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 :doc:`development-process`. Also, read :doc:`submit-checklist`
14for a list of items to check before submitting code.  If you are submitting
15a driver, also read :doc:`submitting-drivers`; for device tree binding patches,
16read :doc:`submitting-patches`.
17
18This documentation assumes that you're using ``git`` to prepare your patches.
19If you're unfamiliar with ``git``, you would be well-advised to learn how to
20use it, it will make your life as a kernel developer and in general much
21easier.
22
23Obtain a current source tree
24----------------------------
25
26If you do not have a repository with the current kernel source handy, use
27``git`` to obtain one.  You'll want to start with the mainline repository,
28which can be grabbed with::
29
30  git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
31
32Note, however, that you may not want to develop against the mainline tree
33directly.  Most subsystem maintainers run their own trees and want to see
34patches prepared against those trees.  See the **T:** entry for the subsystem
35in the MAINTAINERS file to find that tree, or simply ask the maintainer if
36the tree is not listed there.
37
38.. _describe_changes:
39
40Describe your changes
41---------------------
42
43Describe your problem.  Whether your patch is a one-line bug fix or
445000 lines of a new feature, there must be an underlying problem that
45motivated you to do this work.  Convince the reviewer that there is a
46problem worth fixing and that it makes sense for them to read past the
47first paragraph.
48
49Describe user-visible impact.  Straight up crashes and lockups are
50pretty convincing, but not all bugs are that blatant.  Even if the
51problem was spotted during code review, describe the impact you think
52it can have on users.  Keep in mind that the majority of Linux
53installations run kernels from secondary stable trees or
54vendor/product-specific trees that cherry-pick only specific patches
55from upstream, so include anything that could help route your change
56downstream: provoking circumstances, excerpts from dmesg, crash
57descriptions, performance regressions, latency spikes, lockups, etc.
58
59Quantify optimizations and trade-offs.  If you claim improvements in
60performance, memory consumption, stack footprint, or binary size,
61include numbers that back them up.  But also describe non-obvious
62costs.  Optimizations usually aren't free but trade-offs between CPU,
63memory, and readability; or, when it comes to heuristics, between
64different workloads.  Describe the expected downsides of your
65optimization so that the reviewer can weigh costs against benefits.
66
67Once the problem is established, describe what you are actually doing
68about it in technical detail.  It's important to describe the change
69in plain English for the reviewer to verify that the code is behaving
70as you intend it to.
71
72The maintainer will thank you if you write your patch description in a
73form which can be easily pulled into Linux's source code management
74system, ``git``, as a "commit log".  See :ref:`explicit_in_reply_to`.
75
76Solve only one problem per patch.  If your description starts to get
77long, that's a sign that you probably need to split up your patch.
78See :ref:`split_changes`.
79
80When you submit or resubmit a patch or patch series, include the
81complete patch description and justification for it.  Don't just
82say that this is version N of the patch (series).  Don't expect the
83subsystem maintainer to refer back to earlier patch versions or referenced
84URLs to find the patch description and put that into the patch.
85I.e., the patch (series) and its description should be self-contained.
86This benefits both the maintainers and reviewers.  Some reviewers
87probably didn't even receive earlier versions of the patch.
88
89Describe your changes in imperative mood, e.g. "make xyzzy do frotz"
90instead of "[This patch] makes xyzzy do frotz" or "[I] changed xyzzy
91to do frotz", as if you are giving orders to the codebase to change
92its behaviour.
93
94If the patch fixes a logged bug entry, refer to that bug entry by
95number and URL.  If the patch follows from a mailing list discussion,
96give a URL to the mailing list archive; use the https://lkml.kernel.org/
97redirector with a ``Message-Id``, to ensure that the links cannot become
98stale.
99
100However, try to make your explanation understandable without external
101resources.  In addition to giving a URL to a mailing list archive or
102bug, summarize the relevant points of the discussion that led to the
103patch as submitted.
