1.. _submittingpatches:
2
3Submitting patches: the essential guide to getting your code into the kernel
4============================================================================
5
6For a person or company who wishes to submit a change to the Linux
7kernel, the process can sometimes be daunting if you're not familiar
8with "the system."  This text is a collection of suggestions which
9can greatly increase the chances of your change being accepted.
10
11This document contains a large number of suggestions in a relatively terse
12format.  For detailed information on how the kernel development process
13works, see Documentation/process/development-process.rst. Also, read
14Documentation/process/submit-checklist.rst
15for a list of items to check before submitting code.
16For device tree binding patches, read
17Documentation/devicetree/bindings/submitting-patches.rst.
18
19This documentation assumes that you're using ``git`` to prepare your patches.
20If you're unfamiliar with ``git``, you would be well-advised to learn how to
21use it, it will make your life as a kernel developer and in general much
22easier.
23
24Some subsystems and maintainer trees have additional information about
25their workflow and expectations, see
26:ref:`Documentation/process/maintainer-handbooks.rst <maintainer_handbooks_main>`.
27
28Obtain a current source tree
29----------------------------
30
31If you do not have a repository with the current kernel source handy, use
32``git`` to obtain one.  You'll want to start with the mainline repository,
33which can be grabbed with::
34
35  git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
36
37Note, however, that you may not want to develop against the mainline tree
38directly.  Most subsystem maintainers run their own trees and want to see
39patches prepared against those trees.  See the **T:** entry for the subsystem
40in the MAINTAINERS file to find that tree, or simply ask the maintainer if
41the tree is not listed there.
42
43.. _describe_changes:
44
45Describe your changes
46---------------------
47
48Describe your problem.  Whether your patch is a one-line bug fix or
495000 lines of a new feature, there must be an underlying problem that
50motivated you to do this work.  Convince the reviewer that there is a
51problem worth fixing and that it makes sense for them to read past the
52first paragraph.
53
54Describe user-visible impact.  Straight up crashes and lockups are
55pretty convincing, but not all bugs are that blatant.  Even if the
56problem was spotted during code review, describe the impact you think
57it can have on users.  Keep in mind that the majority of Linux
58installations run kernels from secondary stable trees or
59vendor/product-specific trees that cherry-pick only specific patches
60from upstream, so include anything that could help route your change
61downstream: provoking circumstances, excerpts from dmesg, crash
62descriptions, performance regressions, latency spikes, lockups, etc.
63
64Quantify optimizations and trade-offs.  If you claim improvements in
65performance, memory consumption, stack footprint, or binary size,
66include numbers that back them up.  But also describe non-obvious
67costs.  Optimizations usually aren't free but trade-offs between CPU,
68memory, and readability; or, when it comes to heuristics, between
69different workloads.  Describe the expected downsides of your
70optimization so that the reviewer can weigh costs against benefits.
71
72Once the problem is established, describe what you are actually doing
73about it in technical detail.  It's important to describe the change
74in plain English for the reviewer to verify that the code is behaving
75as you intend it to.
76
77The maintainer will thank you if you write your patch description in a
78form which can be easily pulled into Linux's source code management
79system, ``git``, as a "commit log".  See :ref:`the_canonical_patch_format`.
80
81Solve only one problem per patch.  If your description starts to get
82long, that's a sign that you probably need to split up your patch.
83See :ref:`split_changes`.
84
85When you submit or resubmit a patch or patch series, include the
86complete patch description and justification for it.  Don't just
87say that this is version N of the patch (series).  Don't expect the
88subsystem maintainer to refer back to earlier patch versions or referenced
89URLs to find the patch description and put that into the patch.
90I.e., the patch (series) and its description should be self-contained.
91This benefits both the maintainers and reviewers.  Some reviewers
92probably didn't even receive earlier versions of the patch.
93
94Describe your changes in imperative mood, e.g. "make xyzzy do frotz"
95instead of "[This patch] makes xyzzy do frotz" or "[I] changed xyzzy
96to do frotz", as if you are giving orders to the codebase to change
97its behaviour.
98
99If you want to refer to a specific commit, don't just refer to the
100SHA-1 ID of the commit. Please also include the oneline summary of
101the commit, to make it easier for reviewers to know what it is about.
