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Linux/Documentation/SubmittingPatches

  1 .. _submittingpatches:
  2 
  3 How to Get Your Change Into the Linux Kernel or Care And Operation Of Your Linus Torvalds
  4 =========================================================================================
  5 
  6 For a person or company who wishes to submit a change to the Linux
  7 kernel, the process can sometimes be daunting if you're not familiar
  8 with "the system."  This text is a collection of suggestions which
  9 can greatly increase the chances of your change being accepted.
 10 
 11 This document contains a large number of suggestions in a relatively terse
 12 format.  For detailed information on how the kernel development process
 13 works, see :ref:`Documentation/development-process <development_process_main>`.
 14 Also, read :ref:`Documentation/SubmitChecklist <submitchecklist>`
 15 for a list of items to check before
 16 submitting code.  If you are submitting a driver, also read
 17 :ref:`Documentation/SubmittingDrivers <submittingdrivers>`;
 18 for device tree binding patches, read
 19 Documentation/devicetree/bindings/submitting-patches.txt.
 20 
 21 Many of these steps describe the default behavior of the ``git`` version
 22 control system; if you use ``git`` to prepare your patches, you'll find much
 23 of the mechanical work done for you, though you'll still need to prepare
 24 and document a sensible set of patches.  In general, use of ``git`` will make
 25 your life as a kernel developer easier.
 26 
 27 Creating and Sending your Change
 28 ********************************
 29 
 30 
 31 0) Obtain a current source tree
 32 -------------------------------
 33 
 34 If you do not have a repository with the current kernel source handy, use
 35 ``git`` to obtain one.  You'll want to start with the mainline repository,
 36 which can be grabbed with::
 37 
 38   git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
 39 
 40 Note, however, that you may not want to develop against the mainline tree
 41 directly.  Most subsystem maintainers run their own trees and want to see
 42 patches prepared against those trees.  See the **T:** entry for the subsystem
 43 in the MAINTAINERS file to find that tree, or simply ask the maintainer if
 44 the tree is not listed there.
 45 
 46 It is still possible to download kernel releases via tarballs (as described
 47 in the next section), but that is the hard way to do kernel development.
 48 
 49 1) ``diff -up``
 50 ---------------
 51 
 52 If you must generate your patches by hand, use ``diff -up`` or ``diff -uprN``
 53 to create patches.  Git generates patches in this form by default; if
 54 you're using ``git``, you can skip this section entirely.
 55 
 56 All changes to the Linux kernel occur in the form of patches, as
 57 generated by :manpage:`diff(1)`.  When creating your patch, make sure to
 58 create it in "unified diff" format, as supplied by the ``-u`` argument
 59 to :manpage:`diff(1)`.
 60 Also, please use the ``-p`` argument which shows which C function each
 61 change is in - that makes the resultant ``diff`` a lot easier to read.
 62 Patches should be based in the root kernel source directory,
 63 not in any lower subdirectory.
 64 
 65 To create a patch for a single file, it is often sufficient to do::
 66 
 67         SRCTREE= linux
 68         MYFILE=  drivers/net/mydriver.c
 69 
 70         cd $SRCTREE
 71         cp $MYFILE $MYFILE.orig
 72         vi $MYFILE      # make your change
 73         cd ..
 74         diff -up $SRCTREE/$MYFILE{.orig,} > /tmp/patch
 75 
 76 To create a patch for multiple files, you should unpack a "vanilla",
 77 or unmodified kernel source tree, and generate a ``diff`` against your
 78 own source tree.  For example::
 79 
 80         MYSRC= /devel/linux
 81 
 82         tar xvfz linux-3.19.tar.gz
 83         mv linux-3.19 linux-3.19-vanilla
 84         diff -uprN -X linux-3.19-vanilla/Documentation/dontdiff \
 85                 linux-3.19-vanilla $MYSRC > /tmp/patch
 86 
 87 ``dontdiff`` is a list of files which are generated by the kernel during
 88 the build process, and should be ignored in any :manpage:`diff(1)`-generated
 89 patch.
 90 
 91 Make sure your patch does not include any extra files which do not
 92 belong in a patch submission.  Make sure to review your patch -after-
 93 generating it with :manpage:`diff(1)`, to ensure accuracy.
 94 
 95 If your changes produce a lot of deltas, you need to split them into
 96 individual patches which modify things in logical stages; see
 97 :ref:`split_changes`.  This will facilitate review by other kernel developers,
 98 very important if you want your patch accepted.