104
105If you want to refer to a specific commit, don't just refer to the
106SHA-1 ID of the commit. Please also include the oneline summary of
107the commit, to make it easier for reviewers to know what it is about.
108Example::
109
110	Commit e21d2170f36602ae2708 ("video: remove unnecessary
111	platform_set_drvdata()") removed the unnecessary
112	platform_set_drvdata(), but left the variable "dev" unused,
113	delete it.
114
115You should also be sure to use at least the first twelve characters of the
116SHA-1 ID.  The kernel repository holds a *lot* of objects, making
117collisions with shorter IDs a real possibility.  Bear in mind that, even if
118there is no collision with your six-character ID now, that condition may
119change five years from now.
120
121If your patch fixes a bug in a specific commit, e.g. you found an issue using
122``git bisect``, please use the 'Fixes:' tag with the first 12 characters of
123the SHA-1 ID, and the one line summary.  Do not split the tag across multiple
124lines, tags are exempt from the "wrap at 75 columns" rule in order to simplify
125parsing scripts.  For example::
126
127	Fixes: 54a4f0239f2e ("KVM: MMU: make kvm_mmu_zap_page() return the number of pages it actually freed")
128
129The following ``git config`` settings can be used to add a pretty format for
130outputting the above style in the ``git log`` or ``git show`` commands::
131
132	[core]
133		abbrev = 12
134	[pretty]
135		fixes = Fixes: %h (\"%s\")
136
137An example call::
138
139	$ git log -1 --pretty=fixes 54a4f0239f2e
140	Fixes: 54a4f0239f2e ("KVM: MMU: make kvm_mmu_zap_page() return the number of pages it actually freed")
141
142.. _split_changes:
143
144Separate your changes
145---------------------
146
147Separate each **logical change** into a separate patch.
148
149For example, if your changes include both bug fixes and performance
150enhancements for a single driver, separate those changes into two
151or more patches.  If your changes include an API update, and a new
152driver which uses that new API, separate those into two patches.
153
154On the other hand, if you make a single change to numerous files,
155group those changes into a single patch.  Thus a single logical change
156is contained within a single patch.
157
158The point to remember is that each patch should make an easily understood
159change that can be verified by reviewers.  Each patch should be justifiable
160on its own merits.
161
162If one patch depends on another patch in order for a change to be
163complete, that is OK.  Simply note **"this patch depends on patch X"**
164in your patch description.
165
166When dividing your change into a series of patches, take special care to
167ensure that the kernel builds and runs properly after each patch in the
168series.  Developers using ``git bisect`` to track down a problem can end up
169splitting your patch series at any point; they will not thank you if you
170introduce bugs in the middle.
171
172If you cannot condense your patch set into a smaller set of patches,
173then only post say 15 or so at a time and wait for review and integration.
174
175
176
177Style-check your changes
178------------------------
179
180Check your patch for basic style violations, details of which can be
181found in
182:ref:`Documentation/process/coding-style.rst <codingstyle>`.
183Failure to do so simply wastes
184the reviewers time and will get your patch rejected, probably
185without even being read.
186
187One significant exception is when moving code from one file to
188another -- in this case you should not modify the moved code at all in
189the same patch which moves it.  This clearly delineates the act of
190moving the code and your changes.  This greatly aids review of the
191actual differences and allows tools to better track the history of
192the code itself.
193
194Check your patches with the patch style checker prior to submission
195(scripts/checkpatch.pl).  Note, though, that the style checker should be
196viewed as a guide, not as a replacement for human judgment.  If your code
197looks better with a violation then its probably best left alone.
198
199The checker reports at three levels:
200 - ERROR: things that are very likely to be wrong
201 - WARNING: things requiring careful review
202 - CHECK: things requiring thought
203
204You should be able to justify all violations that remain in your
205patch.