102Example::
103
104	Commit e21d2170f36602ae2708 ("video: remove unnecessary
105	platform_set_drvdata()") removed the unnecessary
106	platform_set_drvdata(), but left the variable "dev" unused,
107	delete it.
108
109You should also be sure to use at least the first twelve characters of the
110SHA-1 ID.  The kernel repository holds a *lot* of objects, making
111collisions with shorter IDs a real possibility.  Bear in mind that, even if
112there is no collision with your six-character ID now, that condition may
113change five years from now.
114
115If related discussions or any other background information behind the change
116can be found on the web, add 'Link:' tags pointing to it. In case your patch
117fixes a bug, for example, add a tag with a URL referencing the report in the
118mailing list archives or a bug tracker; if the patch is a result of some
119earlier mailing list discussion or something documented on the web, point to
120it.
121
122When linking to mailing list archives, preferably use the lore.kernel.org
123message archiver service. To create the link URL, use the contents of the
124``Message-Id`` header of the message without the surrounding angle brackets.
125For example::
126
127    Link: https://lore.kernel.org/r/30th.anniversary.repost@klaava.Helsinki.FI/
128
129Please check the link to make sure that it is actually working and points
130to the relevant message.
131
132However, try to make your explanation understandable without external
133resources. In addition to giving a URL to a mailing list archive or bug,
134summarize the relevant points of the discussion that led to the
135patch as submitted.
136
137If your patch fixes a bug in a specific commit, e.g. you found an issue using
138``git bisect``, please use the 'Fixes:' tag with the first 12 characters of
139the SHA-1 ID, and the one line summary.  Do not split the tag across multiple
140lines, tags are exempt from the "wrap at 75 columns" rule in order to simplify
141parsing scripts.  For example::
142
143	Fixes: 54a4f0239f2e ("KVM: MMU: make kvm_mmu_zap_page() return the number of pages it actually freed")
144
145The following ``git config`` settings can be used to add a pretty format for
146outputting the above style in the ``git log`` or ``git show`` commands::
147
148	[core]
149		abbrev = 12
150	[pretty]
151		fixes = Fixes: %h (\"%s\")
152
153An example call::
154
155	$ git log -1 --pretty=fixes 54a4f0239f2e
156	Fixes: 54a4f0239f2e ("KVM: MMU: make kvm_mmu_zap_page() return the number of pages it actually freed")
157
158.. _split_changes:
159
160Separate your changes
161---------------------
162
163Separate each **logical change** into a separate patch.
164
165For example, if your changes include both bug fixes and performance
166enhancements for a single driver, separate those changes into two
167or more patches.  If your changes include an API update, and a new
168driver which uses that new API, separate those into two patches.
169
170On the other hand, if you make a single change to numerous files,
171group those changes into a single patch.  Thus a single logical change
172is contained within a single patch.
173
174The point to remember is that each patch should make an easily understood
175change that can be verified by reviewers.  Each patch should be justifiable
176on its own merits.
177
178If one patch depends on another patch in order for a change to be
179complete, that is OK.  Simply note **"this patch depends on patch X"**
180in your patch description.
181
182When dividing your change into a series of patches, take special care to
183ensure that the kernel builds and runs properly after each patch in the
184series.  Developers using ``git bisect`` to track down a problem can end up
185splitting your patch series at any point; they will not thank you if you
186introduce bugs in the middle.
187
188If you cannot condense your patch set into a smaller set of patches,
189then only post say 15 or so at a time and wait for review and integration.
190
191
192
193Style-check your changes
194------------------------
195
196Check your patch for basic style violations, details of which can be
197found in Documentation/process/coding-style.rst.
198Failure to do so simply wastes
199the reviewers time and will get your patch rejected, probably
200without even being read.
201
202One significant exception is when moving code from one file to
203another -- in this case you should not modify the moved code at all in
204the same patch which moves it.  This clearly delineates the act of
205moving the code and your changes.  This greatly aids review of the
206actual differences and allows tools to better track the history of
207the code itself.
208
209Check your patches with the patch style checker prior to submission
210(scripts/checkpatch.pl).  Note, though, that the style checker should be
211viewed as a guide, not as a replacement for human judgment.  If your code
212looks better with a violation then its probably best left alone.
