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 :ref:`Documentation/process <development_process_main>`.
14Also, read :ref:`Documentation/process/submit-checklist.rst <submitchecklist>`
15for a list of items to check before
16submitting code.  If you are submitting a driver, also read
17:ref:`Documentation/process/submitting-drivers.rst <submittingdrivers>`;
18for device tree binding patches, read
19Documentation/devicetree/bindings/submitting-patches.rst.
20
21Many of these steps describe the default behavior of the ``git`` version
22control system; if you use ``git`` to prepare your patches, you'll find much
23of the mechanical work done for you, though you'll still need to prepare
24and document a sensible set of patches.  In general, use of ``git`` will make
25your life as a kernel developer easier.
26
270) Obtain a current source tree
28-------------------------------
29
30If you do not have a repository with the current kernel source handy, use
31``git`` to obtain one.  You'll want to start with the mainline repository,
32which can be grabbed with::
33
34  git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
35
36Note, however, that you may not want to develop against the mainline tree
37directly.  Most subsystem maintainers run their own trees and want to see
38patches prepared against those trees.  See the **T:** entry for the subsystem
39in the MAINTAINERS file to find that tree, or simply ask the maintainer if
40the tree is not listed there.
41
42It is still possible to download kernel releases via tarballs (as described
43in the next section), but that is the hard way to do kernel development.
44
451) ``diff -up``
46---------------
47
48If you must generate your patches by hand, use ``diff -up`` or ``diff -uprN``
49to create patches.  Git generates patches in this form by default; if
50you're using ``git``, you can skip this section entirely.
51
52All changes to the Linux kernel occur in the form of patches, as
53generated by :manpage:`diff(1)`.  When creating your patch, make sure to
54create it in "unified diff" format, as supplied by the ``-u`` argument
55to :manpage:`diff(1)`.
56Also, please use the ``-p`` argument which shows which C function each
57change is in - that makes the resultant ``diff`` a lot easier to read.
58Patches should be based in the root kernel source directory,
59not in any lower subdirectory.
60
61To create a patch for a single file, it is often sufficient to do::
62
63	SRCTREE=linux
64	MYFILE=drivers/net/mydriver.c
65
66	cd $SRCTREE
67	cp $MYFILE $MYFILE.orig
68	vi $MYFILE	# make your change
69	cd ..
70	diff -up $SRCTREE/$MYFILE{.orig,} > /tmp/patch
71
72To create a patch for multiple files, you should unpack a "vanilla",
73or unmodified kernel source tree, and generate a ``diff`` against your
74own source tree.  For example::
75
76	MYSRC=/devel/linux
77
78	tar xvfz linux-3.19.tar.gz
79	mv linux-3.19 linux-3.19-vanilla
80	diff -uprN -X linux-3.19-vanilla/Documentation/dontdiff \
81		linux-3.19-vanilla $MYSRC > /tmp/patch
82
83``dontdiff`` is a list of files which are generated by the kernel during
84the build process, and should be ignored in any :manpage:`diff(1)`-generated
85patch.
86
87Make sure your patch does not include any extra files which do not
88belong in a patch submission.  Make sure to review your patch -after-
89generating it with :manpage:`diff(1)`, to ensure accuracy.
90
91If your changes produce a lot of deltas, you need to split them into
92individual patches which modify things in logical stages; see
93:ref:`split_changes`.  This will facilitate review by other kernel developers,
94very important if you want your patch accepted.
95
96If you're using ``git``, ``git rebase -i`` can help you with this process.  If
97you're not using ``git``, ``quilt`` <http://savannah.nongnu.org/projects/quilt>
98is another popular alternative.
99
100.. _describe_changes:
101
1022) Describe your changes
103------------------------
104
105Describe your problem.  Whether your patch is a one-line bug fix or
1065000 lines of a new feature, there must be an underlying problem that
107motivated you to do this work.  Convince the reviewer that there is a
108problem worth fixing and that it makes sense for them to read past the
109first paragraph.
110
111Describe user-visible impact.  Straight up crashes and lockups are
112pretty convincing, but not all bugs are that blatant.  Even if the
113problem was spotted during code review, describe the impact you think
114it can have on users.  Keep in mind that the majority of Linux
115installations run kernels from secondary stable trees or
116vendor/product-specific trees that cherry-pick only specific patches
117from upstream, so include anything that could help route your change
118downstream: provoking circumstances, excerpts from dmesg, crash
119descriptions, performance regressions, latency spikes, lockups, etc.
