 308relevant people will see your posting on the mailing lists. In particular, 309copies 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 330When selecting recipients for a patch, it is good to have an idea of who 331you think will eventually accept the patch and get it merged. While it 332is possible to send patches directly to Linus Torvalds and have him merge 333them, things are not normally done that way. Linus is busy, and there are 334subsystem maintainers who watch over specific parts of the kernel. Usually 335you will be wanting that maintainer to merge your patches. If there is no 336obvious maintainer, Andrew Morton is often the patch target of last resort. 337 338Patches need good subject lines. The canonical format for a patch line is 339something like: 340 341:: 342 343 [PATCH nn/mm] subsys: one-line description of the patch 344 345where "nn" is the ordinal number of the patch, "mm" is the total number of 346patches in the series, and "subsys" is the name of the affected subsystem. 347Clearly, nn/mm can be omitted for a single, standalone patch. 348 349If you have a significant series of patches, it is customary to send an 350introductory description as part zero. This convention is not universally 351followed though; if you use it, remember that information in the 352introduction does not make it into the kernel changelogs. So please ensure 353that the patches, themselves, have complete changelog information. 354 355In general, the second and following parts of a multi-part patch should be 356sent as a reply to the first part so that they all thread together at the 357receiving end. Tools like git and quilt have commands to mail out a set of 358patches with the proper threading. If you have a long series, though, and 359are using git, please stay away from the --chain-reply-to option to avoid 360creating exceptionally deep nesting. 361