 99 
100 If you're using ``git``, ``git rebase -i`` can help you with this process.  If
101 you're not using ``git``, ``quilt`` <http://savannah.nongnu.org/projects/quilt>
102 is another popular alternative.
103 
104 .. _describe_changes:
105 
106 2) Describe your changes
107 ------------------------
108 
109 Describe your problem.  Whether your patch is a one-line bug fix or
110 5000 lines of a new feature, there must be an underlying problem that
111 motivated you to do this work.  Convince the reviewer that there is a
112 problem worth fixing and that it makes sense for them to read past the
113 first paragraph.
114 
115 Describe user-visible impact.  Straight up crashes and lockups are
116 pretty convincing, but not all bugs are that blatant.  Even if the
117 problem was spotted during code review, describe the impact you think
118 it can have on users.  Keep in mind that the majority of Linux
119 installations run kernels from secondary stable trees or
120 vendor/product-specific trees that cherry-pick only specific patches
121 from upstream, so include anything that could help route your change
122 downstream: provoking circumstances, excerpts from dmesg, crash
123 descriptions, performance regressions, latency spikes, lockups, etc.
124 
125 Quantify optimizations and trade-offs.  If you claim improvements in
126 performance, memory consumption, stack footprint, or binary size,
127 include numbers that back them up.  But also describe non-obvious
128 costs.  Optimizations usually aren't free but trade-offs between CPU,
129 memory, and readability; or, when it comes to heuristics, between
130 different workloads.  Describe the expected downsides of your
131 optimization so that the reviewer can weigh costs against benefits.
132 
133 Once the problem is established, describe what you are actually doing
134 about it in technical detail.  It's important to describe the change
135 in plain English for the reviewer to verify that the code is behaving
136 as you intend it to.
137 
138 The maintainer will thank you if you write your patch description in a
139 form which can be easily pulled into Linux's source code management
140 system, ``git``, as a "commit log".  See :ref:`explicit_in_reply_to`.
141 
142 Solve only one problem per patch.  If your description starts to get
143 long, that's a sign that you probably need to split up your patch.
144 See :ref:`split_changes`.
145 
146 When you submit or resubmit a patch or patch series, include the
147 complete patch description and justification for it.  Don't just
148 say that this is version N of the patch (series).  Don't expect the
149 subsystem maintainer to refer back to earlier patch versions or referenced
150 URLs to find the patch description and put that into the patch.
151 I.e., the patch (series) and its description should be self-contained.
152 This benefits both the maintainers and reviewers.  Some reviewers
153 probably didn't even receive earlier versions of the patch.
154 
155 Describe your changes in imperative mood, e.g. "make xyzzy do frotz"
156 instead of "[This patch] makes xyzzy do frotz" or "[I] changed xyzzy
157 to do frotz", as if you are giving orders to the codebase to change
158 its behaviour.
159 
160 If the patch fixes a logged bug entry, refer to that bug entry by
161 number and URL.  If the patch follows from a mailing list discussion,
162 give a URL to the mailing list archive; use the https://lkml.kernel.org/
163 redirector with a ``Message-Id``, to ensure that the links cannot become
164 stale.
165 
166 However, try to make your explanation understandable without external
167 resources.  In addition to giving a URL to a mailing list archive or
168 bug, summarize the relevant points of the discussion that led to the
169 patch as submitted.
170 
171 If you want to refer to a specific commit, don't just refer to the
172 SHA-1 ID of the commit. Please also include the oneline summary of
173 the commit, to make it easier for reviewers to know what it is about.
174 Example::
175 
176         Commit e21d2170f36602ae2708 ("video: remove unnecessary
177         platform_set_drvdata()") removed the unnecessary
178         platform_set_drvdata(), but left the variable "dev" unused,
179         delete it.
180 
181 You should also be sure to use at least the first twelve characters of the
182 SHA-1 ID.  The kernel repository holds a *lot* of objects, making
183 collisions with shorter IDs a real possibility.  Bear in mind that, even if
184 there is no collision with your six-character ID now, that condition may
185 change five years from now.
186 
187 If your patch fixes a bug in a specific commit, e.g. you found an issue using
188 ``git bisect``, please use the 'Fixes:' tag with the first 12 characters of
189 the SHA-1 ID, and the one line summary.  For example::
190 
191         Fixes: e21d2170f366 ("video: remove unnecessary platform_set_drvdata()")
192 
193 The following ``git config`` settings can be used to add a pretty format for
194 outputting the above style in the ``git log`` or ``git show`` commands::
195 
196         [core]
197                 abbrev = 12
198         [pretty]
199                 fixes = Fixes: %h (\"%s\")
200 
201 .. _split_changes:
202 
203 3) Separate your changes
204 ------------------------
205 
206 Separate each **logical change** into a separate patch.
207 
208 For example, if your changes include both bug fixes and performance
209 enhancements for a single driver, separate those changes into two
210 or more patches.  If your changes include an API update, and a new
211 driver which uses that new API, separate those into two patches.