120
121Quantify optimizations and trade-offs.  If you claim improvements in
122performance, memory consumption, stack footprint, or binary size,
123include numbers that back them up.  But also describe non-obvious
124costs.  Optimizations usually aren't free but trade-offs between CPU,
125memory, and readability; or, when it comes to heuristics, between
126different workloads.  Describe the expected downsides of your
127optimization so that the reviewer can weigh costs against benefits.
128
129Once the problem is established, describe what you are actually doing
130about it in technical detail.  It's important to describe the change
131in plain English for the reviewer to verify that the code is behaving
132as you intend it to.
133
134The maintainer will thank you if you write your patch description in a
135form which can be easily pulled into Linux's source code management
136system, ``git``, as a "commit log".  See :ref:`explicit_in_reply_to`.
137
138Solve only one problem per patch.  If your description starts to get
139long, that's a sign that you probably need to split up your patch.
140See :ref:`split_changes`.
141
142When you submit or resubmit a patch or patch series, include the
143complete patch description and justification for it.  Don't just
144say that this is version N of the patch (series).  Don't expect the
145subsystem maintainer to refer back to earlier patch versions or referenced
146URLs to find the patch description and put that into the patch.
147I.e., the patch (series) and its description should be self-contained.
148This benefits both the maintainers and reviewers.  Some reviewers
149probably didn't even receive earlier versions of the patch.
150
151Describe your changes in imperative mood, e.g. "make xyzzy do frotz"
152instead of "[This patch] makes xyzzy do frotz" or "[I] changed xyzzy
153to do frotz", as if you are giving orders to the codebase to change
154its behaviour.
155
156If the patch fixes a logged bug entry, refer to that bug entry by
157number and URL.  If the patch follows from a mailing list discussion,
158give a URL to the mailing list archive; use the https://lkml.kernel.org/
159redirector with a ``Message-Id``, to ensure that the links cannot become
160stale.
161
162However, try to make your explanation understandable without external
163resources.  In addition to giving a URL to a mailing list archive or
164bug, summarize the relevant points of the discussion that led to the
165patch as submitted.
166
167If you want to refer to a specific commit, don't just refer to the
168SHA-1 ID of the commit. Please also include the oneline summary of
169the commit, to make it easier for reviewers to know what it is about.
170Example::
171
172	Commit e21d2170f36602ae2708 ("video: remove unnecessary
173	platform_set_drvdata()") removed the unnecessary
174	platform_set_drvdata(), but left the variable "dev" unused,
175	delete it.
176
177You should also be sure to use at least the first twelve characters of the
178SHA-1 ID.  The kernel repository holds a *lot* of objects, making
179collisions with shorter IDs a real possibility.  Bear in mind that, even if
180there is no collision with your six-character ID now, that condition may
181change five years from now.
182
183If your patch fixes a bug in a specific commit, e.g. you found an issue using
184``git bisect``, please use the 'Fixes:' tag with the first 12 characters of
185the SHA-1 ID, and the one line summary.  Do not split the tag across multiple
186lines, tags are exempt from the "wrap at 75 columns" rule in order to simplify
187parsing scripts.  For example::
188
189	Fixes: 54a4f0239f2e ("KVM: MMU: make kvm_mmu_zap_page() return the number of pages it actually freed")
190
191The following ``git config`` settings can be used to add a pretty format for
192outputting the above style in the ``git log`` or ``git show`` commands::
193
194	[core]
195		abbrev = 12
196	[pretty]
197		fixes = Fixes: %h (\"%s\")
198
199.. _split_changes:
200
2013) Separate your changes
202------------------------
203
204Separate each **logical change** into a separate patch.
205
206For example, if your changes include both bug fixes and performance
207enhancements for a single driver, separate those changes into two
208or more patches.  If your changes include an API update, and a new
209driver which uses that new API, separate those into two patches.
210
211On the other hand, if you make a single change to numerous files,
212group those changes into a single patch.  Thus a single logical change
213is contained within a single patch.
214
215The point to remember is that each patch should make an easily understood
216change that can be verified by reviewers.  Each patch should be justifiable
217on its own merits.
218
219If one patch depends on another patch in order for a change to be
220complete, that is OK.  Simply note **"this patch depends on patch X"**
221in your patch description.
222
223When dividing your change into a series of patches, take special care to
224ensure that the kernel builds and runs properly after each patch in the
225series.  Developers using ``git bisect`` to track down a problem can end up
226splitting your patch series at any point; they will not thank you if you
227introduce bugs in the middle.
228
229If you cannot condense your patch set into a smaller set of patches,
230then only post say 15 or so at a time and wait for review and integration.
231
232
233
2344) Style-check your changes
235---------------------------
236
237Check your patch for basic style violations, details of which can be
238found in
239:ref:`Documentation/process/coding-style.rst <codingstyle>`.