206
207
208Select the recipients for your patch
209------------------------------------
210
211You should always copy the appropriate subsystem maintainer(s) on any patch
212to code that they maintain; look through the MAINTAINERS file and the
213source code revision history to see who those maintainers are.  The
214script scripts/get_maintainer.pl can be very useful at this step.  If you
215cannot find a maintainer for the subsystem you are working on, Andrew
216Morton (akpm@linux-foundation.org) serves as a maintainer of last resort.
217
218You should also normally choose at least one mailing list to receive a copy
219of your patch set.  linux-kernel@vger.kernel.org functions as a list of
220last resort, but the volume on that list has caused a number of developers
221to tune it out.  Look in the MAINTAINERS file for a subsystem-specific
222list; your patch will probably get more attention there.  Please do not
223spam unrelated lists, though.
224
225Many kernel-related lists are hosted on vger.kernel.org; you can find a
226list of them at http://vger.kernel.org/vger-lists.html.  There are
227kernel-related lists hosted elsewhere as well, though.
228
229Do not send more than 15 patches at once to the vger mailing lists!!!
230
231Linus Torvalds is the final arbiter of all changes accepted into the
232Linux kernel.  His e-mail address is <torvalds@linux-foundation.org>.
233He gets a lot of e-mail, and, at this point, very few patches go through
234Linus directly, so typically you should do your best to -avoid-
235sending him e-mail.
236
237If you have a patch that fixes an exploitable security bug, send that patch
238to security@kernel.org.  For severe bugs, a short embargo may be considered
239to allow distributors to get the patch out to users; in such cases,
240obviously, the patch should not be sent to any public lists. See also
241:doc:`/admin-guide/security-bugs`.
242
243Patches that fix a severe bug in a released kernel should be directed
244toward the stable maintainers by putting a line like this::
245
246  Cc: stable@vger.kernel.org
247
248into the sign-off area of your patch (note, NOT an email recipient).  You
249should also read
250:ref:`Documentation/process/stable-kernel-rules.rst <stable_kernel_rules>`
251in addition to this file.
252
253Note, however, that some subsystem maintainers want to come to their own
254conclusions on which patches should go to the stable trees.  The networking
255maintainer, in particular, would rather not see individual developers
256adding lines like the above to their patches.
257
258If changes affect userland-kernel interfaces, please send the MAN-PAGES
259maintainer (as listed in the MAINTAINERS file) a man-pages patch, or at
260least a notification of the change, so that some information makes its way
261into the manual pages.  User-space API changes should also be copied to
262linux-api@vger.kernel.org.
263
264For small patches you may want to CC the Trivial Patch Monkey
265trivial@kernel.org which collects "trivial" patches. Have a look
266into the MAINTAINERS file for its current manager.
267
268Trivial patches must qualify for one of the following rules:
269
270- Spelling fixes in documentation
271- Spelling fixes for errors which could break :manpage:`grep(1)`
272- Warning fixes (cluttering with useless warnings is bad)
273- Compilation fixes (only if they are actually correct)
274- Runtime fixes (only if they actually fix things)
275- Removing use of deprecated functions/macros
276- Contact detail and documentation fixes
277- Non-portable code replaced by portable code (even in arch-specific,
278  since people copy, as long as it's trivial)
279- Any fix by the author/maintainer of the file (ie. patch monkey
280  in re-transmission mode)
281
282
283
284No MIME, no links, no compression, no attachments.  Just plain text
285-------------------------------------------------------------------
286
287Linus and other kernel developers need to be able to read and comment
288on the changes you are submitting.  It is important for a kernel
289developer to be able to "quote" your changes, using standard e-mail
290tools, so that they may comment on specific portions of your code.
291
292For this reason, all patches should be submitted by e-mail "inline". The
293easiest way to do this is with ``git send-email``, which is strongly
294recommended.  An interactive tutorial for ``git send-email`` is available at
295https://git-send-email.io.
296
297If you choose not to use ``git send-email``:
298
299.. warning::
300
301  Be wary of your editor's word-wrap corrupting your patch,
302  if you choose to cut-n-paste your patch.