213
214The checker reports at three levels:
215 - ERROR: things that are very likely to be wrong
216 - WARNING: things requiring careful review
217 - CHECK: things requiring thought
218
219You should be able to justify all violations that remain in your
220patch.
221
222
223Select the recipients for your patch
224------------------------------------
225
226You should always copy the appropriate subsystem maintainer(s) on any patch
227to code that they maintain; look through the MAINTAINERS file and the
228source code revision history to see who those maintainers are.  The
229script scripts/get_maintainer.pl can be very useful at this step (pass paths to
230your patches as arguments to scripts/get_maintainer.pl).  If you cannot find a
231maintainer for the subsystem you are working on, Andrew Morton
232(akpm@linux-foundation.org) serves as a maintainer of last resort.
233
234You should also normally choose at least one mailing list to receive a copy
235of your patch set.  linux-kernel@vger.kernel.org should be used by default
236for all patches, but the volume on that list has caused a number of
237developers to tune it out.  Look in the MAINTAINERS file for a
238subsystem-specific list; your patch will probably get more attention there.
239Please do not spam unrelated lists, though.
240
241Many kernel-related lists are hosted on vger.kernel.org; you can find a
242list of them at http://vger.kernel.org/vger-lists.html.  There are
243kernel-related lists hosted elsewhere as well, though.
244
245Do not send more than 15 patches at once to the vger mailing lists!!!
246
247Linus Torvalds is the final arbiter of all changes accepted into the
248Linux kernel.  His e-mail address is <torvalds@linux-foundation.org>.
249He gets a lot of e-mail, and, at this point, very few patches go through
250Linus directly, so typically you should do your best to -avoid-
251sending him e-mail.
252
253If you have a patch that fixes an exploitable security bug, send that patch
254to security@kernel.org.  For severe bugs, a short embargo may be considered
255to allow distributors to get the patch out to users; in such cases,
256obviously, the patch should not be sent to any public lists. See also
257Documentation/admin-guide/security-bugs.rst.
258
259Patches that fix a severe bug in a released kernel should be directed
260toward the stable maintainers by putting a line like this::
261
262  Cc: stable@vger.kernel.org
263
264into the sign-off area of your patch (note, NOT an email recipient).  You
265should also read Documentation/process/stable-kernel-rules.rst
266in addition to this document.
267
268If changes affect userland-kernel interfaces, please send the MAN-PAGES
269maintainer (as listed in the MAINTAINERS file) a man-pages patch, or at
270least a notification of the change, so that some information makes its way
271into the manual pages.  User-space API changes should also be copied to
272linux-api@vger.kernel.org.
273
274
275No MIME, no links, no compression, no attachments.  Just plain text
276-------------------------------------------------------------------
277
278Linus and other kernel developers need to be able to read and comment
279on the changes you are submitting.  It is important for a kernel
280developer to be able to "quote" your changes, using standard e-mail
281tools, so that they may comment on specific portions of your code.
282
283For this reason, all patches should be submitted by e-mail "inline". The
284easiest way to do this is with ``git send-email``, which is strongly
285recommended.  An interactive tutorial for ``git send-email`` is available at
286https://git-send-email.io.
287
288If you choose not to use ``git send-email``:
289
290.. warning::
291
292  Be wary of your editor's word-wrap corrupting your patch,
293  if you choose to cut-n-paste your patch.
294
295Do not attach the patch as a MIME attachment, compressed or not.
296Many popular e-mail applications will not always transmit a MIME
297attachment as plain text, making it impossible to comment on your
298code.  A MIME attachment also takes Linus a bit more time to process,
299decreasing the likelihood of your MIME-attached change being accepted.
300
301Exception:  If your mailer is mangling patches then someone may ask
302you to re-send them using MIME.
303
304See Documentation/process/email-clients.rst for hints about configuring
305your e-mail client so that it sends your patches untouched.
306
307Respond to review comments
308--------------------------
309
310Your patch will almost certainly get comments from reviewers on ways in
311which the patch can be improved, in the form of a reply to your email. You must
312respond to those comments; ignoring reviewers is a good way to get ignored in
313return. You can simply reply to their emails to answer their comments. Review
314comments or questions that do not lead to a code change should almost certainly
315bring about a comment or changelog entry so that the next reviewer better
316understands what is going on.