240Failure to do so simply wastes
241the reviewers time and will get your patch rejected, probably
242without even being read.
243
244One significant exception is when moving code from one file to
245another -- in this case you should not modify the moved code at all in
246the same patch which moves it.  This clearly delineates the act of
247moving the code and your changes.  This greatly aids review of the
248actual differences and allows tools to better track the history of
249the code itself.
250
251Check your patches with the patch style checker prior to submission
252(scripts/checkpatch.pl).  Note, though, that the style checker should be
253viewed as a guide, not as a replacement for human judgment.  If your code
254looks better with a violation then its probably best left alone.
255
256The checker reports at three levels:
257 - ERROR: things that are very likely to be wrong
258 - WARNING: things requiring careful review
259 - CHECK: things requiring thought
260
261You should be able to justify all violations that remain in your
262patch.
263
264
2655) Select the recipients for your patch
266---------------------------------------
267
268You should always copy the appropriate subsystem maintainer(s) on any patch
269to code that they maintain; look through the MAINTAINERS file and the
270source code revision history to see who those maintainers are.  The
271script scripts/get_maintainer.pl can be very useful at this step.  If you
272cannot find a maintainer for the subsystem you are working on, Andrew
273Morton (akpm@linux-foundation.org) serves as a maintainer of last resort.
274
275You should also normally choose at least one mailing list to receive a copy
276of your patch set.  linux-kernel@vger.kernel.org functions as a list of
277last resort, but the volume on that list has caused a number of developers
278to tune it out.  Look in the MAINTAINERS file for a subsystem-specific
279list; your patch will probably get more attention there.  Please do not
280spam unrelated lists, though.
281
282Many kernel-related lists are hosted on vger.kernel.org; you can find a
283list of them at http://vger.kernel.org/vger-lists.html.  There are
284kernel-related lists hosted elsewhere as well, though.
285
286Do not send more than 15 patches at once to the vger mailing lists!!!
287
288Linus Torvalds is the final arbiter of all changes accepted into the
289Linux kernel.  His e-mail address is <torvalds@linux-foundation.org>.
290He gets a lot of e-mail, and, at this point, very few patches go through
291Linus directly, so typically you should do your best to -avoid-
292sending him e-mail.
293
294If you have a patch that fixes an exploitable security bug, send that patch
295to security@kernel.org.  For severe bugs, a short embargo may be considered
296to allow distributors to get the patch out to users; in such cases,
297obviously, the patch should not be sent to any public lists.
298
299Patches that fix a severe bug in a released kernel should be directed
300toward the stable maintainers by putting a line like this::
301
302  Cc: stable@vger.kernel.org
303
304into the sign-off area of your patch (note, NOT an email recipient).  You
305should also read
306:ref:`Documentation/process/stable-kernel-rules.rst <stable_kernel_rules>`
307in addition to this file.
308
309Note, however, that some subsystem maintainers want to come to their own
310conclusions on which patches should go to the stable trees.  The networking
311maintainer, in particular, would rather not see individual developers
312adding lines like the above to their patches.
313
314If changes affect userland-kernel interfaces, please send the MAN-PAGES
315maintainer (as listed in the MAINTAINERS file) a man-pages patch, or at
316least a notification of the change, so that some information makes its way
317into the manual pages.  User-space API changes should also be copied to
318linux-api@vger.kernel.org.
319
320For small patches you may want to CC the Trivial Patch Monkey
321trivial@kernel.org which collects "trivial" patches. Have a look
322into the MAINTAINERS file for its current manager.
323
324Trivial patches must qualify for one of the following rules:
325
326- Spelling fixes in documentation
327- Spelling fixes for errors which could break :manpage:`grep(1)`
328- Warning fixes (cluttering with useless warnings is bad)
329- Compilation fixes (only if they are actually correct)
330- Runtime fixes (only if they actually fix things)
331- Removing use of deprecated functions/macros
332- Contact detail and documentation fixes
333- Non-portable code replaced by portable code (even in arch-specific,
334  since people copy, as long as it's trivial)
335- Any fix by the author/maintainer of the file (ie. patch monkey
336  in re-transmission mode)
337
338
339
3406) No MIME, no links, no compression, no attachments.  Just plain text
341----------------------------------------------------------------------
342
343Linus and other kernel developers need to be able to read and comment
344on the changes you are submitting.  It is important for a kernel
345developer to be able to "quote" your changes, using standard e-mail
346tools, so that they may comment on specific portions of your code.
347
348For this reason, all patches should be submitted by e-mail "inline".
349
350.. warning::
351
352  Be wary of your editor's word-wrap corrupting your patch,
353  if you choose to cut-n-paste your patch.