303
304Do not attach the patch as a MIME attachment, compressed or not.
305Many popular e-mail applications will not always transmit a MIME
306attachment as plain text, making it impossible to comment on your
307code.  A MIME attachment also takes Linus a bit more time to process,
308decreasing the likelihood of your MIME-attached change being accepted.
309
310Exception:  If your mailer is mangling patches then someone may ask
311you to re-send them using MIME.
312
313See :doc:`/process/email-clients` for hints about configuring your e-mail
314client so that it sends your patches untouched.
315
316Respond to review comments
317--------------------------
318
319Your patch will almost certainly get comments from reviewers on ways in
320which the patch can be improved, in the form of a reply to your email. You must
321respond to those comments; ignoring reviewers is a good way to get ignored in
322return. You can simply reply to their emails to answer their comments. Review
323comments or questions that do not lead to a code change should almost certainly
324bring about a comment or changelog entry so that the next reviewer better
325understands what is going on.
326
327Be sure to tell the reviewers what changes you are making and to thank them
328for their time.  Code review is a tiring and time-consuming process, and
329reviewers sometimes get grumpy.  Even in that case, though, respond
330politely and address the problems they have pointed out.
331
332See :doc:`email-clients` for recommendations on email
333clients and mailing list etiquette.
334
335
336Don't get discouraged - or impatient
337------------------------------------
338
339After you have submitted your change, be patient and wait.  Reviewers are
340busy people and may not get to your patch right away.
341
342Once upon a time, patches used to disappear into the void without comment,
343but the development process works more smoothly than that now.  You should
344receive comments within a week or so; if that does not happen, make sure
345that you have sent your patches to the right place.  Wait for a minimum of
346one week before resubmitting or pinging reviewers - possibly longer during
347busy times like merge windows.
348
349
350Include PATCH in the subject
351-----------------------------
352
353Due to high e-mail traffic to Linus, and to linux-kernel, it is common
354convention to prefix your subject line with [PATCH].  This lets Linus
355and other kernel developers more easily distinguish patches from other
356e-mail discussions.
357
358``git send-email`` will do this for you automatically.
359
360
361Sign your work - the Developer's Certificate of Origin
362------------------------------------------------------
363
364To improve tracking of who did what, especially with patches that can
365percolate to their final resting place in the kernel through several
366layers of maintainers, we've introduced a "sign-off" procedure on
367patches that are being emailed around.
368
369The sign-off is a simple line at the end of the explanation for the
370patch, which certifies that you wrote it or otherwise have the right to
371pass it on as an open-source patch.  The rules are pretty simple: if you
372can certify the below:
373
374Developer's Certificate of Origin 1.1
375^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
376
377By making a contribution to this project, I certify that:
378
379        (a) The contribution was created in whole or in part by me and I
380            have the right to submit it under the open source license
381            indicated in the file; or
382
383        (b) The contribution is based upon previous work that, to the best
384            of my knowledge, is covered under an appropriate open source
385            license and I have the right under that license to submit that
386            work with modifications, whether created in whole or in part
387            by me, under the same open source license (unless I am
388            permitted to submit under a different license), as indicated
389            in the file; or
390
391        (c) The contribution was provided directly to me by some other
392            person who certified (a), (b) or (c) and I have not modified
393            it.
394
395        (d) I understand and agree that this project and the contribution
396            are public and that a record of the contribution (including all
397            personal information I submit with it, including my sign-off) is
398            maintained indefinitely and may be redistributed consistent with
399            this project or the open source license(s) involved.
400
401then you just add a line saying::
402
403	Signed-off-by: Random J Developer <random@developer.example.org>
404
405using your real name (sorry, no pseudonyms or anonymous contributions.)
406This will be done for you automatically if you use ``git commit -s``.
407Reverts should also include "Signed-off-by". ``git revert -s`` does that
408for you.
