.. _submittingpatches:

  How to Get Your Change Into the Linux Kernel or Care And Operation Of Your Linus Torvalds
  =========================================================================================

  
  For a person or company who wishes to submit a change to the Linux
  kernel, the process can sometimes be daunting if you're not familiar
  with "the system."  This text is a collection of suggestions which
  can greatly increase the chances of your change being accepted.

  This document contains a large number of suggestions in a relatively terse
  format.  For detailed information on how the kernel development process

  works, see :ref:`Documentation/development-process <development_process_main>`.
  Also, read :ref:`Documentation/SubmitChecklist <submitchecklist>`
  for a list of items to check before

  submitting code.  If you are submitting a driver, also read

  :ref:`Documentation/SubmittingDrivers <submittingdrivers>`;
  for device tree binding patches, read

  Documentation/devicetree/bindings/submitting-patches.txt.

  Many of these steps describe the default behavior of the ``git`` version
  control system; if you use ``git`` to prepare your patches, you'll find much

  of the mechanical work done for you, though you'll still need to prepare

  and document a sensible set of patches.  In general, use of ``git`` will make

  your life as a kernel developer easier.

  Creating and Sending your Change
  ********************************

  0) Obtain a current source tree
  -------------------------------
  
  If you do not have a repository with the current kernel source handy, use

  ``git`` to obtain one.  You'll want to start with the mainline repository,

  which can be grabbed with::

    git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git

  
  Note, however, that you may not want to develop against the mainline tree
  directly.  Most subsystem maintainers run their own trees and want to see

  patches prepared against those trees.  See the **T:** entry for the subsystem

  in the MAINTAINERS file to find that tree, or simply ask the maintainer if
  the tree is not listed there.
  
  It is still possible to download kernel releases via tarballs (as described
  in the next section), but that is the hard way to do kernel development.

  1) ``diff -up``
  ---------------

  If you must generate your patches by hand, use ``diff -up`` or ``diff -uprN``

  to create patches.  Git generates patches in this form by default; if

  you're using ``git``, you can skip this section entirely.

  
  All changes to the Linux kernel occur in the form of patches, as

  generated by :manpage:`diff(1)`.  When creating your patch, make sure to
  create it in "unified diff" format, as supplied by the ``-u`` argument
  to :manpage:`diff(1)`.

  Also, please use the ``-p`` argument which shows which C function each

  change is in - that makes the resultant ``diff`` a lot easier to read.

  Patches should be based in the root kernel source directory,
  not in any lower subdirectory.

  To create a patch for a single file, it is often sufficient to do::

  	SRCTREE= linux

  	MYFILE=  drivers/net/mydriver.c
  
  	cd $SRCTREE
  	cp $MYFILE $MYFILE.orig
  	vi $MYFILE	# make your change
  	cd ..
  	diff -up $SRCTREE/$MYFILE{.orig,} > /tmp/patch
  
  To create a patch for multiple files, you should unpack a "vanilla",

  or unmodified kernel source tree, and generate a ``diff`` against your

  own source tree.  For example::

  	MYSRC= /devel/linux

  	tar xvfz linux-3.19.tar.gz
  	mv linux-3.19 linux-3.19-vanilla
  	diff -uprN -X linux-3.19-vanilla/Documentation/dontdiff \
  		linux-3.19-vanilla $MYSRC > /tmp/patch

  ``dontdiff`` is a list of files which are generated by the kernel during

  the build process, and should be ignored in any :manpage:`diff(1)`-generated

  patch.

  
  Make sure your patch does not include any extra files which do not
  belong in a patch submission.  Make sure to review your patch -after-

  generating it with :manpage:`diff(1)`, to ensure accuracy.

  If your changes produce a lot of deltas, you need to split them into

  individual patches which modify things in logical stages; see
  :ref:`split_changes`.  This will facilitate review by other kernel developers,

  very important if you want your patch accepted.

  If you're using ``git``, ``git rebase -i`` can help you with this process.  If
  you're not using ``git``, ``quilt`` <http://savannah.nongnu.org/projects/quilt>

  is another popular alternative.