354
355Do not attach the patch as a MIME attachment, compressed or not.
356Many popular e-mail applications will not always transmit a MIME
357attachment as plain text, making it impossible to comment on your
358code.  A MIME attachment also takes Linus a bit more time to process,
359decreasing the likelihood of your MIME-attached change being accepted.
360
361Exception:  If your mailer is mangling patches then someone may ask
362you to re-send them using MIME.
363
364See :ref:`Documentation/process/email-clients.rst <email_clients>`
365for hints about configuring your e-mail client so that it sends your patches
366untouched.
367
3687) E-mail size
369--------------
370
371Large changes are not appropriate for mailing lists, and some
372maintainers.  If your patch, uncompressed, exceeds 300 kB in size,
373it is preferred that you store your patch on an Internet-accessible
374server, and provide instead a URL (link) pointing to your patch.  But note
375that if your patch exceeds 300 kB, it almost certainly needs to be broken up
376anyway.
377
3788) Respond to review comments
379-----------------------------
380
381Your patch will almost certainly get comments from reviewers on ways in
382which the patch can be improved.  You must respond to those comments;
383ignoring reviewers is a good way to get ignored in return.  Review comments
384or questions that do not lead to a code change should almost certainly
385bring about a comment or changelog entry so that the next reviewer better
386understands what is going on.
387
388Be sure to tell the reviewers what changes you are making and to thank them
389for their time.  Code review is a tiring and time-consuming process, and
390reviewers sometimes get grumpy.  Even in that case, though, respond
391politely and address the problems they have pointed out.
392
393
3949) Don't get discouraged - or impatient
395---------------------------------------
396
397After you have submitted your change, be patient and wait.  Reviewers are
398busy people and may not get to your patch right away.
399
400Once upon a time, patches used to disappear into the void without comment,
401but the development process works more smoothly than that now.  You should
402receive comments within a week or so; if that does not happen, make sure
403that you have sent your patches to the right place.  Wait for a minimum of
404one week before resubmitting or pinging reviewers - possibly longer during
405busy times like merge windows.
406
407
40810) Include PATCH in the subject
409--------------------------------
410
411Due to high e-mail traffic to Linus, and to linux-kernel, it is common
412convention to prefix your subject line with [PATCH].  This lets Linus
413and other kernel developers more easily distinguish patches from other
414e-mail discussions.
415
416
417
41811) Sign your work - the Developer's Certificate of Origin
419----------------------------------------------------------
420
421To improve tracking of who did what, especially with patches that can
422percolate to their final resting place in the kernel through several
423layers of maintainers, we've introduced a "sign-off" procedure on
424patches that are being emailed around.
425
426The sign-off is a simple line at the end of the explanation for the
427patch, which certifies that you wrote it or otherwise have the right to
428pass it on as an open-source patch.  The rules are pretty simple: if you
429can certify the below:
430
431Developer's Certificate of Origin 1.1
432^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
433
434By making a contribution to this project, I certify that:
435
436        (a) The contribution was created in whole or in part by me and I
437            have the right to submit it under the open source license
438            indicated in the file; or
439
440        (b) The contribution is based upon previous work that, to the best
441            of my knowledge, is covered under an appropriate open source
442            license and I have the right under that license to submit that
443            work with modifications, whether created in whole or in part
444            by me, under the same open source license (unless I am
445            permitted to submit under a different license), as indicated
446            in the file; or
447
448        (c) The contribution was provided directly to me by some other
449            person who certified (a), (b) or (c) and I have not modified
450            it.
451
452        (d) I understand and agree that this project and the contribution
453            are public and that a record of the contribution (including all
454            personal information I submit with it, including my sign-off) is
455            maintained indefinitely and may be redistributed consistent with
456            this project or the open source license(s) involved.
457
458then you just add a line saying::
459
460	Signed-off-by: Random J Developer <random@developer.example.org>
461
462using your real name (sorry, no pseudonyms or anonymous contributions.)
463
464Some people also put extra tags at the end.  They'll just be ignored for
465now, but you can do this to mark internal company procedures or just
466point out some special detail about the sign-off.