409
410Some people also put extra tags at the end.  They'll just be ignored for
411now, but you can do this to mark internal company procedures or just
412point out some special detail about the sign-off.
413
414Any further SoBs (Signed-off-by:'s) following the author's SoB are from
415people handling and transporting the patch, but were not involved in its
416development. SoB chains should reflect the **real** route a patch took
417as it was propagated to the maintainers and ultimately to Linus, with
418the first SoB entry signalling primary authorship of a single author.
419
420
421When to use Acked-by:, Cc:, and Co-developed-by:
422------------------------------------------------
423
424The Signed-off-by: tag indicates that the signer was involved in the
425development of the patch, or that he/she was in the patch's delivery path.
426
427If a person was not directly involved in the preparation or handling of a
428patch but wishes to signify and record their approval of it then they can
429ask to have an Acked-by: line added to the patch's changelog.
430
431Acked-by: is often used by the maintainer of the affected code when that
432maintainer neither contributed to nor forwarded the patch.
433
434Acked-by: is not as formal as Signed-off-by:.  It is a record that the acker
435has at least reviewed the patch and has indicated acceptance.  Hence patch
436mergers will sometimes manually convert an acker's "yep, looks good to me"
437into an Acked-by: (but note that it is usually better to ask for an
438explicit ack).
439
440Acked-by: does not necessarily indicate acknowledgement of the entire patch.
441For example, if a patch affects multiple subsystems and has an Acked-by: from
442one subsystem maintainer then this usually indicates acknowledgement of just
443the part which affects that maintainer's code.  Judgement should be used here.
444When in doubt people should refer to the original discussion in the mailing
445list archives.
446
447If a person has had the opportunity to comment on a patch, but has not
448provided such comments, you may optionally add a ``Cc:`` tag to the patch.
449This is the only tag which might be added without an explicit action by the
450person it names - but it should indicate that this person was copied on the
451patch.  This tag documents that potentially interested parties
452have been included in the discussion.
453
454Co-developed-by: states that the patch was co-created by multiple developers;
455it is used to give attribution to co-authors (in addition to the author
456attributed by the From: tag) when several people work on a single patch.  Since
457Co-developed-by: denotes authorship, every Co-developed-by: must be immediately
458followed by a Signed-off-by: of the associated co-author.  Standard sign-off
459procedure applies, i.e. the ordering of Signed-off-by: tags should reflect the
460chronological history of the patch insofar as possible, regardless of whether
461the author is attributed via From: or Co-developed-by:.  Notably, the last
462Signed-off-by: must always be that of the developer submitting the patch.
463
464Note, the From: tag is optional when the From: author is also the person (and
465email) listed in the From: line of the email header.
466
467Example of a patch submitted by the From: author::
468
469	<changelog>
470
471	Co-developed-by: First Co-Author <first@coauthor.example.org>
472	Signed-off-by: First Co-Author <first@coauthor.example.org>
473	Co-developed-by: Second Co-Author <second@coauthor.example.org>
474	Signed-off-by: Second Co-Author <second@coauthor.example.org>
475	Signed-off-by: From Author <from@author.example.org>
476
477Example of a patch submitted by a Co-developed-by: author::
478
479	From: From Author <from@author.example.org>
480
481	<changelog>
482
483	Co-developed-by: Random Co-Author <random@coauthor.example.org>
484	Signed-off-by: Random Co-Author <random@coauthor.example.org>
485	Signed-off-by: From Author <from@author.example.org>
486	Co-developed-by: Submitting Co-Author <sub@coauthor.example.org>
487	Signed-off-by: Submitting Co-Author <sub@coauthor.example.org>
488
489
490Using Reported-by:, Tested-by:, Reviewed-by:, Suggested-by: and Fixes:
491----------------------------------------------------------------------
492
493The Reported-by tag gives credit to people who find bugs and report them and it
494hopefully inspires them to help us again in the future.  Please note that if
495the bug was reported in private, then ask for permission first before using the
496Reported-by tag.