  .. _describe_changes:

  2) Describe your changes
  ------------------------

  Describe your problem.  Whether your patch is a one-line bug fix or
  5000 lines of a new feature, there must be an underlying problem that
  motivated you to do this work.  Convince the reviewer that there is a
  problem worth fixing and that it makes sense for them to read past the
  first paragraph.
  
  Describe user-visible impact.  Straight up crashes and lockups are
  pretty convincing, but not all bugs are that blatant.  Even if the
  problem was spotted during code review, describe the impact you think
  it can have on users.  Keep in mind that the majority of Linux
  installations run kernels from secondary stable trees or
  vendor/product-specific trees that cherry-pick only specific patches
  from upstream, so include anything that could help route your change
  downstream: provoking circumstances, excerpts from dmesg, crash
  descriptions, performance regressions, latency spikes, lockups, etc.
  
  Quantify optimizations and trade-offs.  If you claim improvements in
  performance, memory consumption, stack footprint, or binary size,
  include numbers that back them up.  But also describe non-obvious
  costs.  Optimizations usually aren't free but trade-offs between CPU,
  memory, and readability; or, when it comes to heuristics, between
  different workloads.  Describe the expected downsides of your
  optimization so that the reviewer can weigh costs against benefits.
  
  Once the problem is established, describe what you are actually doing
  about it in technical detail.  It's important to describe the change
  in plain English for the reviewer to verify that the code is behaving
  as you intend it to.

  The maintainer will thank you if you write your patch description in a
  form which can be easily pulled into Linux's source code management

  system, ``git``, as a "commit log".  See :ref:`explicit_in_reply_to`.

  Solve only one problem per patch.  If your description starts to get
  long, that's a sign that you probably need to split up your patch.

  See :ref:`split_changes`.

  When you submit or resubmit a patch or patch series, include the
  complete patch description and justification for it.  Don't just
  say that this is version N of the patch (series).  Don't expect the

  subsystem maintainer to refer back to earlier patch versions or referenced

  URLs to find the patch description and put that into the patch.
  I.e., the patch (series) and its description should be self-contained.

  This benefits both the maintainers and reviewers.  Some reviewers

  probably didn't even receive earlier versions of the patch.

  Describe your changes in imperative mood, e.g. "make xyzzy do frotz"
  instead of "[This patch] makes xyzzy do frotz" or "[I] changed xyzzy
  to do frotz", as if you are giving orders to the codebase to change
  its behaviour.

  If the patch fixes a logged bug entry, refer to that bug entry by

  number and URL.  If the patch follows from a mailing list discussion,
  give a URL to the mailing list archive; use the https://lkml.kernel.org/

  redirector with a ``Message-Id``, to ensure that the links cannot become

  stale.
  
  However, try to make your explanation understandable without external
  resources.  In addition to giving a URL to a mailing list archive or
  bug, summarize the relevant points of the discussion that led to the
  patch as submitted.

  If you want to refer to a specific commit, don't just refer to the
  SHA-1 ID of the commit. Please also include the oneline summary of
  the commit, to make it easier for reviewers to know what it is about.

  Example::

  
  	Commit e21d2170f36602ae2708 ("video: remove unnecessary
  	platform_set_drvdata()") removed the unnecessary
  	platform_set_drvdata(), but left the variable "dev" unused,
  	delete it.

  You should also be sure to use at least the first twelve characters of the
  SHA-1 ID.  The kernel repository holds a *lot* of objects, making
  collisions with shorter IDs a real possibility.  Bear in mind that, even if
  there is no collision with your six-character ID now, that condition may
  change five years from now.