212 
213 On the other hand, if you make a single change to numerous files,
214 group those changes into a single patch.  Thus a single logical change
215 is contained within a single patch.
216 
217 The point to remember is that each patch should make an easily understood
218 change that can be verified by reviewers.  Each patch should be justifiable
219 on its own merits.
220 
221 If one patch depends on another patch in order for a change to be
222 complete, that is OK.  Simply note **"this patch depends on patch X"**
223 in your patch description.
224 
225 When dividing your change into a series of patches, take special care to
226 ensure that the kernel builds and runs properly after each patch in the
227 series.  Developers using ``git bisect`` to track down a problem can end up
228 splitting your patch series at any point; they will not thank you if you
229 introduce bugs in the middle.
230 
231 If you cannot condense your patch set into a smaller set of patches,
232 then only post say 15 or so at a time and wait for review and integration.
233 
234 
235 
236 4) Style-check your changes
237 ---------------------------
238 
239 Check your patch for basic style violations, details of which can be
240 found in
241 :ref:`Documentation/CodingStyle <codingstyle>`.
242 Failure to do so simply wastes
243 the reviewers time and will get your patch rejected, probably
244 without even being read.
245 
246 One significant exception is when moving code from one file to
247 another -- in this case you should not modify the moved code at all in
248 the same patch which moves it.  This clearly delineates the act of
249 moving the code and your changes.  This greatly aids review of the
250 actual differences and allows tools to better track the history of
251 the code itself.
252 
253 Check your patches with the patch style checker prior to submission
254 (scripts/checkpatch.pl).  Note, though, that the style checker should be
255 viewed as a guide, not as a replacement for human judgment.  If your code
256 looks better with a violation then its probably best left alone.
257 
258 The checker reports at three levels:
259  - ERROR: things that are very likely to be wrong
260  - WARNING: things requiring careful review
261  - CHECK: things requiring thought
262 
263 You should be able to justify all violations that remain in your
264 patch.
265 
266 
267 5) Select the recipients for your patch
268 ---------------------------------------
269 
270 You should always copy the appropriate subsystem maintainer(s) on any patch
271 to code that they maintain; look through the MAINTAINERS file and the
272 source code revision history to see who those maintainers are.  The
273 script scripts/get_maintainer.pl can be very useful at this step.  If you
274 cannot find a maintainer for the subsystem you are working on, Andrew
275 Morton (akpm@linux-foundation.org) serves as a maintainer of last resort.
276 
277 You should also normally choose at least one mailing list to receive a copy
278 of your patch set.  linux-kernel@vger.kernel.org functions as a list of
279 last resort, but the volume on that list has caused a number of developers
280 to tune it out.  Look in the MAINTAINERS file for a subsystem-specific
281 list; your patch will probably get more attention there.  Please do not
282 spam unrelated lists, though.
283 
284 Many kernel-related lists are hosted on vger.kernel.org; you can find a
285 list of them at http://vger.kernel.org/vger-lists.html.  There are
286 kernel-related lists hosted elsewhere as well, though.
287 
288 Do not send more than 15 patches at once to the vger mailing lists!!!
289 
290 Linus Torvalds is the final arbiter of all changes accepted into the
291 Linux kernel.  His e-mail address is <torvalds@linux-foundation.org>.
292 He gets a lot of e-mail, and, at this point, very few patches go through
293 Linus directly, so typically you should do your best to -avoid-
294 sending him e-mail.
295 
296 If you have a patch that fixes an exploitable security bug, send that patch
297 to security@kernel.org.  For severe bugs, a short embargo may be considered
298 to allow distributors to get the patch out to users; in such cases,
299 obviously, the patch should not be sent to any public lists.
300 
301 Patches that fix a severe bug in a released kernel should be directed
302 toward the stable maintainers by putting a line like this::
303 
304   Cc: stable@vger.kernel.org
305 
306 into the sign-off area of your patch (note, NOT an email recipient).  You
307 should also read
308 :ref:`Documentation/stable_kernel_rules.txt <stable_kernel_rules>`
309 in addition to this file.