317
318Be sure to tell the reviewers what changes you are making and to thank them
319for their time.  Code review is a tiring and time-consuming process, and
320reviewers sometimes get grumpy.  Even in that case, though, respond
321politely and address the problems they have pointed out.  When sending a next
322version, add a ``patch changelog`` to the cover letter or to individual patches
323explaining difference aganst previous submission (see
324:ref:`the_canonical_patch_format`).
325
326See Documentation/process/email-clients.rst for recommendations on email
327clients and mailing list etiquette.
328
329.. _resend_reminders:
330
331Don't get discouraged - or impatient
332------------------------------------
333
334After you have submitted your change, be patient and wait.  Reviewers are
335busy people and may not get to your patch right away.
336
337Once upon a time, patches used to disappear into the void without comment,
338but the development process works more smoothly than that now.  You should
339receive comments within a week or so; if that does not happen, make sure
340that you have sent your patches to the right place.  Wait for a minimum of
341one week before resubmitting or pinging reviewers - possibly longer during
342busy times like merge windows.
343
344It's also ok to resend the patch or the patch series after a couple of
345weeks with the word "RESEND" added to the subject line::
346
347   [PATCH Vx RESEND] sub/sys: Condensed patch summary
348
349Don't add "RESEND" when you are submitting a modified version of your
350patch or patch series - "RESEND" only applies to resubmission of a
351patch or patch series which have not been modified in any way from the
352previous submission.
353
354
355Include PATCH in the subject
356-----------------------------
357
358Due to high e-mail traffic to Linus, and to linux-kernel, it is common
359convention to prefix your subject line with [PATCH].  This lets Linus
360and other kernel developers more easily distinguish patches from other
361e-mail discussions.
362
363``git send-email`` will do this for you automatically.
364
365
366Sign your work - the Developer's Certificate of Origin
367------------------------------------------------------
368
369To improve tracking of who did what, especially with patches that can
370percolate to their final resting place in the kernel through several
371layers of maintainers, we've introduced a "sign-off" procedure on
372patches that are being emailed around.
373
374The sign-off is a simple line at the end of the explanation for the
375patch, which certifies that you wrote it or otherwise have the right to
376pass it on as an open-source patch.  The rules are pretty simple: if you
377can certify the below:
378
379Developer's Certificate of Origin 1.1
380^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
381
382By making a contribution to this project, I certify that:
383
384        (a) The contribution was created in whole or in part by me and I
385            have the right to submit it under the open source license
386            indicated in the file; or
387
388        (b) The contribution is based upon previous work that, to the best
389            of my knowledge, is covered under an appropriate open source
390            license and I have the right under that license to submit that
391            work with modifications, whether created in whole or in part
392            by me, under the same open source license (unless I am
393            permitted to submit under a different license), as indicated
394            in the file; or
395
396        (c) The contribution was provided directly to me by some other
397            person who certified (a), (b) or (c) and I have not modified
398            it.
399
400        (d) I understand and agree that this project and the contribution
401            are public and that a record of the contribution (including all
402            personal information I submit with it, including my sign-off) is
403            maintained indefinitely and may be redistributed consistent with
404            this project or the open source license(s) involved.
405
406then you just add a line saying::
407
408	Signed-off-by: Random J Developer <random@developer.example.org>
409
410using your real name (sorry, no pseudonyms or anonymous contributions.)
411This will be done for you automatically if you use ``git commit -s``.
412Reverts should also include "Signed-off-by". ``git revert -s`` does that
413for you.
414
415Some people also put extra tags at the end.  They'll just be ignored for
416now, but you can do this to mark internal company procedures or just
417point out some special detail about the sign-off.
418
419Any further SoBs (Signed-off-by:'s) following the author's SoB are from
420people handling and transporting the patch, but were not involved in its
421development. SoB chains should reflect the **real** route a patch took
422as it was propagated to the maintainers and ultimately to Linus, with
423the first SoB entry signalling primary authorship of a single author.
424
425
426When to use Acked-by:, Cc:, and Co-developed-by:
427------------------------------------------------
428
429The Signed-off-by: tag indicates that the signer was involved in the
430development of the patch, or that he/she was in the patch's delivery path.
