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. If the patch is a
117result of some earlier mailing list discussions or something documented on the
118web, point to it.
119
120When linking to mailing list archives, preferably use the lore.kernel.org
121message archiver service. To create the link URL, use the contents of the
122``Message-ID`` header of the message without the surrounding angle brackets.
123For example::
124
125    Link: https://lore.kernel.org/30th.anniversary.repost@klaava.Helsinki.FI
126
127Please check the link to make sure that it is actually working and points
128to the relevant message.
129
130However, try to make your explanation understandable without external
131resources. In addition to giving a URL to a mailing list archive or bug,
132summarize the relevant points of the discussion that led to the
133patch as submitted.
134
135In case your patch fixes a bug, use the 'Closes:' tag with a URL referencing
136the report in the mailing list archives or a public bug tracker. For example::
137
138	Closes: https://example.com/issues/1234
139
140Some bug trackers have the ability to close issues automatically when a
141commit with such a tag is applied. Some bots monitoring mailing lists can
142also track such tags and take certain actions. Private bug trackers and
143invalid URLs are forbidden.
144
145If your patch fixes a bug in a specific commit, e.g. you found an issue using
146``git bisect``, please use the 'Fixes:' tag with at least the first 12
147characters of the SHA-1 ID, and the one line summary.  Do not split the tag
148across multiple lines, tags are exempt from the "wrap at 75 columns" rule in
149order to simplify parsing scripts.  For example::
150
151	Fixes: 54a4f0239f2e ("KVM: MMU: make kvm_mmu_zap_page() return the number of pages it actually freed")
152
153The following ``git config`` settings can be used to add a pretty format for
154outputting the above style in the ``git log`` or ``git show`` commands::
155
156	[core]
157		abbrev = 12
158	[pretty]
159		fixes = Fixes: %h (\"%s\")
160
161An example call::
162
163	$ git log -1 --pretty=fixes 54a4f0239f2e
164	Fixes: 54a4f0239f2e ("KVM: MMU: make kvm_mmu_zap_page() return the number of pages it actually freed")
165
166.. _split_changes:
167
168Separate your changes
169---------------------
170
171Separate each **logical change** into a separate patch.
172
173For example, if your changes include both bug fixes and performance
174enhancements for a single driver, separate those changes into two
175or more patches.  If your changes include an API update, and a new
176driver which uses that new API, separate those into two patches.
177
178On the other hand, if you make a single change to numerous files,
179group those changes into a single patch.  Thus a single logical change
180is contained within a single patch.
181
182The point to remember is that each patch should make an easily understood
183change that can be verified by reviewers.  Each patch should be justifiable
184on its own merits.
185
186If one patch depends on another patch in order for a change to be
187complete, that is OK.  Simply note **"this patch depends on patch X"**
188in your patch description.
189
190When dividing your change into a series of patches, take special care to
191ensure that the kernel builds and runs properly after each patch in the
192series.  Developers using ``git bisect`` to track down a problem can end up
193splitting your patch series at any point; they will not thank you if you
194introduce bugs in the middle.
195
196If you cannot condense your patch set into a smaller set of patches,
197then only post say 15 or so at a time and wait for review and integration.
198
199
200
201Style-check your changes
202------------------------
203
204Check your patch for basic style violations, details of which can be
205found in Documentation/process/coding-style.rst.
206Failure to do so simply wastes
207the reviewers time and will get your patch rejected, probably
208without even being read.
209
210One significant exception is when moving code from one file to
211another -- in this case you should not modify the moved code at all in
212the same patch which moves it.  This clearly delineates the act of
213moving the code and your changes.  This greatly aids review of the
214actual differences and allows tools to better track the history of
215the code itself.
216
217Check your patches with the patch style checker prior to submission
218(scripts/checkpatch.pl).  Note, though, that the style checker should be
219viewed as a guide, not as a replacement for human judgment.  If your code
220looks better with a violation then its probably best left alone.
221
222The checker reports at three levels:
223 - ERROR: things that are very likely to be wrong
224 - WARNING: things requiring careful review
225 - CHECK: things requiring thought
226
227You should be able to justify all violations that remain in your
228patch.