497
498A Tested-by: tag indicates that the patch has been successfully tested (in
499some environment) by the person named.  This tag informs maintainers that
500some testing has been performed, provides a means to locate testers for
501future patches, and ensures credit for the testers.
502
503Reviewed-by:, instead, indicates that the patch has been reviewed and found
504acceptable according to the Reviewer's Statement:
505
506Reviewer's statement of oversight
507^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
508
509By offering my Reviewed-by: tag, I state that:
510
511	 (a) I have carried out a technical review of this patch to
512	     evaluate its appropriateness and readiness for inclusion into
513	     the mainline kernel.
514
515	 (b) Any problems, concerns, or questions relating to the patch
516	     have been communicated back to the submitter.  I am satisfied
517	     with the submitter's response to my comments.
518
519	 (c) While there may be things that could be improved with this
520	     submission, I believe that it is, at this time, (1) a
521	     worthwhile modification to the kernel, and (2) free of known
522	     issues which would argue against its inclusion.
523
524	 (d) While I have reviewed the patch and believe it to be sound, I
525	     do not (unless explicitly stated elsewhere) make any
526	     warranties or guarantees that it will achieve its stated
527	     purpose or function properly in any given situation.
528
529A Reviewed-by tag is a statement of opinion that the patch is an
530appropriate modification of the kernel without any remaining serious
531technical issues.  Any interested reviewer (who has done the work) can
532offer a Reviewed-by tag for a patch.  This tag serves to give credit to
533reviewers and to inform maintainers of the degree of review which has been
534done on the patch.  Reviewed-by: tags, when supplied by reviewers known to
535understand the subject area and to perform thorough reviews, will normally
536increase the likelihood of your patch getting into the kernel.
537
538Both Tested-by and Reviewed-by tags, once received on mailing list from tester
539or reviewer, should be added by author to the applicable patches when sending
540next versions.  However if the patch has changed substantially in following
541version, these tags might not be applicable anymore and thus should be removed.
542Usually removal of someone's Tested-by or Reviewed-by tags should be mentioned
543in the patch changelog (after the '---' separator).
544
545A Suggested-by: tag indicates that the patch idea is suggested by the person
546named and ensures credit to the person for the idea. Please note that this
547tag should not be added without the reporter's permission, especially if the
548idea was not posted in a public forum. That said, if we diligently credit our
549idea reporters, they will, hopefully, be inspired to help us again in the
550future.
551
552A Fixes: tag indicates that the patch fixes an issue in a previous commit. It
553is used to make it easy to determine where a bug originated, which can help
554review a bug fix. This tag also assists the stable kernel team in determining
555which stable kernel versions should receive your fix. This is the preferred
556method for indicating a bug fixed by the patch. See :ref:`describe_changes`
557for more details.
558
559.. _the_canonical_patch_format:
560
561The canonical patch format
562--------------------------
563
564This section describes how the patch itself should be formatted.  Note
565that, if you have your patches stored in a ``git`` repository, proper patch
566formatting can be had with ``git format-patch``.  The tools cannot create
567the necessary text, though, so read the instructions below anyway.
568
569The canonical patch subject line is::
570
571    Subject: [PATCH 001/123] subsystem: summary phrase
572
573The canonical patch message body contains the following:
574
575  - A ``from`` line specifying the patch author, followed by an empty
576    line (only needed if the person sending the patch is not the author).
577
578  - The body of the explanation, line wrapped at 75 columns, which will
579    be copied to the permanent changelog to describe this patch.
580
581  - An empty line.
582
583  - The ``Signed-off-by:`` lines, described above, which will
584    also go in the changelog.
585
586  - A marker line containing simply ``---``.
587
588  - Any additional comments not suitable for the changelog.
589
590  - The actual patch (``diff`` output).
591
592The Subject line format makes it very easy to sort the emails
593alphabetically by subject line - pretty much any email reader will
594support that - since because the sequence number is zero-padded,
595the numerical and alphabetic sort is the same.