431
432If a person was not directly involved in the preparation or handling of a
433patch but wishes to signify and record their approval of it then they can
434ask to have an Acked-by: line added to the patch's changelog.
435
436Acked-by: is often used by the maintainer of the affected code when that
437maintainer neither contributed to nor forwarded the patch.
438
439Acked-by: is not as formal as Signed-off-by:.  It is a record that the acker
440has at least reviewed the patch and has indicated acceptance.  Hence patch
441mergers will sometimes manually convert an acker's "yep, looks good to me"
442into an Acked-by: (but note that it is usually better to ask for an
443explicit ack).
444
445Acked-by: does not necessarily indicate acknowledgement of the entire patch.
446For example, if a patch affects multiple subsystems and has an Acked-by: from
447one subsystem maintainer then this usually indicates acknowledgement of just
448the part which affects that maintainer's code.  Judgement should be used here.
449When in doubt people should refer to the original discussion in the mailing
450list archives.
451
452If a person has had the opportunity to comment on a patch, but has not
453provided such comments, you may optionally add a ``Cc:`` tag to the patch.
454This is the only tag which might be added without an explicit action by the
455person it names - but it should indicate that this person was copied on the
456patch.  This tag documents that potentially interested parties
457have been included in the discussion.
458
459Co-developed-by: states that the patch was co-created by multiple developers;
460it is used to give attribution to co-authors (in addition to the author
461attributed by the From: tag) when several people work on a single patch.  Since
462Co-developed-by: denotes authorship, every Co-developed-by: must be immediately
463followed by a Signed-off-by: of the associated co-author.  Standard sign-off
464procedure applies, i.e. the ordering of Signed-off-by: tags should reflect the
465chronological history of the patch insofar as possible, regardless of whether
466the author is attributed via From: or Co-developed-by:.  Notably, the last
467Signed-off-by: must always be that of the developer submitting the patch.
468
469Note, the From: tag is optional when the From: author is also the person (and
470email) listed in the From: line of the email header.
471
472Example of a patch submitted by the From: author::
473
474	<changelog>
475
476	Co-developed-by: First Co-Author <first@coauthor.example.org>
477	Signed-off-by: First Co-Author <first@coauthor.example.org>
478	Co-developed-by: Second Co-Author <second@coauthor.example.org>
479	Signed-off-by: Second Co-Author <second@coauthor.example.org>
480	Signed-off-by: From Author <from@author.example.org>
481
482Example of a patch submitted by a Co-developed-by: author::
483
484	From: From Author <from@author.example.org>
485
486	<changelog>
487
488	Co-developed-by: Random Co-Author <random@coauthor.example.org>
489	Signed-off-by: Random Co-Author <random@coauthor.example.org>
490	Signed-off-by: From Author <from@author.example.org>
491	Co-developed-by: Submitting Co-Author <sub@coauthor.example.org>
492	Signed-off-by: Submitting Co-Author <sub@coauthor.example.org>
493
494
495Using Reported-by:, Tested-by:, Reviewed-by:, Suggested-by: and Fixes:
496----------------------------------------------------------------------
497
498The Reported-by tag gives credit to people who find bugs and report them and it
499hopefully inspires them to help us again in the future.  Please note that if
500the bug was reported in private, then ask for permission first before using the
501Reported-by tag. The tag is intended for bugs; please do not use it to credit
502feature requests.
503
504A Tested-by: tag indicates that the patch has been successfully tested (in
505some environment) by the person named.  This tag informs maintainers that
506some testing has been performed, provides a means to locate testers for
507future patches, and ensures credit for the testers.
508
509Reviewed-by:, instead, indicates that the patch has been reviewed and found
510acceptable according to the Reviewer's Statement:
511
512Reviewer's statement of oversight
513^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
514
515By offering my Reviewed-by: tag, I state that:
516
517	 (a) I have carried out a technical review of this patch to
518	     evaluate its appropriateness and readiness for inclusion into
519	     the mainline kernel.
520
521	 (b) Any problems, concerns, or questions relating to the patch
522	     have been communicated back to the submitter.  I am satisfied
523	     with the submitter's response to my comments.
524
525	 (c) While there may be things that could be improved with this
526	     submission, I believe that it is, at this time, (1) a
527	     worthwhile modification to the kernel, and (2) free of known
528	     issues which would argue against its inclusion.