229
230
231Select the recipients for your patch
232------------------------------------
233
234You should always copy the appropriate subsystem maintainer(s) and list(s) on
235any patch to code that they maintain; look through the MAINTAINERS file and the
236source code revision history to see who those maintainers are.  The script
237scripts/get_maintainer.pl can be very useful at this step (pass paths to your
238patches as arguments to scripts/get_maintainer.pl).  If you cannot find a
239maintainer for the subsystem you are working on, Andrew Morton
240(akpm@linux-foundation.org) serves as a maintainer of last resort.
241
242linux-kernel@vger.kernel.org should be used by default for all patches, but the
243volume on that list has caused a number of developers to tune it out.  Please
244do not spam unrelated lists and unrelated people, though.
245
246Many kernel-related lists are hosted at kernel.org; you can find a list
247of them at https://subspace.kernel.org.  There are kernel-related lists
248hosted elsewhere as well, though.
249
250Linus Torvalds is the final arbiter of all changes accepted into the
251Linux kernel.  His e-mail address is <torvalds@linux-foundation.org>.
252He gets a lot of e-mail, and, at this point, very few patches go through
253Linus directly, so typically you should do your best to -avoid-
254sending him e-mail.
255
256If you have a patch that fixes an exploitable security bug, send that patch
257to security@kernel.org.  For severe bugs, a short embargo may be considered
258to allow distributors to get the patch out to users; in such cases,
259obviously, the patch should not be sent to any public lists. See also
260Documentation/process/security-bugs.rst.
261
262Patches that fix a severe bug in a released kernel should be directed
263toward the stable maintainers by putting a line like this::
264
265  Cc: stable@vger.kernel.org
266
267into the sign-off area of your patch (note, NOT an email recipient).  You
268should also read Documentation/process/stable-kernel-rules.rst
269in addition to this document.
270
271If changes affect userland-kernel interfaces, please send the MAN-PAGES
272maintainer (as listed in the MAINTAINERS file) a man-pages patch, or at
273least a notification of the change, so that some information makes its way
274into the manual pages.  User-space API changes should also be copied to
275linux-api@vger.kernel.org.
276
277
278No MIME, no links, no compression, no attachments.  Just plain text
279-------------------------------------------------------------------
280
281Linus and other kernel developers need to be able to read and comment
282on the changes you are submitting.  It is important for a kernel
283developer to be able to "quote" your changes, using standard e-mail
284tools, so that they may comment on specific portions of your code.
285
286For this reason, all patches should be submitted by e-mail "inline". The
287easiest way to do this is with ``git send-email``, which is strongly
288recommended.  An interactive tutorial for ``git send-email`` is available at
289https://git-send-email.io.
290
291If you choose not to use ``git send-email``:
292
293.. warning::
294
295  Be wary of your editor's word-wrap corrupting your patch,
296  if you choose to cut-n-paste your patch.
297
298Do not attach the patch as a MIME attachment, compressed or not.
299Many popular e-mail applications will not always transmit a MIME
300attachment as plain text, making it impossible to comment on your
301code.  A MIME attachment also takes Linus a bit more time to process,
302decreasing the likelihood of your MIME-attached change being accepted.
303
304Exception:  If your mailer is mangling patches then someone may ask
305you to re-send them using MIME.
306
307See Documentation/process/email-clients.rst for hints about configuring
308your e-mail client so that it sends your patches untouched.
309
310Respond to review comments
311--------------------------
312
313Your patch will almost certainly get comments from reviewers on ways in
314which the patch can be improved, in the form of a reply to your email. You must
315respond to those comments; ignoring reviewers is a good way to get ignored in
316return. You can simply reply to their emails to answer their comments. Review
317comments or questions that do not lead to a code change should almost certainly
318bring about a comment or changelog entry so that the next reviewer better
319understands what is going on.
320
321Be sure to tell the reviewers what changes you are making and to thank them
322for their time.  Code review is a tiring and time-consuming process, and
323reviewers sometimes get grumpy.  Even in that case, though, respond
324politely and address the problems they have pointed out.  When sending a next
325version, add a ``patch changelog`` to the cover letter or to individual patches
326explaining difference against previous submission (see
327:ref:`the_canonical_patch_format`).