  If your patch fixes a bug in a specific commit, e.g. you found an issue using

  ``git bisect``, please use the 'Fixes:' tag with the first 12 characters of
  the SHA-1 ID, and the one line summary.  For example::

  
  	Fixes: e21d2170f366 ("video: remove unnecessary platform_set_drvdata()")

  The following ``git config`` settings can be used to add a pretty format for
  outputting the above style in the ``git log`` or ``git show`` commands::

  
  	[core]
  		abbrev = 12
  	[pretty]
  		fixes = Fixes: %h (\"%s\")

  .. _split_changes:
  
  3) Separate your changes
  ------------------------

  Separate each **logical change** into a separate patch.

  
  For example, if your changes include both bug fixes and performance
  enhancements for a single driver, separate those changes into two
  or more patches.  If your changes include an API update, and a new
  driver which uses that new API, separate those into two patches.
  
  On the other hand, if you make a single change to numerous files,
  group those changes into a single patch.  Thus a single logical change
  is contained within a single patch.

  The point to remember is that each patch should make an easily understood
  change that can be verified by reviewers.  Each patch should be justifiable
  on its own merits.

  If one patch depends on another patch in order for a change to be

  complete, that is OK.  Simply note **"this patch depends on patch X"**

  in your patch description.

  When dividing your change into a series of patches, take special care to
  ensure that the kernel builds and runs properly after each patch in the

  series.  Developers using ``git bisect`` to track down a problem can end up

  splitting your patch series at any point; they will not thank you if you
  introduce bugs in the middle.

  If you cannot condense your patch set into a smaller set of patches,
  then only post say 15 or so at a time and wait for review and integration.

  4) Style-check your changes
  ---------------------------

  
  Check your patch for basic style violations, details of which can be

  found in
  :ref:`Documentation/CodingStyle <codingstyle>`.
  Failure to do so simply wastes

  the reviewers time and will get your patch rejected, probably

  without even being read.

  One significant exception is when moving code from one file to
  another -- in this case you should not modify the moved code at all in
  the same patch which moves it.  This clearly delineates the act of
  moving the code and your changes.  This greatly aids review of the
  actual differences and allows tools to better track the history of
  the code itself.
  
  Check your patches with the patch style checker prior to submission
  (scripts/checkpatch.pl).  Note, though, that the style checker should be
  viewed as a guide, not as a replacement for human judgment.  If your code
  looks better with a violation then its probably best left alone.

  The checker reports at three levels:
   - ERROR: things that are very likely to be wrong
   - WARNING: things requiring careful review
   - CHECK: things requiring thought

  You should be able to justify all violations that remain in your
  patch.

  5) Select the recipients for your patch
  ---------------------------------------

  You should always copy the appropriate subsystem maintainer(s) on any patch
  to code that they maintain; look through the MAINTAINERS file and the
  source code revision history to see who those maintainers are.  The
  script scripts/get_maintainer.pl can be very useful at this step.  If you

  cannot find a maintainer for the subsystem you are working on, Andrew

  Morton (akpm@linux-foundation.org) serves as a maintainer of last resort.

  You should also normally choose at least one mailing list to receive a copy
  of your patch set.  linux-kernel@vger.kernel.org functions as a list of
  last resort, but the volume on that list has caused a number of developers
  to tune it out.  Look in the MAINTAINERS file for a subsystem-specific
  list; your patch will probably get more attention there.  Please do not
  spam unrelated lists, though.

  Many kernel-related lists are hosted on vger.kernel.org; you can find a
  list of them at http://vger.kernel.org/vger-lists.html.  There are
  kernel-related lists hosted elsewhere as well, though.

  
  Do not send more than 15 patches at once to the vger mailing lists!!!

  Linus Torvalds is the final arbiter of all changes accepted into the

  Linux kernel.  His e-mail address is <torvalds@linux-foundation.org>.

  He gets a lot of e-mail, and, at this point, very few patches go through
  Linus directly, so typically you should do your best to -avoid-

  sending him e-mail.

  If you have a patch that fixes an exploitable security bug, send that patch
  to security@kernel.org.  For severe bugs, a short embargo may be considered

  to allow distributors to get the patch out to users; in such cases,

  obviously, the patch should not be sent to any public lists.