310 
311 Note, however, that some subsystem maintainers want to come to their own
312 conclusions on which patches should go to the stable trees.  The networking
313 maintainer, in particular, would rather not see individual developers
314 adding lines like the above to their patches.
315 
316 If changes affect userland-kernel interfaces, please send the MAN-PAGES
317 maintainer (as listed in the MAINTAINERS file) a man-pages patch, or at
318 least a notification of the change, so that some information makes its way
319 into the manual pages.  User-space API changes should also be copied to
320 linux-api@vger.kernel.org.
321 
322 For small patches you may want to CC the Trivial Patch Monkey
323 trivial@kernel.org which collects "trivial" patches. Have a look
324 into the MAINTAINERS file for its current manager.
325 
326 Trivial patches must qualify for one of the following rules:
327 
328 - Spelling fixes in documentation
329 - Spelling fixes for errors which could break :manpage:`grep(1)`
330 - Warning fixes (cluttering with useless warnings is bad)
331 - Compilation fixes (only if they are actually correct)
332 - Runtime fixes (only if they actually fix things)
333 - Removing use of deprecated functions/macros
334 - Contact detail and documentation fixes
335 - Non-portable code replaced by portable code (even in arch-specific,
336   since people copy, as long as it's trivial)
337 - Any fix by the author/maintainer of the file (ie. patch monkey
338   in re-transmission mode)
339 
340 
341 
342 6) No MIME, no links, no compression, no attachments.  Just plain text
343 ----------------------------------------------------------------------
344 
345 Linus and other kernel developers need to be able to read and comment
346 on the changes you are submitting.  It is important for a kernel
347 developer to be able to "quote" your changes, using standard e-mail
348 tools, so that they may comment on specific portions of your code.
349 
350 For this reason, all patches should be submitted by e-mail "inline".
351 
352 .. warning::
353 
354   Be wary of your editor's word-wrap corrupting your patch,
355   if you choose to cut-n-paste your patch.
356 
357 Do not attach the patch as a MIME attachment, compressed or not.
358 Many popular e-mail applications will not always transmit a MIME
359 attachment as plain text, making it impossible to comment on your
360 code.  A MIME attachment also takes Linus a bit more time to process,
361 decreasing the likelihood of your MIME-attached change being accepted.
362 
363 Exception:  If your mailer is mangling patches then someone may ask
364 you to re-send them using MIME.
365 
366 See :ref:`Documentation/email-clients.txt <email_clients>`
367 for hints about configuring your e-mail client so that it sends your patches
368 untouched.
369 
370 7) E-mail size
371 --------------
372 
373 Large changes are not appropriate for mailing lists, and some
374 maintainers.  If your patch, uncompressed, exceeds 300 kB in size,
375 it is preferred that you store your patch on an Internet-accessible
376 server, and provide instead a URL (link) pointing to your patch.  But note
377 that if your patch exceeds 300 kB, it almost certainly needs to be broken up
378 anyway.
379 
380 8) Respond to review comments
381 -----------------------------
382 
383 Your patch will almost certainly get comments from reviewers on ways in
384 which the patch can be improved.  You must respond to those comments;
385 ignoring reviewers is a good way to get ignored in return.  Review comments
386 or questions that do not lead to a code change should almost certainly
387 bring about a comment or changelog entry so that the next reviewer better
388 understands what is going on.
389 
390 Be sure to tell the reviewers what changes you are making and to thank them
391 for their time.  Code review is a tiring and time-consuming process, and
392 reviewers sometimes get grumpy.  Even in that case, though, respond
393 politely and address the problems they have pointed out.
394 
395 
396 9) Don't get discouraged - or impatient
397 ---------------------------------------
398 
399 After you have submitted your change, be patient and wait.  Reviewers are
400 busy people and may not get to your patch right away.
401 
402 Once upon a time, patches used to disappear into the void without comment,
403 but the development process works more smoothly than that now.  You should
404 receive comments within a week or so; if that does not happen, make sure
405 that you have sent your patches to the right place.  Wait for a minimum of
406 one week before resubmitting or pinging reviewers - possibly longer during
407 busy times like merge windows.
408 
409 
410 10) Include PATCH in the subject
411 --------------------------------
412 
413 Due to high e-mail traffic to Linus, and to linux-kernel, it is common
414 convention to prefix your subject line with [PATCH].  This lets Linus
415 and other kernel developers more easily distinguish patches from other
416 e-mail discussions.