328Notify people that commented on your patch about new versions by adding them to
329the patches CC list.
330
331See Documentation/process/email-clients.rst for recommendations on email
332clients and mailing list etiquette.
333
334.. _interleaved_replies:
335
336Use trimmed interleaved replies in email discussions
337----------------------------------------------------
338Top-posting is strongly discouraged in Linux kernel development
339discussions. Interleaved (or "inline") replies make conversations much
340easier to follow. For more details see:
341https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
342
343As is frequently quoted on the mailing list::
344
345  A: http://en.wikipedia.org/wiki/Top_post
346  Q: Were do I find info about this thing called top-posting?
347  A: Because it messes up the order in which people normally read text.
348  Q: Why is top-posting such a bad thing?
349  A: Top-posting.
350  Q: What is the most annoying thing in e-mail?
351
352Similarly, please trim all unneeded quotations that aren't relevant
353to your reply. This makes responses easier to find, and saves time and
354space. For more details see: http://daringfireball.net/2007/07/on_top ::
355
356  A: No.
357  Q: Should I include quotations after my reply?
358
359.. _resend_reminders:
360
361Don't get discouraged - or impatient
362------------------------------------
363
364After you have submitted your change, be patient and wait.  Reviewers are
365busy people and may not get to your patch right away.
366
367Once upon a time, patches used to disappear into the void without comment,
368but the development process works more smoothly than that now.  You should
369receive comments within a few weeks (typically 2-3); if that does not
370happen, make sure that you have sent your patches to the right place.
371Wait for a minimum of one week before resubmitting or pinging reviewers
372- possibly longer during busy times like merge windows.
373
374It's also ok to resend the patch or the patch series after a couple of
375weeks with the word "RESEND" added to the subject line::
376
377   [PATCH Vx RESEND] sub/sys: Condensed patch summary
378
379Don't add "RESEND" when you are submitting a modified version of your
380patch or patch series - "RESEND" only applies to resubmission of a
381patch or patch series which have not been modified in any way from the
382previous submission.
383
384
385Include PATCH in the subject
386-----------------------------
387
388Due to high e-mail traffic to Linus, and to linux-kernel, it is common
389convention to prefix your subject line with [PATCH].  This lets Linus
390and other kernel developers more easily distinguish patches from other
391e-mail discussions.
392
393``git send-email`` will do this for you automatically.
394
395
396Sign your work - the Developer's Certificate of Origin
397------------------------------------------------------
398
399To improve tracking of who did what, especially with patches that can
400percolate to their final resting place in the kernel through several
401layers of maintainers, we've introduced a "sign-off" procedure on
402patches that are being emailed around.
403
404The sign-off is a simple line at the end of the explanation for the
405patch, which certifies that you wrote it or otherwise have the right to
406pass it on as an open-source patch.  The rules are pretty simple: if you
407can certify the below:
408
409Developer's Certificate of Origin 1.1
410^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
411
412By making a contribution to this project, I certify that:
413
414        (a) The contribution was created in whole or in part by me and I
415            have the right to submit it under the open source license
416            indicated in the file; or
417
418        (b) The contribution is based upon previous work that, to the best
419            of my knowledge, is covered under an appropriate open source
420            license and I have the right under that license to submit that
421            work with modifications, whether created in whole or in part
422            by me, under the same open source license (unless I am
423            permitted to submit under a different license), as indicated
424            in the file; or
425
426        (c) The contribution was provided directly to me by some other
427            person who certified (a), (b) or (c) and I have not modified
428            it.
429
430        (d) I understand and agree that this project and the contribution
431            are public and that a record of the contribution (including all
432            personal information I submit with it, including my sign-off) is
433            maintained indefinitely and may be redistributed consistent with
434            this project or the open source license(s) involved.
435
436then you just add a line saying::
437
438	Signed-off-by: Random J Developer <random@developer.example.org>
439
440using a known identity (sorry, no anonymous contributions.)