467
468If you are a subsystem or branch maintainer, sometimes you need to slightly
469modify patches you receive in order to merge them, because the code is not
470exactly the same in your tree and the submitters'. If you stick strictly to
471rule (c), you should ask the submitter to rediff, but this is a totally
472counter-productive waste of time and energy. Rule (b) allows you to adjust
473the code, but then it is very impolite to change one submitter's code and
474make him endorse your bugs. To solve this problem, it is recommended that
475you add a line between the last Signed-off-by header and yours, indicating
476the nature of your changes. While there is nothing mandatory about this, it
477seems like prepending the description with your mail and/or name, all
478enclosed in square brackets, is noticeable enough to make it obvious that
479you are responsible for last-minute changes. Example::
480
481	Signed-off-by: Random J Developer <random@developer.example.org>
482	[lucky@maintainer.example.org: struct foo moved from foo.c to foo.h]
483	Signed-off-by: Lucky K Maintainer <lucky@maintainer.example.org>
484
485This practice is particularly helpful if you maintain a stable branch and
486want at the same time to credit the author, track changes, merge the fix,
487and protect the submitter from complaints. Note that under no circumstances
488can you change the author's identity (the From header), as it is the one
489which appears in the changelog.
490
491Special note to back-porters: It seems to be a common and useful practice
492to insert an indication of the origin of a patch at the top of the commit
493message (just after the subject line) to facilitate tracking. For instance,
494here's what we see in a 3.x-stable release::
495
496  Date:   Tue Oct 7 07:26:38 2014 -0400
497
498    libata: Un-break ATA blacklist
499
500    commit 1c40279960bcd7d52dbdf1d466b20d24b99176c8 upstream.
501
502And here's what might appear in an older kernel once a patch is backported::
503
504    Date:   Tue May 13 22:12:27 2008 +0200
505
506        wireless, airo: waitbusy() won't delay
507
508        [backport of 2.6 commit b7acbdfbd1f277c1eb23f344f899cfa4cd0bf36a]
509
510Whatever the format, this information provides a valuable help to people
511tracking your trees, and to people trying to troubleshoot bugs in your
512tree.
513
514
51512) When to use Acked-by:, Cc:, and Co-developed-by:
516-------------------------------------------------------
517
518The Signed-off-by: tag indicates that the signer was involved in the
519development of the patch, or that he/she was in the patch's delivery path.
520
521If a person was not directly involved in the preparation or handling of a
522patch but wishes to signify and record their approval of it then they can
523ask to have an Acked-by: line added to the patch's changelog.
524
525Acked-by: is often used by the maintainer of the affected code when that
526maintainer neither contributed to nor forwarded the patch.
527
528Acked-by: is not as formal as Signed-off-by:.  It is a record that the acker
529has at least reviewed the patch and has indicated acceptance.  Hence patch
530mergers will sometimes manually convert an acker's "yep, looks good to me"
531into an Acked-by: (but note that it is usually better to ask for an
532explicit ack).
533
534Acked-by: does not necessarily indicate acknowledgement of the entire patch.
535For example, if a patch affects multiple subsystems and has an Acked-by: from
536one subsystem maintainer then this usually indicates acknowledgement of just
537the part which affects that maintainer's code.  Judgement should be used here.
538When in doubt people should refer to the original discussion in the mailing
539list archives.
540
541If a person has had the opportunity to comment on a patch, but has not
542provided such comments, you may optionally add a ``Cc:`` tag to the patch.
543This is the only tag which might be added without an explicit action by the
544person it names - but it should indicate that this person was copied on the
545patch.  This tag documents that potentially interested parties
546have been included in the discussion.
547
548Co-developed-by: states that the patch was co-created by multiple developers;
549it is a used to give attribution to co-authors (in addition to the author
550attributed by the From: tag) when several people work on a single patch.  Since
551Co-developed-by: denotes authorship, every Co-developed-by: must be immediately
552followed by a Signed-off-by: of the associated co-author.  Standard sign-off
553procedure applies, i.e. the ordering of Signed-off-by: tags should reflect the
554chronological history of the patch insofar as possible, regardless of whether
555the author is attributed via From: or Co-developed-by:.  Notably, the last
556Signed-off-by: must always be that of the developer submitting the patch.
557
558Note, the From: tag is optional when the From: author is also the person (and
559email) listed in the From: line of the email header.
560
561Example of a patch submitted by the From: author::
562
563	<changelog>
564
565	Co-developed-by: First Co-Author <first@coauthor.example.org>
566	Signed-off-by: First Co-Author <first@coauthor.example.org>
567	Co-developed-by: Second Co-Author <second@coauthor.example.org>
568	Signed-off-by: Second Co-Author <second@coauthor.example.org>
569	Signed-off-by: From Author <from@author.example.org>
570
571Example of a patch submitted by a Co-developed-by: author::
572
573	From: From Author <from@author.example.org>
574
575	<changelog>
576
577	Co-developed-by: Random Co-Author <random@coauthor.example.org>
578	Signed-off-by: Random Co-Author <random@coauthor.example.org>
579	Signed-off-by: From Author <from@author.example.org>
580	Co-developed-by: Submitting Co-Author <sub@coauthor.example.org>
581	Signed-off-by: Submitting Co-Author <sub@coauthor.example.org>
582
583
58413) Using Reported-by:, Tested-by:, Reviewed-by:, Suggested-by: and Fixes:
585--------------------------------------------------------------------------
586
587The Reported-by tag gives credit to people who find bugs and report them and it
588hopefully inspires them to help us again in the future.  Please note that if
589the bug was reported in private, then ask for permission first before using the
590Reported-by tag.