596
597The ``subsystem`` in the email's Subject should identify which
598area or subsystem of the kernel is being patched.
599
600The ``summary phrase`` in the email's Subject should concisely
601describe the patch which that email contains.  The ``summary
602phrase`` should not be a filename.  Do not use the same ``summary
603phrase`` for every patch in a whole patch series (where a ``patch
604series`` is an ordered sequence of multiple, related patches).
605
606Bear in mind that the ``summary phrase`` of your email becomes a
607globally-unique identifier for that patch.  It propagates all the way
608into the ``git`` changelog.  The ``summary phrase`` may later be used in
609developer discussions which refer to the patch.  People will want to
610google for the ``summary phrase`` to read discussion regarding that
611patch.  It will also be the only thing that people may quickly see
612when, two or three months later, they are going through perhaps
613thousands of patches using tools such as ``gitk`` or ``git log
614--oneline``.
615
616For these reasons, the ``summary`` must be no more than 70-75
617characters, and it must describe both what the patch changes, as well
618as why the patch might be necessary.  It is challenging to be both
619succinct and descriptive, but that is what a well-written summary
620should do.
621
622The ``summary phrase`` may be prefixed by tags enclosed in square
623brackets: "Subject: [PATCH <tag>...] <summary phrase>".  The tags are
624not considered part of the summary phrase, but describe how the patch
625should be treated.  Common tags might include a version descriptor if
626the multiple versions of the patch have been sent out in response to
627comments (i.e., "v1, v2, v3"), or "RFC" to indicate a request for
628comments.  If there are four patches in a patch series the individual
629patches may be numbered like this: 1/4, 2/4, 3/4, 4/4.  This assures
630that developers understand the order in which the patches should be
631applied and that they have reviewed or applied all of the patches in
632the patch series.
633
634A couple of example Subjects::
635
636    Subject: [PATCH 2/5] ext2: improve scalability of bitmap searching
637    Subject: [PATCH v2 01/27] x86: fix eflags tracking
638
639The ``from`` line must be the very first line in the message body,
640and has the form:
641
642        From: Patch Author <author@example.com>
643
644The ``from`` line specifies who will be credited as the author of the
645patch in the permanent changelog.  If the ``from`` line is missing,
646then the ``From:`` line from the email header will be used to determine
647the patch author in the changelog.
648
649The explanation body will be committed to the permanent source
650changelog, so should make sense to a competent reader who has long
651since forgotten the immediate details of the discussion that might
652have led to this patch.  Including symptoms of the failure which the
653patch addresses (kernel log messages, oops messages, etc.) is
654especially useful for people who might be searching the commit logs
655looking for the applicable patch.  If a patch fixes a compile failure,
656it may not be necessary to include _all_ of the compile failures; just
657enough that it is likely that someone searching for the patch can find
658it.  As in the ``summary phrase``, it is important to be both succinct as
659well as descriptive.
660
661The ``---`` marker line serves the essential purpose of marking for patch
662handling tools where the changelog message ends.
663
664One good use for the additional comments after the ``---`` marker is for
665a ``diffstat``, to show what files have changed, and the number of
666inserted and deleted lines per file.  A ``diffstat`` is especially useful
667on bigger patches.  Other comments relevant only to the moment or the
668maintainer, not suitable for the permanent changelog, should also go
669here.  A good example of such comments might be ``patch changelogs``
670which describe what has changed between the v1 and v2 version of the
671patch.
672
673If you are going to include a ``diffstat`` after the ``---`` marker, please
674use ``diffstat`` options ``-p 1 -w 70`` so that filenames are listed from
675the top of the kernel source tree and don't use too much horizontal
676space (easily fit in 80 columns, maybe with some indentation).  (``git``
677generates appropriate diffstats by default.)
678
679See more details on the proper patch format in the following
680references.