441This will be done for you automatically if you use ``git commit -s``.
442Reverts should also include "Signed-off-by". ``git revert -s`` does that
443for you.
444
445Some people also put extra tags at the end.  They'll just be ignored for
446now, but you can do this to mark internal company procedures or just
447point out some special detail about the sign-off.
448
449Any further SoBs (Signed-off-by:'s) following the author's SoB are from
450people handling and transporting the patch, but were not involved in its
451development. SoB chains should reflect the **real** route a patch took
452as it was propagated to the maintainers and ultimately to Linus, with
453the first SoB entry signalling primary authorship of a single author.
454
455
456When to use Acked-by:, Cc:, and Co-developed-by:
457------------------------------------------------
458
459The Signed-off-by: tag indicates that the signer was involved in the
460development of the patch, or that he/she was in the patch's delivery path.
461
462If a person was not directly involved in the preparation or handling of a
463patch but wishes to signify and record their approval of it then they can
464ask to have an Acked-by: line added to the patch's changelog.
465
466Acked-by: is meant to be used by those responsible for or involved with the
467affected code in one way or another.  Most commonly, the maintainer when that
468maintainer neither contributed to nor forwarded the patch.
469
470Acked-by: may also be used by other stakeholders, such as people with domain
471knowledge (e.g. the original author of the code being modified), userspace-side
472reviewers for a kernel uAPI patch or key users of a feature.  Optionally, in
473these cases, it can be useful to add a "# Suffix" to clarify its meaning::
474
475	Acked-by: The Stakeholder <stakeholder@example.org> # As primary user
476
477Acked-by: is not as formal as Signed-off-by:.  It is a record that the acker
478has at least reviewed the patch and has indicated acceptance.  Hence patch
479mergers will sometimes manually convert an acker's "yep, looks good to me"
480into an Acked-by: (but note that it is usually better to ask for an
481explicit ack).
482
483Acked-by: is also less formal than Reviewed-by:.  For instance, maintainers may
484use it to signify that they are OK with a patch landing, but they may not have
485reviewed it as thoroughly as if a Reviewed-by: was provided.  Similarly, a key
486user may not have carried out a technical review of the patch, yet they may be
487satisfied with the general approach, the feature or the user-facing interface.
488
489Acked-by: does not necessarily indicate acknowledgement of the entire patch.
490For example, if a patch affects multiple subsystems and has an Acked-by: from
491one subsystem maintainer then this usually indicates acknowledgement of just
492the part which affects that maintainer's code.  Judgement should be used here.
493When in doubt people should refer to the original discussion in the mailing
494list archives.  A "# Suffix" may also be used in this case to clarify.
495
496If a person has had the opportunity to comment on a patch, but has not
497provided such comments, you may optionally add a ``Cc:`` tag to the patch.
498This tag documents that potentially interested parties have been included in
499the discussion. Note, this is one of only three tags you might be able to use
500without explicit permission of the person named (see 'Tagging people requires
501permission' below for details).
502
503Co-developed-by: states that the patch was co-created by multiple developers;
504it is used to give attribution to co-authors (in addition to the author
505attributed by the From: tag) when several people work on a single patch.  Since
506Co-developed-by: denotes authorship, every Co-developed-by: must be immediately
507followed by a Signed-off-by: of the associated co-author.  Standard sign-off
508procedure applies, i.e. the ordering of Signed-off-by: tags should reflect the
509chronological history of the patch insofar as possible, regardless of whether
510the author is attributed via From: or Co-developed-by:.  Notably, the last
511Signed-off-by: must always be that of the developer submitting the patch.
512
513Note, the From: tag is optional when the From: author is also the person (and
514email) listed in the From: line of the email header.