  Patches that fix a severe bug in a released kernel should be directed

  toward the stable maintainers by putting a line like this::

    Cc: stable@vger.kernel.org

  into the sign-off area of your patch (note, NOT an email recipient).  You

  should also read
  :ref:`Documentation/stable_kernel_rules.txt <stable_kernel_rules>`
  in addition to this file.

  Note, however, that some subsystem maintainers want to come to their own
  conclusions on which patches should go to the stable trees.  The networking
  maintainer, in particular, would rather not see individual developers
  adding lines like the above to their patches.

  If changes affect userland-kernel interfaces, please send the MAN-PAGES
  maintainer (as listed in the MAINTAINERS file) a man-pages patch, or at
  least a notification of the change, so that some information makes its way
  into the manual pages.  User-space API changes should also be copied to

  linux-api@vger.kernel.org.

  
  For small patches you may want to CC the Trivial Patch Monkey

  trivial@kernel.org which collects "trivial" patches. Have a look
  into the MAINTAINERS file for its current manager.

  Trivial patches must qualify for one of the following rules:

  - Spelling fixes in documentation
  - Spelling fixes for errors which could break :manpage:`grep(1)`
  - Warning fixes (cluttering with useless warnings is bad)
  - Compilation fixes (only if they are actually correct)
  - Runtime fixes (only if they actually fix things)
  - Removing use of deprecated functions/macros
  - Contact detail and documentation fixes
  - Non-portable code replaced by portable code (even in arch-specific,
    since people copy, as long as it's trivial)
  - Any fix by the author/maintainer of the file (ie. patch monkey
    in re-transmission mode)

  6) No MIME, no links, no compression, no attachments.  Just plain text
  ----------------------------------------------------------------------

  
  Linus and other kernel developers need to be able to read and comment
  on the changes you are submitting.  It is important for a kernel
  developer to be able to "quote" your changes, using standard e-mail
  tools, so that they may comment on specific portions of your code.

  For this reason, all patches should be submitted by e-mail "inline".

  
  .. warning::
  
    Be wary of your editor's word-wrap corrupting your patch,
    if you choose to cut-n-paste your patch.

  
  Do not attach the patch as a MIME attachment, compressed or not.
  Many popular e-mail applications will not always transmit a MIME
  attachment as plain text, making it impossible to comment on your
  code.  A MIME attachment also takes Linus a bit more time to process,
  decreasing the likelihood of your MIME-attached change being accepted.
  
  Exception:  If your mailer is mangling patches then someone may ask
  you to re-send them using MIME.

  See :ref:`Documentation/email-clients.txt <email_clients>`
  for hints about configuring your e-mail client so that it sends your patches
  untouched.

  7) E-mail size
  --------------

  
  Large changes are not appropriate for mailing lists, and some

  maintainers.  If your patch, uncompressed, exceeds 300 kB in size,

  it is preferred that you store your patch on an Internet-accessible

  server, and provide instead a URL (link) pointing to your patch.  But note
  that if your patch exceeds 300 kB, it almost certainly needs to be broken up
  anyway.

  8) Respond to review comments
  -----------------------------

  Your patch will almost certainly get comments from reviewers on ways in
  which the patch can be improved.  You must respond to those comments;
  ignoring reviewers is a good way to get ignored in return.  Review comments
  or questions that do not lead to a code change should almost certainly
  bring about a comment or changelog entry so that the next reviewer better
  understands what is going on.

  Be sure to tell the reviewers what changes you are making and to thank them
  for their time.  Code review is a tiring and time-consuming process, and
  reviewers sometimes get grumpy.  Even in that case, though, respond
  politely and address the problems they have pointed out.

  9) Don't get discouraged - or impatient
  ---------------------------------------

  After you have submitted your change, be patient and wait.  Reviewers are
  busy people and may not get to your patch right away.

  Once upon a time, patches used to disappear into the void without comment,
  but the development process works more smoothly than that now.  You should
  receive comments within a week or so; if that does not happen, make sure
  that you have sent your patches to the right place.  Wait for a minimum of
  one week before resubmitting or pinging reviewers - possibly longer during
  busy times like merge windows.