591
592A Tested-by: tag indicates that the patch has been successfully tested (in
593some environment) by the person named.  This tag informs maintainers that
594some testing has been performed, provides a means to locate testers for
595future patches, and ensures credit for the testers.
596
597Reviewed-by:, instead, indicates that the patch has been reviewed and found
598acceptable according to the Reviewer's Statement:
599
600Reviewer's statement of oversight
601^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
602
603By offering my Reviewed-by: tag, I state that:
604
605	 (a) I have carried out a technical review of this patch to
606	     evaluate its appropriateness and readiness for inclusion into
607	     the mainline kernel.
608
609	 (b) Any problems, concerns, or questions relating to the patch
610	     have been communicated back to the submitter.  I am satisfied
611	     with the submitter's response to my comments.
612
613	 (c) While there may be things that could be improved with this
614	     submission, I believe that it is, at this time, (1) a
615	     worthwhile modification to the kernel, and (2) free of known
616	     issues which would argue against its inclusion.
617
618	 (d) While I have reviewed the patch and believe it to be sound, I
619	     do not (unless explicitly stated elsewhere) make any
620	     warranties or guarantees that it will achieve its stated
621	     purpose or function properly in any given situation.
622
623A Reviewed-by tag is a statement of opinion that the patch is an
624appropriate modification of the kernel without any remaining serious
625technical issues.  Any interested reviewer (who has done the work) can
626offer a Reviewed-by tag for a patch.  This tag serves to give credit to
627reviewers and to inform maintainers of the degree of review which has been
628done on the patch.  Reviewed-by: tags, when supplied by reviewers known to
629understand the subject area and to perform thorough reviews, will normally
630increase the likelihood of your patch getting into the kernel.
631
632A Suggested-by: tag indicates that the patch idea is suggested by the person
633named and ensures credit to the person for the idea. Please note that this
634tag should not be added without the reporter's permission, especially if the
635idea was not posted in a public forum. That said, if we diligently credit our
636idea reporters, they will, hopefully, be inspired to help us again in the
637future.
638
639A Fixes: tag indicates that the patch fixes an issue in a previous commit. It
640is used to make it easy to determine where a bug originated, which can help
641review a bug fix. This tag also assists the stable kernel team in determining
642which stable kernel versions should receive your fix. This is the preferred
643method for indicating a bug fixed by the patch. See :ref:`describe_changes`
644for more details.
645
646.. _the_canonical_patch_format:
647
64814) The canonical patch format
649------------------------------
650
651This section describes how the patch itself should be formatted.  Note
652that, if you have your patches stored in a ``git`` repository, proper patch
653formatting can be had with ``git format-patch``.  The tools cannot create
654the necessary text, though, so read the instructions below anyway.
655
656The canonical patch subject line is::
657
658    Subject: [PATCH 001/123] subsystem: summary phrase
659
660The canonical patch message body contains the following:
661
662  - A ``from`` line specifying the patch author, followed by an empty
663    line (only needed if the person sending the patch is not the author).
664
665  - The body of the explanation, line wrapped at 75 columns, which will
666    be copied to the permanent changelog to describe this patch.
667
668  - An empty line.
669
670  - The ``Signed-off-by:`` lines, described above, which will
671    also go in the changelog.
672
673  - A marker line containing simply ``---``.
674
675  - Any additional comments not suitable for the changelog.
676
677  - The actual patch (``diff`` output).
678
679The Subject line format makes it very easy to sort the emails
680alphabetically by subject line - pretty much any email reader will
681support that - since because the sequence number is zero-padded,
682the numerical and alphabetic sort is the same.
683
684The ``subsystem`` in the email's Subject should identify which
685area or subsystem of the kernel is being patched.
686
687The ``summary phrase`` in the email's Subject should concisely
688describe the patch which that email contains.  The ``summary
689phrase`` should not be a filename.  Do not use the same ``summary
690phrase`` for every patch in a whole patch series (where a ``patch
691series`` is an ordered sequence of multiple, related patches).