529
530	 (d) While I have reviewed the patch and believe it to be sound, I
531	     do not (unless explicitly stated elsewhere) make any
532	     warranties or guarantees that it will achieve its stated
533	     purpose or function properly in any given situation.
534
535A Reviewed-by tag is a statement of opinion that the patch is an
536appropriate modification of the kernel without any remaining serious
537technical issues.  Any interested reviewer (who has done the work) can
538offer a Reviewed-by tag for a patch.  This tag serves to give credit to
539reviewers and to inform maintainers of the degree of review which has been
540done on the patch.  Reviewed-by: tags, when supplied by reviewers known to
541understand the subject area and to perform thorough reviews, will normally
542increase the likelihood of your patch getting into the kernel.
543
544Both Tested-by and Reviewed-by tags, once received on mailing list from tester
545or reviewer, should be added by author to the applicable patches when sending
546next versions.  However if the patch has changed substantially in following
547version, these tags might not be applicable anymore and thus should be removed.
548Usually removal of someone's Tested-by or Reviewed-by tags should be mentioned
549in the patch changelog (after the '---' separator).
550
551A Suggested-by: tag indicates that the patch idea is suggested by the person
552named and ensures credit to the person for the idea. Please note that this
553tag should not be added without the reporter's permission, especially if the
554idea was not posted in a public forum. That said, if we diligently credit our
555idea reporters, they will, hopefully, be inspired to help us again in the
556future.
557
558A Fixes: tag indicates that the patch fixes an issue in a previous commit. It
559is used to make it easy to determine where a bug originated, which can help
560review a bug fix. This tag also assists the stable kernel team in determining
561which stable kernel versions should receive your fix. This is the preferred
562method for indicating a bug fixed by the patch. See :ref:`describe_changes`
563for more details.
564
565Note: Attaching a Fixes: tag does not subvert the stable kernel rules
566process nor the requirement to Cc: stable@vger.kernel.org on all stable
567patch candidates. For more information, please read
568Documentation/process/stable-kernel-rules.rst.
569
570.. _the_canonical_patch_format:
571
572The canonical patch format
573--------------------------
574
575This section describes how the patch itself should be formatted.  Note
576that, if you have your patches stored in a ``git`` repository, proper patch
577formatting can be had with ``git format-patch``.  The tools cannot create
578the necessary text, though, so read the instructions below anyway.
579
580The canonical patch subject line is::
581
582    Subject: [PATCH 001/123] subsystem: summary phrase
583
584The canonical patch message body contains the following:
585
586  - A ``from`` line specifying the patch author, followed by an empty
587    line (only needed if the person sending the patch is not the author).
588
589  - The body of the explanation, line wrapped at 75 columns, which will
590    be copied to the permanent changelog to describe this patch.
591
592  - An empty line.
593
594  - The ``Signed-off-by:`` lines, described above, which will
595    also go in the changelog.
596
597  - A marker line containing simply ``---``.
598
599  - Any additional comments not suitable for the changelog.
600
601  - The actual patch (``diff`` output).
602
603The Subject line format makes it very easy to sort the emails
604alphabetically by subject line - pretty much any email reader will
605support that - since because the sequence number is zero-padded,
606the numerical and alphabetic sort is the same.
607
608The ``subsystem`` in the email's Subject should identify which
609area or subsystem of the kernel is being patched.
610
611The ``summary phrase`` in the email's Subject should concisely
612describe the patch which that email contains.  The ``summary
613phrase`` should not be a filename.  Do not use the same ``summary
614phrase`` for every patch in a whole patch series (where a ``patch
615series`` is an ordered sequence of multiple, related patches).
616
617Bear in mind that the ``summary phrase`` of your email becomes a
618globally-unique identifier for that patch.  It propagates all the way
619into the ``git`` changelog.  The ``summary phrase`` may later be used in
620developer discussions which refer to the patch.  People will want to
621google for the ``summary phrase`` to read discussion regarding that
622patch.  It will also be the only thing that people may quickly see
623when, two or three months later, they are going through perhaps
624thousands of patches using tools such as ``gitk`` or ``git log
625--oneline``.