  10) Include PATCH in the subject

  --------------------------------

  
  Due to high e-mail traffic to Linus, and to linux-kernel, it is common
  convention to prefix your subject line with [PATCH].  This lets Linus
  and other kernel developers more easily distinguish patches from other
  e-mail discussions.

  11) Sign your work

  ------------------

  
  To improve tracking of who did what, especially with patches that can
  percolate to their final resting place in the kernel through several
  layers of maintainers, we've introduced a "sign-off" procedure on
  patches that are being emailed around.
  
  The sign-off is a simple line at the end of the explanation for the
  patch, which certifies that you wrote it or otherwise have the right to

  pass it on as an open-source patch.  The rules are pretty simple: if you

  can certify the below:

  Developer's Certificate of Origin 1.1
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

  By making a contribution to this project, I certify that:

  
          (a) The contribution was created in whole or in part by me and I
              have the right to submit it under the open source license
              indicated in the file; or
  
          (b) The contribution is based upon previous work that, to the best
              of my knowledge, is covered under an appropriate open source
              license and I have the right under that license to submit that
              work with modifications, whether created in whole or in part
              by me, under the same open source license (unless I am
              permitted to submit under a different license), as indicated
              in the file; or
  
          (c) The contribution was provided directly to me by some other
              person who certified (a), (b) or (c) and I have not modified
              it.

          (d) I understand and agree that this project and the contribution
              are public and that a record of the contribution (including all
              personal information I submit with it, including my sign-off) is
              maintained indefinitely and may be redistributed consistent with
              this project or the open source license(s) involved.

  then you just add a line saying::

  	Signed-off-by: Random J Developer <random@developer.example.org>

  using your real name (sorry, no pseudonyms or anonymous contributions.)

  Some people also put extra tags at the end.  They'll just be ignored for
  now, but you can do this to mark internal company procedures or just

  point out some special detail about the sign-off.

  If you are a subsystem or branch maintainer, sometimes you need to slightly
  modify patches you receive in order to merge them, because the code is not
  exactly the same in your tree and the submitters'. If you stick strictly to
  rule (c), you should ask the submitter to rediff, but this is a totally
  counter-productive waste of time and energy. Rule (b) allows you to adjust
  the code, but then it is very impolite to change one submitter's code and
  make him endorse your bugs. To solve this problem, it is recommended that
  you add a line between the last Signed-off-by header and yours, indicating
  the nature of your changes. While there is nothing mandatory about this, it
  seems like prepending the description with your mail and/or name, all
  enclosed in square brackets, is noticeable enough to make it obvious that

  you are responsible for last-minute changes. Example::

  
  	Signed-off-by: Random J Developer <random@developer.example.org>
  	[lucky@maintainer.example.org: struct foo moved from foo.c to foo.h]
  	Signed-off-by: Lucky K Maintainer <lucky@maintainer.example.org>

  This practice is particularly helpful if you maintain a stable branch and

  want at the same time to credit the author, track changes, merge the fix,
  and protect the submitter from complaints. Note that under no circumstances
  can you change the author's identity (the From header), as it is the one
  which appears in the changelog.

  Special note to back-porters: It seems to be a common and useful practice

  to insert an indication of the origin of a patch at the top of the commit
  message (just after the subject line) to facilitate tracking. For instance,

  here's what we see in a 3.x-stable release::

    Date:   Tue Oct 7 07:26:38 2014 -0400

      libata: Un-break ATA blacklist

      commit 1c40279960bcd7d52dbdf1d466b20d24b99176c8 upstream.

  And here's what might appear in an older kernel once a patch is backported::

  
      Date:   Tue May 13 22:12:27 2008 +0200
  
          wireless, airo: waitbusy() won't delay
  
          [backport of 2.6 commit b7acbdfbd1f277c1eb23f344f899cfa4cd0bf36a]
  
  Whatever the format, this information provides a valuable help to people

  tracking your trees, and to people trying to troubleshoot bugs in your

  tree.