692
693Bear in mind that the ``summary phrase`` of your email becomes a
694globally-unique identifier for that patch.  It propagates all the way
695into the ``git`` changelog.  The ``summary phrase`` may later be used in
696developer discussions which refer to the patch.  People will want to
697google for the ``summary phrase`` to read discussion regarding that
698patch.  It will also be the only thing that people may quickly see
699when, two or three months later, they are going through perhaps
700thousands of patches using tools such as ``gitk`` or ``git log
701--oneline``.
702
703For these reasons, the ``summary`` must be no more than 70-75
704characters, and it must describe both what the patch changes, as well
705as why the patch might be necessary.  It is challenging to be both
706succinct and descriptive, but that is what a well-written summary
707should do.
708
709The ``summary phrase`` may be prefixed by tags enclosed in square
710brackets: "Subject: [PATCH <tag>...] <summary phrase>".  The tags are
711not considered part of the summary phrase, but describe how the patch
712should be treated.  Common tags might include a version descriptor if
713the multiple versions of the patch have been sent out in response to
714comments (i.e., "v1, v2, v3"), or "RFC" to indicate a request for
715comments.  If there are four patches in a patch series the individual
716patches may be numbered like this: 1/4, 2/4, 3/4, 4/4.  This assures
717that developers understand the order in which the patches should be
718applied and that they have reviewed or applied all of the patches in
719the patch series.
720
721A couple of example Subjects::
722
723    Subject: [PATCH 2/5] ext2: improve scalability of bitmap searching
724    Subject: [PATCH v2 01/27] x86: fix eflags tracking
725
726The ``from`` line must be the very first line in the message body,
727and has the form:
728
729        From: Patch Author <author@example.com>
730
731The ``from`` line specifies who will be credited as the author of the
732patch in the permanent changelog.  If the ``from`` line is missing,
733then the ``From:`` line from the email header will be used to determine
734the patch author in the changelog.
735
736The explanation body will be committed to the permanent source
737changelog, so should make sense to a competent reader who has long
738since forgotten the immediate details of the discussion that might
739have led to this patch.  Including symptoms of the failure which the
740patch addresses (kernel log messages, oops messages, etc.) is
741especially useful for people who might be searching the commit logs
742looking for the applicable patch.  If a patch fixes a compile failure,
743it may not be necessary to include _all_ of the compile failures; just
744enough that it is likely that someone searching for the patch can find
745it.  As in the ``summary phrase``, it is important to be both succinct as
746well as descriptive.
747
748The ``---`` marker line serves the essential purpose of marking for patch
749handling tools where the changelog message ends.
750
751One good use for the additional comments after the ``---`` marker is for
752a ``diffstat``, to show what files have changed, and the number of
753inserted and deleted lines per file.  A ``diffstat`` is especially useful
754on bigger patches.  Other comments relevant only to the moment or the
755maintainer, not suitable for the permanent changelog, should also go
756here.  A good example of such comments might be ``patch changelogs``
757which describe what has changed between the v1 and v2 version of the
758patch.
759
760If you are going to include a ``diffstat`` after the ``---`` marker, please
761use ``diffstat`` options ``-p 1 -w 70`` so that filenames are listed from
762the top of the kernel source tree and don't use too much horizontal
763space (easily fit in 80 columns, maybe with some indentation).  (``git``
764generates appropriate diffstats by default.)
765
766See more details on the proper patch format in the following
767references.
768
769.. _explicit_in_reply_to:
770
77115) Explicit In-Reply-To headers
772--------------------------------
773
774It can be helpful to manually add In-Reply-To: headers to a patch
775(e.g., when using ``git send-email``) to associate the patch with
776previous relevant discussion, e.g. to link a bug fix to the email with
777the bug report.  However, for a multi-patch series, it is generally
778best to avoid using In-Reply-To: to link to older versions of the
779series.  This way multiple versions of the patch don't become an
780unmanageable forest of references in email clients.  If a link is
781helpful, you can use the https://lkml.kernel.org/ redirector (e.g., in
782the cover email text) to link to an earlier version of the patch series.
783
784
78516) Providing base tree information
786-----------------------------------
787
788When other developers receive your patches and start the review process,
789it is often useful for them to know where in the tree history they
790should place your work. This is particularly useful for automated CI
791processes that attempt to run a series of tests in order to establish
792the quality of your submission before the maintainer starts the review.
793
794If you are using ``git format-patch`` to generate your patches, you can
795automatically include the base tree information in your submission by
796using the ``--base`` flag. The easiest and most convenient way to use
797this option is with topical branches::
798
799    $ git checkout -t -b my-topical-branch master
800    Branch 'my-topical-branch' set up to track local branch 'master'.
801    Switched to a new branch 'my-topical-branch'
802
803    [perform your edits and commits]
804
805    $ git format-patch --base=auto --cover-letter -o outgoing/ master
806    outgoing/0000-cover-letter.patch
807    outgoing/0001-First-Commit.patch
808    outgoing/...