515
516Example of a patch submitted by the From: author::
517
518	<changelog>
519
520	Co-developed-by: First Co-Author <first@coauthor.example.org>
521	Signed-off-by: First Co-Author <first@coauthor.example.org>
522	Co-developed-by: Second Co-Author <second@coauthor.example.org>
523	Signed-off-by: Second Co-Author <second@coauthor.example.org>
524	Signed-off-by: From Author <from@author.example.org>
525
526Example of a patch submitted by a Co-developed-by: author::
527
528	From: From Author <from@author.example.org>
529
530	<changelog>
531
532	Co-developed-by: Random Co-Author <random@coauthor.example.org>
533	Signed-off-by: Random Co-Author <random@coauthor.example.org>
534	Signed-off-by: From Author <from@author.example.org>
535	Co-developed-by: Submitting Co-Author <sub@coauthor.example.org>
536	Signed-off-by: Submitting Co-Author <sub@coauthor.example.org>
537
538
539Using Reported-by:, Tested-by:, Reviewed-by:, Suggested-by: and Fixes:
540----------------------------------------------------------------------
541
542The Reported-by tag gives credit to people who find bugs and report them and it
543hopefully inspires them to help us again in the future. The tag is intended for
544bugs; please do not use it to credit feature requests. The tag should be
545followed by a Closes: tag pointing to the report, unless the report is not
546available on the web. The Link: tag can be used instead of Closes: if the patch
547fixes a part of the issue(s) being reported. Note, the Reported-by tag is one
548of only three tags you might be able to use without explicit permission of the
549person named (see 'Tagging people requires permission' below for details).
550
551A Tested-by: tag indicates that the patch has been successfully tested (in
552some environment) by the person named.  This tag informs maintainers that
553some testing has been performed, provides a means to locate testers for
554future patches, and ensures credit for the testers.
555
556Reviewed-by:, instead, indicates that the patch has been reviewed and found
557acceptable according to the Reviewer's Statement:
558
559Reviewer's statement of oversight
560^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
561
562By offering my Reviewed-by: tag, I state that:
563
564	 (a) I have carried out a technical review of this patch to
565	     evaluate its appropriateness and readiness for inclusion into
566	     the mainline kernel.
567
568	 (b) Any problems, concerns, or questions relating to the patch
569	     have been communicated back to the submitter.  I am satisfied
570	     with the submitter's response to my comments.
571
572	 (c) While there may be things that could be improved with this
573	     submission, I believe that it is, at this time, (1) a
574	     worthwhile modification to the kernel, and (2) free of known
575	     issues which would argue against its inclusion.
576
577	 (d) While I have reviewed the patch and believe it to be sound, I
578	     do not (unless explicitly stated elsewhere) make any
579	     warranties or guarantees that it will achieve its stated
580	     purpose or function properly in any given situation.
581
582A Reviewed-by tag is a statement of opinion that the patch is an
583appropriate modification of the kernel without any remaining serious
584technical issues.  Any interested reviewer (who has done the work) can
585offer a Reviewed-by tag for a patch.  This tag serves to give credit to
586reviewers and to inform maintainers of the degree of review which has been
587done on the patch.  Reviewed-by: tags, when supplied by reviewers known to
588understand the subject area and to perform thorough reviews, will normally
589increase the likelihood of your patch getting into the kernel.
590
591Both Tested-by and Reviewed-by tags, once received on mailing list from tester
592or reviewer, should be added by author to the applicable patches when sending
593next versions.  However if the patch has changed substantially in following
594version, these tags might not be applicable anymore and thus should be removed.
595Usually removal of someone's Tested-by or Reviewed-by tags should be mentioned
596in the patch changelog (after the '---' separator).
597
598A Suggested-by: tag indicates that the patch idea is suggested by the person
599named and ensures credit to the person for the idea: if we diligently credit
600our idea reporters, they will, hopefully, be inspired to help us again in the
601future. Note, this is one of only three tags you might be able to use without
602explicit permission of the person named (see 'Tagging people requires
603permission' below for details).
604
605A Fixes: tag indicates that the patch fixes an issue in a previous commit. It
606is used to make it easy to determine where a bug originated, which can help
607review a bug fix. This tag also assists the stable kernel team in determining
608which stable kernel versions should receive your fix. This is the preferred
609method for indicating a bug fixed by the patch. See :ref:`describe_changes`
610for more details.
611
612Note: Attaching a Fixes: tag does not subvert the stable kernel rules
613process nor the requirement to Cc: stable@vger.kernel.org on all stable
614patch candidates. For more information, please read
615Documentation/process/stable-kernel-rules.rst.