417 
418 
419 
420 11) Sign your work
421 ------------------
422 
423 To improve tracking of who did what, especially with patches that can
424 percolate to their final resting place in the kernel through several
425 layers of maintainers, we've introduced a "sign-off" procedure on
426 patches that are being emailed around.
427 
428 The sign-off is a simple line at the end of the explanation for the
429 patch, which certifies that you wrote it or otherwise have the right to
430 pass it on as an open-source patch.  The rules are pretty simple: if you
431 can certify the below:
432 
433 Developer's Certificate of Origin 1.1
434 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
435 
436 By making a contribution to this project, I certify that:
437 
438         (a) The contribution was created in whole or in part by me and I
439             have the right to submit it under the open source license
440             indicated in the file; or
441 
442         (b) The contribution is based upon previous work that, to the best
443             of my knowledge, is covered under an appropriate open source
444             license and I have the right under that license to submit that
445             work with modifications, whether created in whole or in part
446             by me, under the same open source license (unless I am
447             permitted to submit under a different license), as indicated
448             in the file; or
449 
450         (c) The contribution was provided directly to me by some other
451             person who certified (a), (b) or (c) and I have not modified
452             it.
453 
454         (d) I understand and agree that this project and the contribution
455             are public and that a record of the contribution (including all
456             personal information I submit with it, including my sign-off) is
457             maintained indefinitely and may be redistributed consistent with
458             this project or the open source license(s) involved.
459 
460 then you just add a line saying::
461 
462         Signed-off-by: Random J Developer <random@developer.example.org>
463 
464 using your real name (sorry, no pseudonyms or anonymous contributions.)
465 
466 Some people also put extra tags at the end.  They'll just be ignored for
467 now, but you can do this to mark internal company procedures or just
468 point out some special detail about the sign-off.
469 
470 If you are a subsystem or branch maintainer, sometimes you need to slightly
471 modify patches you receive in order to merge them, because the code is not
472 exactly the same in your tree and the submitters'. If you stick strictly to
473 rule (c), you should ask the submitter to rediff, but this is a totally
474 counter-productive waste of time and energy. Rule (b) allows you to adjust
475 the code, but then it is very impolite to change one submitter's code and
476 make him endorse your bugs. To solve this problem, it is recommended that
477 you add a line between the last Signed-off-by header and yours, indicating
478 the nature of your changes. While there is nothing mandatory about this, it
479 seems like prepending the description with your mail and/or name, all
480 enclosed in square brackets, is noticeable enough to make it obvious that
481 you are responsible for last-minute changes. Example::
482 
483         Signed-off-by: Random J Developer <random@developer.example.org>
484         [lucky@maintainer.example.org: struct foo moved from foo.c to foo.h]
485         Signed-off-by: Lucky K Maintainer <lucky@maintainer.example.org>
486 
487 This practice is particularly helpful if you maintain a stable branch and
488 want at the same time to credit the author, track changes, merge the fix,
489 and protect the submitter from complaints. Note that under no circumstances
490 can you change the author's identity (the From header), as it is the one
491 which appears in the changelog.
492 
493 Special note to back-porters: It seems to be a common and useful practice
494 to insert an indication of the origin of a patch at the top of the commit
495 message (just after the subject line) to facilitate tracking. For instance,
496 here's what we see in a 3.x-stable release::
497 
498   Date:   Tue Oct 7 07:26:38 2014 -0400
499 
500     libata: Un-break ATA blacklist
501 
502     commit 1c40279960bcd7d52dbdf1d466b20d24b99176c8 upstream.
503 
504 And here's what might appear in an older kernel once a patch is backported::
505 
506     Date:   Tue May 13 22:12:27 2008 +0200
507 
508         wireless, airo: waitbusy() won't delay
509 
510         [backport of 2.6 commit b7acbdfbd1f277c1eb23f344f899cfa4cd0bf36a]
511 
512 Whatever the format, this information provides a valuable help to people
513 tracking your trees, and to people trying to troubleshoot bugs in your
514 tree.
515 
516 
517 12) When to use Acked-by: and Cc:
518 ---------------------------------
519 
520 The Signed-off-by: tag indicates that the signer was involved in the
521 development of the patch, or that he/she was in the patch's delivery path.
522 
523 If a person was not directly involved in the preparation or handling of a
524 patch but wishes to signify and record their approval of it then they can
525 ask to have an Acked-by: line added to the patch's changelog.
526 
527 Acked-by: is often used by the maintainer of the affected code when that
528 maintainer neither contributed to nor forwarded the patch.