809
810When you open ``outgoing/0000-cover-letter.patch`` for editing, you will
811notice that it will have the ``base-commit:`` trailer at the very
812bottom, which provides the reviewer and the CI tools enough information
813to properly perform ``git am`` without worrying about conflicts::
814
815    $ git checkout -b patch-review [base-commit-id]
816    Switched to a new branch 'patch-review'
817    $ git am patches.mbox
818    Applying: First Commit
819    Applying: ...
820
821Please see ``man git-format-patch`` for more information about this
822option.
823
824.. note::
825
826    The ``--base`` feature was introduced in git version 2.9.0.
827
828If you are not using git to format your patches, you can still include
829the same ``base-commit`` trailer to indicate the commit hash of the tree
830on which your work is based. You should add it either in the cover
831letter or in the first patch of the series and it should be placed
832either below the ``---`` line or at the very bottom of all other
833content, right before your email signature.
834
835
83617) Sending ``git pull`` requests
837---------------------------------
838
839If you have a series of patches, it may be most convenient to have the
840maintainer pull them directly into the subsystem repository with a
841``git pull`` operation.  Note, however, that pulling patches from a developer
842requires a higher degree of trust than taking patches from a mailing list.
843As a result, many subsystem maintainers are reluctant to take pull
844requests, especially from new, unknown developers.  If in doubt you can use
845the pull request as the cover letter for a normal posting of the patch
846series, giving the maintainer the option of using either.
847
848A pull request should have [GIT PULL] in the subject line.  The
849request itself should include the repository name and the branch of
850interest on a single line; it should look something like::
851
852  Please pull from
853
854      git://jdelvare.pck.nerim.net/jdelvare-2.6 i2c-for-linus
855
856  to get these changes:
857
858A pull request should also include an overall message saying what will be
859included in the request, a ``git shortlog`` listing of the patches
860themselves, and a ``diffstat`` showing the overall effect of the patch series.
861The easiest way to get all this information together is, of course, to let
862``git`` do it for you with the ``git request-pull`` command.
863
864Some maintainers (including Linus) want to see pull requests from signed
865commits; that increases their confidence that the request actually came
866from you.  Linus, in particular, will not pull from public hosting sites
867like GitHub in the absence of a signed tag.
868
869The first step toward creating such tags is to make a GNUPG key and get it
870signed by one or more core kernel developers.  This step can be hard for
871new developers, but there is no way around it.  Attending conferences can
872be a good way to find developers who can sign your key.
873
874Once you have prepared a patch series in ``git`` that you wish to have somebody
875pull, create a signed tag with ``git tag -s``.  This will create a new tag
876identifying the last commit in the series and containing a signature
877created with your private key.  You will also have the opportunity to add a
878changelog-style message to the tag; this is an ideal place to describe the
879effects of the pull request as a whole.
880
881If the tree the maintainer will be pulling from is not the repository you
882are working from, don't forget to push the signed tag explicitly to the
883public tree.
884
885When generating your pull request, use the signed tag as the target.  A
886command like this will do the trick::
887
888  git request-pull master git://my.public.tree/linux.git my-signed-tag
889
890
891References
892----------
893
894Andrew Morton, "The perfect patch" (tpp).
895  <http://www.ozlabs.org/~akpm/stuff/tpp.txt>
896
897Jeff Garzik, "Linux kernel patch submission format".
898  <https://web.archive.org/web/20180829112450/http://linux.yyz.us/patch-format.html>
899
900Greg Kroah-Hartman, "How to piss off a kernel subsystem maintainer".
901  <http://www.kroah.com/log/linux/maintainer.html>
902
903  <http://www.kroah.com/log/linux/maintainer-02.html>
904
905  <http://www.kroah.com/log/linux/maintainer-03.html>
906
907  <http://www.kroah.com/log/linux/maintainer-04.html>
908
909  <http://www.kroah.com/log/linux/maintainer-05.html>
910
911  <http://www.kroah.com/log/linux/maintainer-06.html>
912
913NO!!!! No more huge patch bombs to linux-kernel@vger.kernel.org people!
914  <https://lkml.org/lkml/2005/7/11/336>
915
916Kernel Documentation/process/coding-style.rst:
917  :ref:`Documentation/process/coding-style.rst <codingstyle>`
918
919Linus Torvalds's mail on the canonical patch format:
920  <http://lkml.org/lkml/2005/4/7/183>
921
922Andi Kleen, "On submitting kernel patches"
923  Some strategies to get difficult or controversial changes in.
924
925  http://halobates.de/on-submitting-patches.pdf
926