  12) When to use Acked-by: and Cc:

  ---------------------------------

  The Signed-off-by: tag indicates that the signer was involved in the
  development of the patch, or that he/she was in the patch's delivery path.
  
  If a person was not directly involved in the preparation or handling of a
  patch but wishes to signify and record their approval of it then they can

  ask to have an Acked-by: line added to the patch's changelog.

  
  Acked-by: is often used by the maintainer of the affected code when that
  maintainer neither contributed to nor forwarded the patch.
  
  Acked-by: is not as formal as Signed-off-by:.  It is a record that the acker
  has at least reviewed the patch and has indicated acceptance.  Hence patch
  mergers will sometimes manually convert an acker's "yep, looks good to me"

  into an Acked-by: (but note that it is usually better to ask for an
  explicit ack).

  
  Acked-by: does not necessarily indicate acknowledgement of the entire patch.
  For example, if a patch affects multiple subsystems and has an Acked-by: from
  one subsystem maintainer then this usually indicates acknowledgement of just
  the part which affects that maintainer's code.  Judgement should be used here.

  When in doubt people should refer to the original discussion in the mailing

  list archives.

  If a person has had the opportunity to comment on a patch, but has not

  provided such comments, you may optionally add a ``Cc:`` tag to the patch.

  This is the only tag which might be added without an explicit action by the

  person it names - but it should indicate that this person was copied on the
  patch.  This tag documents that potentially interested parties
  have been included in the discussion.

  13) Using Reported-by:, Tested-by:, Reviewed-by:, Suggested-by: and Fixes:

  --------------------------------------------------------------------------

  The Reported-by tag gives credit to people who find bugs and report them and it
  hopefully inspires them to help us again in the future.  Please note that if
  the bug was reported in private, then ask for permission first before using the
  Reported-by tag.

  
  A Tested-by: tag indicates that the patch has been successfully tested (in
  some environment) by the person named.  This tag informs maintainers that
  some testing has been performed, provides a means to locate testers for
  future patches, and ensures credit for the testers.
  
  Reviewed-by:, instead, indicates that the patch has been reviewed and found
  acceptable according to the Reviewer's Statement:

  Reviewer's statement of oversight
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

  By offering my Reviewed-by: tag, I state that:

  	 (a) I have carried out a technical review of this patch to

  	     evaluate its appropriateness and readiness for inclusion into
  	     the mainline kernel.
  
  	 (b) Any problems, concerns, or questions relating to the patch
  	     have been communicated back to the submitter.  I am satisfied
  	     with the submitter's response to my comments.
  
  	 (c) While there may be things that could be improved with this
  	     submission, I believe that it is, at this time, (1) a
  	     worthwhile modification to the kernel, and (2) free of known
  	     issues which would argue against its inclusion.
  
  	 (d) While I have reviewed the patch and believe it to be sound, I
  	     do not (unless explicitly stated elsewhere) make any
  	     warranties or guarantees that it will achieve its stated
  	     purpose or function properly in any given situation.
  
  A Reviewed-by tag is a statement of opinion that the patch is an
  appropriate modification of the kernel without any remaining serious
  technical issues.  Any interested reviewer (who has done the work) can
  offer a Reviewed-by tag for a patch.  This tag serves to give credit to
  reviewers and to inform maintainers of the degree of review which has been
  done on the patch.  Reviewed-by: tags, when supplied by reviewers known to
  understand the subject area and to perform thorough reviews, will normally

  increase the likelihood of your patch getting into the kernel.

  A Suggested-by: tag indicates that the patch idea is suggested by the person
  named and ensures credit to the person for the idea. Please note that this
  tag should not be added without the reporter's permission, especially if the
  idea was not posted in a public forum. That said, if we diligently credit our
  idea reporters, they will, hopefully, be inspired to help us again in the
  future.