626
627For these reasons, the ``summary`` must be no more than 70-75
628characters, and it must describe both what the patch changes, as well
629as why the patch might be necessary.  It is challenging to be both
630succinct and descriptive, but that is what a well-written summary
631should do.
632
633The ``summary phrase`` may be prefixed by tags enclosed in square
634brackets: "Subject: [PATCH <tag>...] <summary phrase>".  The tags are
635not considered part of the summary phrase, but describe how the patch
636should be treated.  Common tags might include a version descriptor if
637the multiple versions of the patch have been sent out in response to
638comments (i.e., "v1, v2, v3"), or "RFC" to indicate a request for
639comments.
640
641If there are four patches in a patch series the individual patches may
642be numbered like this: 1/4, 2/4, 3/4, 4/4. This assures that developers
643understand the order in which the patches should be applied and that
644they have reviewed or applied all of the patches in the patch series.
645
646Here are some good example Subjects::
647
648    Subject: [PATCH 2/5] ext2: improve scalability of bitmap searching
649    Subject: [PATCH v2 01/27] x86: fix eflags tracking
650    Subject: [PATCH v2] sub/sys: Condensed patch summary
651    Subject: [PATCH v2 M/N] sub/sys: Condensed patch summary
652
653The ``from`` line must be the very first line in the message body,
654and has the form:
655
656        From: Patch Author <author@example.com>
657
658The ``from`` line specifies who will be credited as the author of the
659patch in the permanent changelog.  If the ``from`` line is missing,
660then the ``From:`` line from the email header will be used to determine
661the patch author in the changelog.
662
663The explanation body will be committed to the permanent source
664changelog, so should make sense to a competent reader who has long since
665forgotten the immediate details of the discussion that might have led to
666this patch. Including symptoms of the failure which the patch addresses
667(kernel log messages, oops messages, etc.) are especially useful for
668people who might be searching the commit logs looking for the applicable
669patch. The text should be written in such detail so that when read
670weeks, months or even years later, it can give the reader the needed
671details to grasp the reasoning for **why** the patch was created.
672
673If a patch fixes a compile failure, it may not be necessary to include
674_all_ of the compile failures; just enough that it is likely that
675someone searching for the patch can find it. As in the ``summary
676phrase``, it is important to be both succinct as well as descriptive.
677
678The ``---`` marker line serves the essential purpose of marking for
679patch handling tools where the changelog message ends.
680
681One good use for the additional comments after the ``---`` marker is
682for a ``diffstat``, to show what files have changed, and the number of
683inserted and deleted lines per file. A ``diffstat`` is especially useful
684on bigger patches. If you are going to include a ``diffstat`` after the
685``---`` marker, please use ``diffstat`` options ``-p 1 -w 70`` so that
686filenames are listed from the top of the kernel source tree and don't
687use too much horizontal space (easily fit in 80 columns, maybe with some
688indentation). (``git`` generates appropriate diffstats by default.)
689
690Other comments relevant only to the moment or the maintainer, not
691suitable for the permanent changelog, should also go here. A good
692example of such comments might be ``patch changelogs`` which describe
693what has changed between the v1 and v2 version of the patch.
694
695Please put this information **after** the ``---`` line which separates
696the changelog from the rest of the patch. The version information is
697not part of the changelog which gets committed to the git tree. It is
698additional information for the reviewers. If it's placed above the
699commit tags, it needs manual interaction to remove it. If it is below
700the separator line, it gets automatically stripped off when applying the
701patch::
702
703  <commit message>
704  ...
705  Signed-off-by: Author <author@mail>
706  ---
707  V2 -> V3: Removed redundant helper function
708  V1 -> V2: Cleaned up coding style and addressed review comments
709
710  path/to/file | 5+++--
711  ...
712
713See more details on the proper patch format in the following
714references.
715
716.. _backtraces:
717
718Backtraces in commit messages
719^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
720
721Backtraces help document the call chain leading to a problem. However,
722not all backtraces are helpful. For example, early boot call chains are
723unique and obvious. Copying the full dmesg output verbatim, however,
724adds distracting information like timestamps, module lists, register and
725stack dumps.