616
617Finally, while providing tags is welcome and typically very appreciated, please
618note that signers (i.e. submitters and maintainers) may use their discretion in
619applying offered tags.
620
621.. _tagging_people:
622
623Tagging people requires permission
624----------------------------------
625
626Be careful in the addition of the aforementioned tags to your patches, as all
627except for Cc:, Reported-by:, and Suggested-by: need explicit permission of the
628person named. For those three implicit permission is sufficient if the person
629contributed to the Linux kernel using that name and email address according
630to the lore archives or the commit history -- and in case of Reported-by:
631and Suggested-by: did the reporting or suggestion in public. Note,
632bugzilla.kernel.org is a public place in this sense, but email addresses
633used there are private; so do not expose them in tags, unless the person
634used them in earlier contributions.
635
636.. _the_canonical_patch_format:
637
638The canonical patch format
639--------------------------
640
641This section describes how the patch itself should be formatted.  Note
642that, if you have your patches stored in a ``git`` repository, proper patch
643formatting can be had with ``git format-patch``.  The tools cannot create
644the necessary text, though, so read the instructions below anyway.
645
646Subject Line
647^^^^^^^^^^^^
648
649The canonical patch subject line is::
650
651    Subject: [PATCH 001/123] subsystem: summary phrase
652
653The canonical patch message body contains the following:
654
655  - A ``from`` line specifying the patch author, followed by an empty
656    line (only needed if the person sending the patch is not the author).
657
658  - The body of the explanation, line wrapped at 75 columns, which will
659    be copied to the permanent changelog to describe this patch.
660
661  - An empty line.
662
663  - The ``Signed-off-by:`` lines, described above, which will
664    also go in the changelog.
665
666  - A marker line containing simply ``---``.
667
668  - Any additional comments not suitable for the changelog.
669
670  - The actual patch (``diff`` output).
671
672The Subject line format makes it very easy to sort the emails
673alphabetically by subject line - pretty much any email reader will
674support that - since because the sequence number is zero-padded,
675the numerical and alphabetic sort is the same.
676
677The ``subsystem`` in the email's Subject should identify which
678area or subsystem of the kernel is being patched.
679
680The ``summary phrase`` in the email's Subject should concisely
681describe the patch which that email contains.  The ``summary
682phrase`` should not be a filename.  Do not use the same ``summary
683phrase`` for every patch in a whole patch series (where a ``patch
684series`` is an ordered sequence of multiple, related patches).
685
686Bear in mind that the ``summary phrase`` of your email becomes a
687globally-unique identifier for that patch.  It propagates all the way
688into the ``git`` changelog.  The ``summary phrase`` may later be used in
689developer discussions which refer to the patch.  People will want to
690google for the ``summary phrase`` to read discussion regarding that
691patch.  It will also be the only thing that people may quickly see
692when, two or three months later, they are going through perhaps
693thousands of patches using tools such as ``gitk`` or ``git log
694--oneline``.
695
696For these reasons, the ``summary`` must be no more than 70-75
697characters, and it must describe both what the patch changes, as well
698as why the patch might be necessary.  It is challenging to be both
699succinct and descriptive, but that is what a well-written summary
700should do.
701
702The ``summary phrase`` may be prefixed by tags enclosed in square
703brackets: "Subject: [PATCH <tag>...] <summary phrase>".  The tags are
704not considered part of the summary phrase, but describe how the patch
705should be treated.  Common tags might include a version descriptor if
706the multiple versions of the patch have been sent out in response to
707comments (i.e., "v1, v2, v3"), or "RFC" to indicate a request for
708comments.
709
710If there are four patches in a patch series the individual patches may
711be numbered like this: 1/4, 2/4, 3/4, 4/4. This assures that developers
712understand the order in which the patches should be applied and that
713they have reviewed or applied all of the patches in the patch series.