  A Fixes: tag indicates that the patch fixes an issue in a previous commit. It
  is used to make it easy to determine where a bug originated, which can help
  review a bug fix. This tag also assists the stable kernel team in determining
  which stable kernel versions should receive your fix. This is the preferred

  method for indicating a bug fixed by the patch. See :ref:`describe_changes`
  for more details.

  14) The canonical patch format

  ------------------------------
  
  This section describes how the patch itself should be formatted.  Note

  that, if you have your patches stored in a ``git`` repository, proper patch

  formatting can be had with ``git format-patch``.  The tools cannot create

  the necessary text, though, so read the instructions below anyway.

  The canonical patch subject line is::

      Subject: [PATCH 001/123] subsystem: summary phrase

  
  The canonical patch message body contains the following:

    - A ``from`` line specifying the patch author (only needed if the person

      sending the patch is not the author).

  
    - An empty line.

    - The body of the explanation, line wrapped at 75 columns, which will
      be copied to the permanent changelog to describe this patch.

    - The ``Signed-off-by:`` lines, described above, which will

      also go in the changelog.

    - A marker line containing simply ``---``.

  
    - Any additional comments not suitable for the changelog.

    - The actual patch (``diff`` output).

  
  The Subject line format makes it very easy to sort the emails
  alphabetically by subject line - pretty much any email reader will
  support that - since because the sequence number is zero-padded,
  the numerical and alphabetic sort is the same.

  The ``subsystem`` in the email's Subject should identify which

  area or subsystem of the kernel is being patched.

  The ``summary phrase`` in the email's Subject should concisely
  describe the patch which that email contains.  The ``summary
  phrase`` should not be a filename.  Do not use the same ``summary
  phrase`` for every patch in a whole patch series (where a ``patch
  series`` is an ordered sequence of multiple, related patches).

  Bear in mind that the ``summary phrase`` of your email becomes a

  globally-unique identifier for that patch.  It propagates all the way

  into the ``git`` changelog.  The ``summary phrase`` may later be used in

  developer discussions which refer to the patch.  People will want to

  google for the ``summary phrase`` to read discussion regarding that

  patch.  It will also be the only thing that people may quickly see
  when, two or three months later, they are going through perhaps

  thousands of patches using tools such as ``gitk`` or ``git log
  --oneline``.

  For these reasons, the ``summary`` must be no more than 70-75

  characters, and it must describe both what the patch changes, as well
  as why the patch might be necessary.  It is challenging to be both
  succinct and descriptive, but that is what a well-written summary
  should do.

  The ``summary phrase`` may be prefixed by tags enclosed in square

  brackets: "Subject: [PATCH <tag>...] <summary phrase>".  The tags are
  not considered part of the summary phrase, but describe how the patch

  should be treated.  Common tags might include a version descriptor if
  the multiple versions of the patch have been sent out in response to
  comments (i.e., "v1, v2, v3"), or "RFC" to indicate a request for
  comments.  If there are four patches in a patch series the individual
  patches may be numbered like this: 1/4, 2/4, 3/4, 4/4.  This assures
  that developers understand the order in which the patches should be
  applied and that they have reviewed or applied all of the patches in
  the patch series.

  A couple of example Subjects::

      Subject: [PATCH 2/5] ext2: improve scalability of bitmap searching
      Subject: [PATCH v2 01/27] x86: fix eflags tracking

  The ``from`` line must be the very first line in the message body,

  and has the form:
  
          From: Original Author <author@example.com>

  The ``from`` line specifies who will be credited as the author of the
  patch in the permanent changelog.  If the ``from`` line is missing,
  then the ``From:`` line from the email header will be used to determine

  the patch author in the changelog.
  
  The explanation body will be committed to the permanent source
  changelog, so should make sense to a competent reader who has long
  since forgotten the immediate details of the discussion that might

  have led to this patch.  Including symptoms of the failure which the
  patch addresses (kernel log messages, oops messages, etc.) is
  especially useful for people who might be searching the commit logs
  looking for the applicable patch.  If a patch fixes a compile failure,
  it may not be necessary to include _all_ of the compile failures; just
  enough that it is likely that someone searching for the patch can find

  it.  As in the ``summary phrase``, it is important to be both succinct as

  well as descriptive.