529 
530 Acked-by: is not as formal as Signed-off-by:.  It is a record that the acker
531 has at least reviewed the patch and has indicated acceptance.  Hence patch
532 mergers will sometimes manually convert an acker's "yep, looks good to me"
533 into an Acked-by: (but note that it is usually better to ask for an
534 explicit ack).
535 
536 Acked-by: does not necessarily indicate acknowledgement of the entire patch.
537 For example, if a patch affects multiple subsystems and has an Acked-by: from
538 one subsystem maintainer then this usually indicates acknowledgement of just
539 the part which affects that maintainer's code.  Judgement should be used here.
540 When in doubt people should refer to the original discussion in the mailing
541 list archives.
542 
543 If a person has had the opportunity to comment on a patch, but has not
544 provided such comments, you may optionally add a ``Cc:`` tag to the patch.
545 This is the only tag which might be added without an explicit action by the
546 person it names - but it should indicate that this person was copied on the
547 patch.  This tag documents that potentially interested parties
548 have been included in the discussion.
549 
550 
551 13) Using Reported-by:, Tested-by:, Reviewed-by:, Suggested-by: and Fixes:
552 --------------------------------------------------------------------------
553 
554 The Reported-by tag gives credit to people who find bugs and report them and it
555 hopefully inspires them to help us again in the future.  Please note that if
556 the bug was reported in private, then ask for permission first before using the
557 Reported-by tag.
558 
559 A Tested-by: tag indicates that the patch has been successfully tested (in
560 some environment) by the person named.  This tag informs maintainers that
561 some testing has been performed, provides a means to locate testers for
562 future patches, and ensures credit for the testers.
563 
564 Reviewed-by:, instead, indicates that the patch has been reviewed and found
565 acceptable according to the Reviewer's Statement:
566 
567 Reviewer's statement of oversight
568 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
569 
570 By offering my Reviewed-by: tag, I state that:
571 
572          (a) I have carried out a technical review of this patch to
573              evaluate its appropriateness and readiness for inclusion into
574              the mainline kernel.
575 
576          (b) Any problems, concerns, or questions relating to the patch
577              have been communicated back to the submitter.  I am satisfied
578              with the submitter's response to my comments.
579 
580          (c) While there may be things that could be improved with this
581              submission, I believe that it is, at this time, (1) a
582              worthwhile modification to the kernel, and (2) free of known
583              issues which would argue against its inclusion.
584 
585          (d) While I have reviewed the patch and believe it to be sound, I
586              do not (unless explicitly stated elsewhere) make any
587              warranties or guarantees that it will achieve its stated
588              purpose or function properly in any given situation.
589 
590 A Reviewed-by tag is a statement of opinion that the patch is an
591 appropriate modification of the kernel without any remaining serious
592 technical issues.  Any interested reviewer (who has done the work) can
593 offer a Reviewed-by tag for a patch.  This tag serves to give credit to
594 reviewers and to inform maintainers of the degree of review which has been
595 done on the patch.  Reviewed-by: tags, when supplied by reviewers known to
596 understand the subject area and to perform thorough reviews, will normally
597 increase the likelihood of your patch getting into the kernel.
598 
599 A Suggested-by: tag indicates that the patch idea is suggested by the person
600 named and ensures credit to the person for the idea. Please note that this
601 tag should not be added without the reporter's permission, especially if the
602 idea was not posted in a public forum. That said, if we diligently credit our
603 idea reporters, they will, hopefully, be inspired to help us again in the
604 future.
605 
606 A Fixes: tag indicates that the patch fixes an issue in a previous commit. It
607 is used to make it easy to determine where a bug originated, which can help
608 review a bug fix. This tag also assists the stable kernel team in determining
609 which stable kernel versions should receive your fix. This is the preferred
610 method for indicating a bug fixed by the patch. See :ref:`describe_changes`
611 for more details.
612 
613 
614 14) The canonical patch format
615 ------------------------------
616 
617 This section describes how the patch itself should be formatted.  Note
618 that, if you have your patches stored in a ``git`` repository, proper patch
619 formatting can be had with ``git format-patch``.  The tools cannot create
620 the necessary text, though, so read the instructions below anyway.
621 
622 The canonical patch subject line is::
623 
624     Subject: [PATCH 001/123] subsystem: summary phrase
625 
626 The canonical patch message body contains the following:
627 
628   - A ``from`` line specifying the patch author (only needed if the person
629     sending the patch is not the author).
630 
631   - An empty line.
632 
633   - The body of the explanation, line wrapped at 75 columns, which will
634     be copied to the permanent changelog to describe this patch.