714
715Here are some good example Subjects::
716
717    Subject: [PATCH 2/5] ext2: improve scalability of bitmap searching
718    Subject: [PATCH v2 01/27] x86: fix eflags tracking
719    Subject: [PATCH v2] sub/sys: Condensed patch summary
720    Subject: [PATCH v2 M/N] sub/sys: Condensed patch summary
721
722From Line
723^^^^^^^^^
724
725The ``from`` line must be the very first line in the message body,
726and has the form:
727
728        From: Patch Author <author@example.com>
729
730The ``from`` line specifies who will be credited as the author of the
731patch in the permanent changelog.  If the ``from`` line is missing,
732then the ``From:`` line from the email header will be used to determine
733the patch author in the changelog.
734
735The author may indicate their affiliation or the sponsor of the work
736by adding the name of an organization to the ``from`` and ``SoB`` lines,
737e.g.:
738
739	From: Patch Author (Company) <author@example.com>
740
741Explanation Body
742^^^^^^^^^^^^^^^^
743
744The explanation body will be committed to the permanent source
745changelog, so should make sense to a competent reader who has long since
746forgotten the immediate details of the discussion that might have led to
747this patch. Including symptoms of the failure which the patch addresses
748(kernel log messages, oops messages, etc.) are especially useful for
749people who might be searching the commit logs looking for the applicable
750patch. The text should be written in such detail so that when read
751weeks, months or even years later, it can give the reader the needed
752details to grasp the reasoning for **why** the patch was created.
753
754If a patch fixes a compile failure, it may not be necessary to include
755_all_ of the compile failures; just enough that it is likely that
756someone searching for the patch can find it. As in the ``summary
757phrase``, it is important to be both succinct as well as descriptive.
758
759.. _backtraces:
760
761Backtraces in commit messages
762"""""""""""""""""""""""""""""
763
764Backtraces help document the call chain leading to a problem. However,
765not all backtraces are helpful. For example, early boot call chains are
766unique and obvious. Copying the full dmesg output verbatim, however,
767adds distracting information like timestamps, module lists, register and
768stack dumps.
769
770Therefore, the most useful backtraces should distill the relevant
771information from the dump, which makes it easier to focus on the real
772issue. Here is an example of a well-trimmed backtrace::
773
774  unchecked MSR access error: WRMSR to 0xd51 (tried to write 0x0000000000000064)
775  at rIP: 0xffffffffae059994 (native_write_msr+0x4/0x20)
776  Call Trace:
777  mba_wrmsr
778  update_domains
779  rdtgroup_mkdir
780
781Commentary
782^^^^^^^^^^
783
784The ``---`` marker line serves the essential purpose of marking for
785patch handling tools where the changelog message ends.
786
787One good use for the additional comments after the ``---`` marker is
788for a ``diffstat``, to show what files have changed, and the number of
789inserted and deleted lines per file. A ``diffstat`` is especially useful
790on bigger patches. If you are going to include a ``diffstat`` after the
791``---`` marker, please use ``diffstat`` options ``-p 1 -w 70`` so that
792filenames are listed from the top of the kernel source tree and don't
793use too much horizontal space (easily fit in 80 columns, maybe with some
794indentation). (``git`` generates appropriate diffstats by default.)
795
796Other comments relevant only to the moment or the maintainer, not
797suitable for the permanent changelog, should also go here. A good
798example of such comments might be ``patch changelogs`` which describe
799what has changed between the v1 and v2 version of the patch.
800
801Please put this information **after** the ``---`` line which separates
802the changelog from the rest of the patch. The version information is
803not part of the changelog which gets committed to the git tree. It is
804additional information for the reviewers. If it's placed above the
805commit tags, it needs manual interaction to remove it. If it is below
806the separator line, it gets automatically stripped off when applying the
807patch::
808
809  <commit message>
810  ...
811  Signed-off-by: Author <author@mail>
812  ---
813  V2 -> V3: Removed redundant helper function
814  V1 -> V2: Cleaned up coding style and addressed review comments
815
816  path/to/file | 5+++--
817  ...
818
819See more details on the proper patch format in the following
820references.