726
727Therefore, the most useful backtraces should distill the relevant
728information from the dump, which makes it easier to focus on the real
729issue. Here is an example of a well-trimmed backtrace::
730
731  unchecked MSR access error: WRMSR to 0xd51 (tried to write 0x0000000000000064)
732  at rIP: 0xffffffffae059994 (native_write_msr+0x4/0x20)
733  Call Trace:
734  mba_wrmsr
735  update_domains
736  rdtgroup_mkdir
737
738.. _explicit_in_reply_to:
739
740Explicit In-Reply-To headers
741----------------------------
742
743It can be helpful to manually add In-Reply-To: headers to a patch
744(e.g., when using ``git send-email``) to associate the patch with
745previous relevant discussion, e.g. to link a bug fix to the email with
746the bug report.  However, for a multi-patch series, it is generally
747best to avoid using In-Reply-To: to link to older versions of the
748series.  This way multiple versions of the patch don't become an
749unmanageable forest of references in email clients.  If a link is
750helpful, you can use the https://lore.kernel.org/ redirector (e.g., in
751the cover email text) to link to an earlier version of the patch series.
752
753
754Providing base tree information
755-------------------------------
756
757When other developers receive your patches and start the review process,
758it is often useful for them to know where in the tree history they
759should place your work. This is particularly useful for automated CI
760processes that attempt to run a series of tests in order to establish
761the quality of your submission before the maintainer starts the review.
762
763If you are using ``git format-patch`` to generate your patches, you can
764automatically include the base tree information in your submission by
765using the ``--base`` flag. The easiest and most convenient way to use
766this option is with topical branches::
767
768    $ git checkout -t -b my-topical-branch master
769    Branch 'my-topical-branch' set up to track local branch 'master'.
770    Switched to a new branch 'my-topical-branch'
771
772    [perform your edits and commits]
773
774    $ git format-patch --base=auto --cover-letter -o outgoing/ master
775    outgoing/0000-cover-letter.patch
776    outgoing/0001-First-Commit.patch
777    outgoing/...
778
779When you open ``outgoing/0000-cover-letter.patch`` for editing, you will
780notice that it will have the ``base-commit:`` trailer at the very
781bottom, which provides the reviewer and the CI tools enough information
782to properly perform ``git am`` without worrying about conflicts::
783
784    $ git checkout -b patch-review [base-commit-id]
785    Switched to a new branch 'patch-review'
786    $ git am patches.mbox
787    Applying: First Commit
788    Applying: ...
789
790Please see ``man git-format-patch`` for more information about this
791option.
792
793.. note::
794
795    The ``--base`` feature was introduced in git version 2.9.0.
796
797If you are not using git to format your patches, you can still include
798the same ``base-commit`` trailer to indicate the commit hash of the tree
799on which your work is based. You should add it either in the cover
800letter or in the first patch of the series and it should be placed
801either below the ``---`` line or at the very bottom of all other
802content, right before your email signature.
803
804
805References
806----------
807
808Andrew Morton, "The perfect patch" (tpp).
809  <https://www.ozlabs.org/~akpm/stuff/tpp.txt>
810
811Jeff Garzik, "Linux kernel patch submission format".
812  <https://web.archive.org/web/20180829112450/http://linux.yyz.us/patch-format.html>
813
814Greg Kroah-Hartman, "How to piss off a kernel subsystem maintainer".
815  <http://www.kroah.com/log/linux/maintainer.html>
816
817  <http://www.kroah.com/log/linux/maintainer-02.html>
818
819  <http://www.kroah.com/log/linux/maintainer-03.html>
820
821  <http://www.kroah.com/log/linux/maintainer-04.html>
822
823  <http://www.kroah.com/log/linux/maintainer-05.html>
824
825  <http://www.kroah.com/log/linux/maintainer-06.html>
826
827NO!!!! No more huge patch bombs to linux-kernel@vger.kernel.org people!
828  <https://lore.kernel.org/r/20050711.125305.08322243.davem@davemloft.net>
829
830Kernel Documentation/process/coding-style.rst
831
832Linus Torvalds's mail on the canonical patch format:
833  <https://lore.kernel.org/r/Pine.LNX.4.58.0504071023190.28951@ppc970.osdl.org>
834
835Andi Kleen, "On submitting kernel patches"
836  Some strategies to get difficult or controversial changes in.
837
838  http://halobates.de/on-submitting-patches.pdf
839