635 
636   - The ``Signed-off-by:`` lines, described above, which will
637     also go in the changelog.
638 
639   - A marker line containing simply ``---``.
640 
641   - Any additional comments not suitable for the changelog.
642 
643   - The actual patch (``diff`` output).
644 
645 The Subject line format makes it very easy to sort the emails
646 alphabetically by subject line - pretty much any email reader will
647 support that - since because the sequence number is zero-padded,
648 the numerical and alphabetic sort is the same.
649 
650 The ``subsystem`` in the email's Subject should identify which
651 area or subsystem of the kernel is being patched.
652 
653 The ``summary phrase`` in the email's Subject should concisely
654 describe the patch which that email contains.  The ``summary
655 phrase`` should not be a filename.  Do not use the same ``summary
656 phrase`` for every patch in a whole patch series (where a ``patch
657 series`` is an ordered sequence of multiple, related patches).
658 
659 Bear in mind that the ``summary phrase`` of your email becomes a
660 globally-unique identifier for that patch.  It propagates all the way
661 into the ``git`` changelog.  The ``summary phrase`` may later be used in
662 developer discussions which refer to the patch.  People will want to
663 google for the ``summary phrase`` to read discussion regarding that
664 patch.  It will also be the only thing that people may quickly see
665 when, two or three months later, they are going through perhaps
666 thousands of patches using tools such as ``gitk`` or ``git log
667 --oneline``.
668 
669 For these reasons, the ``summary`` must be no more than 70-75
670 characters, and it must describe both what the patch changes, as well
671 as why the patch might be necessary.  It is challenging to be both
672 succinct and descriptive, but that is what a well-written summary
673 should do.
674 
675 The ``summary phrase`` may be prefixed by tags enclosed in square
676 brackets: "Subject: [PATCH <tag>...] <summary phrase>".  The tags are
677 not considered part of the summary phrase, but describe how the patch
678 should be treated.  Common tags might include a version descriptor if
679 the multiple versions of the patch have been sent out in response to
680 comments (i.e., "v1, v2, v3"), or "RFC" to indicate a request for
681 comments.  If there are four patches in a patch series the individual
682 patches may be numbered like this: 1/4, 2/4, 3/4, 4/4.  This assures
683 that developers understand the order in which the patches should be
684 applied and that they have reviewed or applied all of the patches in
685 the patch series.
686 
687 A couple of example Subjects::
688 
689     Subject: [PATCH 2/5] ext2: improve scalability of bitmap searching
690     Subject: [PATCH v2 01/27] x86: fix eflags tracking
691 
692 The ``from`` line must be the very first line in the message body,
693 and has the form:
694 
695         From: Original Author <author@example.com>
696 
697 The ``from`` line specifies who will be credited as the author of the
698 patch in the permanent changelog.  If the ``from`` line is missing,
699 then the ``From:`` line from the email header will be used to determine
700 the patch author in the changelog.
701 
702 The explanation body will be committed to the permanent source
703 changelog, so should make sense to a competent reader who has long
704 since forgotten the immediate details of the discussion that might
705 have led to this patch.  Including symptoms of the failure which the
706 patch addresses (kernel log messages, oops messages, etc.) is
707 especially useful for people who might be searching the commit logs
708 looking for the applicable patch.  If a patch fixes a compile failure,
709 it may not be necessary to include _all_ of the compile failures; just
710 enough that it is likely that someone searching for the patch can find
711 it.  As in the ``summary phrase``, it is important to be both succinct as
712 well as descriptive.
713 
714 The ``---`` marker line serves the essential purpose of marking for patch
715 handling tools where the changelog message ends.
716 
717 One good use for the additional comments after the ``---`` marker is for
718 a ``diffstat``, to show what files have changed, and the number of
719 inserted and deleted lines per file.  A ``diffstat`` is especially useful
720 on bigger patches.  Other comments relevant only to the moment or the
721 maintainer, not suitable for the permanent changelog, should also go
722 here.  A good example of such comments might be ``patch changelogs``
723 which describe what has changed between the v1 and v2 version of the
724 patch.
725 
726 If you are going to include a ``diffstat`` after the ``---`` marker, please
727 use ``diffstat`` options ``-p 1 -w 70`` so that filenames are listed from
728 the top of the kernel source tree and don't use too much horizontal
729 space (easily fit in 80 columns, maybe with some indentation).  (``git``
730 generates appropriate diffstats by default.)
731 
732 See more details on the proper patch format in the following
733 references.