821
822.. _explicit_in_reply_to:
823
824Explicit In-Reply-To headers
825----------------------------
826
827It can be helpful to manually add In-Reply-To: headers to a patch
828(e.g., when using ``git send-email``) to associate the patch with
829previous relevant discussion, e.g. to link a bug fix to the email with
830the bug report.  However, for a multi-patch series, it is generally
831best to avoid using In-Reply-To: to link to older versions of the
832series.  This way multiple versions of the patch don't become an
833unmanageable forest of references in email clients.  If a link is
834helpful, you can use the https://lore.kernel.org/ redirector (e.g., in
835the cover email text) to link to an earlier version of the patch series.
836
837
838Providing base tree information
839-------------------------------
840
841When other developers receive your patches and start the review process,
842it is absolutely necessary for them to know what is the base
843commit/branch your work applies on, considering the sheer amount of
844maintainer trees present nowadays. Note again the **T:** entry in the
845MAINTAINERS file explained above.
846
847This is even more important for automated CI processes that attempt to
848run a series of tests in order to establish the quality of your
849submission before the maintainer starts the review.
850
851If you are using ``git format-patch`` to generate your patches, you can
852automatically include the base tree information in your submission by
853using the ``--base`` flag. The easiest and most convenient way to use
854this option is with topical branches::
855
856    $ git checkout -t -b my-topical-branch master
857    Branch 'my-topical-branch' set up to track local branch 'master'.
858    Switched to a new branch 'my-topical-branch'
859
860    [perform your edits and commits]
861
862    $ git format-patch --base=auto --cover-letter -o outgoing/ master
863    outgoing/0000-cover-letter.patch
864    outgoing/0001-First-Commit.patch
865    outgoing/...
866
867When you open ``outgoing/0000-cover-letter.patch`` for editing, you will
868notice that it will have the ``base-commit:`` trailer at the very
869bottom, which provides the reviewer and the CI tools enough information
870to properly perform ``git am`` without worrying about conflicts::
871
872    $ git checkout -b patch-review [base-commit-id]
873    Switched to a new branch 'patch-review'
874    $ git am patches.mbox
875    Applying: First Commit
876    Applying: ...
877
878Please see ``man git-format-patch`` for more information about this
879option.
880
881.. note::
882
883    The ``--base`` feature was introduced in git version 2.9.0.
884
885If you are not using git to format your patches, you can still include
886the same ``base-commit`` trailer to indicate the commit hash of the tree
887on which your work is based. You should add it either in the cover
888letter or in the first patch of the series and it should be placed
889either below the ``---`` line or at the very bottom of all other
890content, right before your email signature.
891
892Make sure that base commit is in an official maintainer/mainline tree
893and not in some internal, accessible only to you tree - otherwise it
894would be worthless.
895
896Tooling
897-------
898
899Many of the technical aspects of this process can be automated using
900b4, documented at <https://b4.docs.kernel.org/en/latest/>. This can
901help with things like tracking dependencies, running checkpatch and
902with formatting and sending mails.
903
904References
905----------
906
907Andrew Morton, "The perfect patch" (tpp).
908  <https://www.ozlabs.org/~akpm/stuff/tpp.txt>
909
910Jeff Garzik, "Linux kernel patch submission format".
911  <https://web.archive.org/web/20180829112450/http://linux.yyz.us/patch-format.html>
912
913Greg Kroah-Hartman, "How to piss off a kernel subsystem maintainer".
914  <http://www.kroah.com/log/linux/maintainer.html>
915
916  <http://www.kroah.com/log/linux/maintainer-02.html>
917
918  <http://www.kroah.com/log/linux/maintainer-03.html>
919
920  <http://www.kroah.com/log/linux/maintainer-04.html>
921
922  <http://www.kroah.com/log/linux/maintainer-05.html>
923
924  <http://www.kroah.com/log/linux/maintainer-06.html>
925
926Kernel Documentation/process/coding-style.rst
927
928Linus Torvalds's mail on the canonical patch format:
929  <https://lore.kernel.org/r/Pine.LNX.4.58.0504071023190.28951@ppc970.osdl.org>
930
931Andi Kleen, "On submitting kernel patches"
932  Some strategies to get difficult or controversial changes in.
933
934  http://halobates.de/on-submitting-patches.pdf
935