681
682.. _explicit_in_reply_to:
683
684Explicit In-Reply-To headers
685----------------------------
686
687It can be helpful to manually add In-Reply-To: headers to a patch
688(e.g., when using ``git send-email``) to associate the patch with
689previous relevant discussion, e.g. to link a bug fix to the email with
690the bug report.  However, for a multi-patch series, it is generally
691best to avoid using In-Reply-To: to link to older versions of the
692series.  This way multiple versions of the patch don't become an
693unmanageable forest of references in email clients.  If a link is
694helpful, you can use the https://lkml.kernel.org/ redirector (e.g., in
695the cover email text) to link to an earlier version of the patch series.
696
697
698Providing base tree information
699-------------------------------
700
701When other developers receive your patches and start the review process,
702it is often useful for them to know where in the tree history they
703should place your work. This is particularly useful for automated CI
704processes that attempt to run a series of tests in order to establish
705the quality of your submission before the maintainer starts the review.
706
707If you are using ``git format-patch`` to generate your patches, you can
708automatically include the base tree information in your submission by
709using the ``--base`` flag. The easiest and most convenient way to use
710this option is with topical branches::
711
712    $ git checkout -t -b my-topical-branch master
713    Branch 'my-topical-branch' set up to track local branch 'master'.
714    Switched to a new branch 'my-topical-branch'
715
716    [perform your edits and commits]
717
718    $ git format-patch --base=auto --cover-letter -o outgoing/ master
719    outgoing/0000-cover-letter.patch
720    outgoing/0001-First-Commit.patch
721    outgoing/...
722
723When you open ``outgoing/0000-cover-letter.patch`` for editing, you will
724notice that it will have the ``base-commit:`` trailer at the very
725bottom, which provides the reviewer and the CI tools enough information
726to properly perform ``git am`` without worrying about conflicts::
727
728    $ git checkout -b patch-review [base-commit-id]
729    Switched to a new branch 'patch-review'
730    $ git am patches.mbox
731    Applying: First Commit
732    Applying: ...
733
734Please see ``man git-format-patch`` for more information about this
735option.
736
737.. note::
738
739    The ``--base`` feature was introduced in git version 2.9.0.
740
741If you are not using git to format your patches, you can still include
742the same ``base-commit`` trailer to indicate the commit hash of the tree
743on which your work is based. You should add it either in the cover
744letter or in the first patch of the series and it should be placed
745either below the ``---`` line or at the very bottom of all other
746content, right before your email signature.
747
748
749References
750----------
751
752Andrew Morton, "The perfect patch" (tpp).
753  <https://www.ozlabs.org/~akpm/stuff/tpp.txt>
754
755Jeff Garzik, "Linux kernel patch submission format".
756  <https://web.archive.org/web/20180829112450/http://linux.yyz.us/patch-format.html>
757
758Greg Kroah-Hartman, "How to piss off a kernel subsystem maintainer".
759  <http://www.kroah.com/log/linux/maintainer.html>
760
761  <http://www.kroah.com/log/linux/maintainer-02.html>
762
763  <http://www.kroah.com/log/linux/maintainer-03.html>
764
765  <http://www.kroah.com/log/linux/maintainer-04.html>
766
767  <http://www.kroah.com/log/linux/maintainer-05.html>
768
769  <http://www.kroah.com/log/linux/maintainer-06.html>
770
771NO!!!! No more huge patch bombs to linux-kernel@vger.kernel.org people!
772  <https://lkml.org/lkml/2005/7/11/336>
773
774Kernel Documentation/process/coding-style.rst:
775  :ref:`Documentation/process/coding-style.rst <codingstyle>`
776
777Linus Torvalds's mail on the canonical patch format:
778  <http://lkml.org/lkml/2005/4/7/183>
779
780Andi Kleen, "On submitting kernel patches"
781  Some strategies to get difficult or controversial changes in.
782
783  http://halobates.de/on-submitting-patches.pdf
784