734 
735 .. _explicit_in_reply_to:
736 
737 15) Explicit In-Reply-To headers
738 --------------------------------
739 
740 It can be helpful to manually add In-Reply-To: headers to a patch
741 (e.g., when using ``git send-email``) to associate the patch with
742 previous relevant discussion, e.g. to link a bug fix to the email with
743 the bug report.  However, for a multi-patch series, it is generally
744 best to avoid using In-Reply-To: to link to older versions of the
745 series.  This way multiple versions of the patch don't become an
746 unmanageable forest of references in email clients.  If a link is
747 helpful, you can use the https://lkml.kernel.org/ redirector (e.g., in
748 the cover email text) to link to an earlier version of the patch series.
749 
750 
751 16) Sending ``git pull`` requests
752 ---------------------------------
753 
754 If you have a series of patches, it may be most convenient to have the
755 maintainer pull them directly into the subsystem repository with a
756 ``git pull`` operation.  Note, however, that pulling patches from a developer
757 requires a higher degree of trust than taking patches from a mailing list.
758 As a result, many subsystem maintainers are reluctant to take pull
759 requests, especially from new, unknown developers.  If in doubt you can use
760 the pull request as the cover letter for a normal posting of the patch
761 series, giving the maintainer the option of using either.
762 
763 A pull request should have [GIT] or [PULL] in the subject line.  The
764 request itself should include the repository name and the branch of
765 interest on a single line; it should look something like::
766 
767   Please pull from
768 
769       git://jdelvare.pck.nerim.net/jdelvare-2.6 i2c-for-linus
770 
771   to get these changes:
772 
773 A pull request should also include an overall message saying what will be
774 included in the request, a ``git shortlog`` listing of the patches
775 themselves, and a ``diffstat`` showing the overall effect of the patch series.
776 The easiest way to get all this information together is, of course, to let
777 ``git`` do it for you with the ``git request-pull`` command.
778 
779 Some maintainers (including Linus) want to see pull requests from signed
780 commits; that increases their confidence that the request actually came
781 from you.  Linus, in particular, will not pull from public hosting sites
782 like GitHub in the absence of a signed tag.
783 
784 The first step toward creating such tags is to make a GNUPG key and get it
785 signed by one or more core kernel developers.  This step can be hard for
786 new developers, but there is no way around it.  Attending conferences can
787 be a good way to find developers who can sign your key.
788 
789 Once you have prepared a patch series in ``git`` that you wish to have somebody
790 pull, create a signed tag with ``git tag -s``.  This will create a new tag
791 identifying the last commit in the series and containing a signature
792 created with your private key.  You will also have the opportunity to add a
793 changelog-style message to the tag; this is an ideal place to describe the
794 effects of the pull request as a whole.
795 
796 If the tree the maintainer will be pulling from is not the repository you
797 are working from, don't forget to push the signed tag explicitly to the
798 public tree.
799 
800 When generating your pull request, use the signed tag as the target.  A
801 command like this will do the trick::
802 
803   git request-pull master git://my.public.tree/linux.git my-signed-tag
804 
805 
806 REFERENCES
807 **********
808 
809 Andrew Morton, "The perfect patch" (tpp).
810   <http://www.ozlabs.org/~akpm/stuff/tpp.txt>
811 
812 Jeff Garzik, "Linux kernel patch submission format".
813   <http://linux.yyz.us/patch-format.html>
814 
815 Greg Kroah-Hartman, "How to piss off a kernel subsystem maintainer".
816   <http://www.kroah.com/log/linux/maintainer.html>
817 
818   <http://www.kroah.com/log/linux/maintainer-02.html>
819 
820   <http://www.kroah.com/log/linux/maintainer-03.html>
821 
822   <http://www.kroah.com/log/linux/maintainer-04.html>
823 
824   <http://www.kroah.com/log/linux/maintainer-05.html>
825 
826   <http://www.kroah.com/log/linux/maintainer-06.html>
827 
828 NO!!!! No more huge patch bombs to linux-kernel@vger.kernel.org people!
829   <https://lkml.org/lkml/2005/7/11/336>
830 
831 Kernel Documentation/CodingStyle:
832   :ref:`Documentation/CodingStyle <codingstyle>`
833 
834 Linus Torvalds's mail on the canonical patch format:
835   <http://lkml.org/lkml/2005/4/7/183>
836 
837 Andi Kleen, "On submitting kernel patches"
838   Some strategies to get difficult or controversial changes in.
839 
840   http://halobates.de/on-submitting-patches.pdf
841 

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