  The ``---`` marker line serves the essential purpose of marking for patch

  handling tools where the changelog message ends.

  One good use for the additional comments after the ``---`` marker is for

  a ``diffstat``, to show what files have changed, and the number of
  inserted and deleted lines per file.  A ``diffstat`` is especially useful

  on bigger patches.  Other comments relevant only to the moment or the
  maintainer, not suitable for the permanent changelog, should also go

  here.  A good example of such comments might be ``patch changelogs``

  which describe what has changed between the v1 and v2 version of the
  patch.

  If you are going to include a ``diffstat`` after the ``---`` marker, please
  use ``diffstat`` options ``-p 1 -w 70`` so that filenames are listed from

  the top of the kernel source tree and don't use too much horizontal

  space (easily fit in 80 columns, maybe with some indentation).  (``git``

  generates appropriate diffstats by default.)

  
  See more details on the proper patch format in the following
  references.

  .. _explicit_in_reply_to:

  15) Explicit In-Reply-To headers
  --------------------------------
  
  It can be helpful to manually add In-Reply-To: headers to a patch

  (e.g., when using ``git send-email``) to associate the patch with

  previous relevant discussion, e.g. to link a bug fix to the email with
  the bug report.  However, for a multi-patch series, it is generally
  best to avoid using In-Reply-To: to link to older versions of the
  series.  This way multiple versions of the patch don't become an
  unmanageable forest of references in email clients.  If a link is
  helpful, you can use the https://lkml.kernel.org/ redirector (e.g., in
  the cover email text) to link to an earlier version of the patch series.

  16) Sending ``git pull`` requests
  ---------------------------------

  If you have a series of patches, it may be most convenient to have the
  maintainer pull them directly into the subsystem repository with a

  ``git pull`` operation.  Note, however, that pulling patches from a developer

  requires a higher degree of trust than taking patches from a mailing list.
  As a result, many subsystem maintainers are reluctant to take pull

  requests, especially from new, unknown developers.  If in doubt you can use
  the pull request as the cover letter for a normal posting of the patch
  series, giving the maintainer the option of using either.

  A pull request should have [GIT] or [PULL] in the subject line.  The
  request itself should include the repository name and the branch of

  interest on a single line; it should look something like::

    Please pull from

        git://jdelvare.pck.nerim.net/jdelvare-2.6 i2c-for-linus

    to get these changes:

  A pull request should also include an overall message saying what will be

  included in the request, a ``git shortlog`` listing of the patches

  themselves, and a ``diffstat`` showing the overall effect of the patch series.

  The easiest way to get all this information together is, of course, to let

  ``git`` do it for you with the ``git request-pull`` command.

  Some maintainers (including Linus) want to see pull requests from signed
  commits; that increases their confidence that the request actually came
  from you.  Linus, in particular, will not pull from public hosting sites
  like GitHub in the absence of a signed tag.

  The first step toward creating such tags is to make a GNUPG key and get it
  signed by one or more core kernel developers.  This step can be hard for
  new developers, but there is no way around it.  Attending conferences can
  be a good way to find developers who can sign your key.

  Once you have prepared a patch series in ``git`` that you wish to have somebody

  pull, create a signed tag with ``git tag -s``.  This will create a new tag

  identifying the last commit in the series and containing a signature
  created with your private key.  You will also have the opportunity to add a
  changelog-style message to the tag; this is an ideal place to describe the
  effects of the pull request as a whole.

  If the tree the maintainer will be pulling from is not the repository you
  are working from, don't forget to push the signed tag explicitly to the
  public tree.

  When generating your pull request, use the signed tag as the target.  A

  command like this will do the trick::

    git request-pull master git://my.public.tree/linux.git my-signed-tag

  REFERENCES
  **********

  
  Andrew Morton, "The perfect patch" (tpp).

    <http://www.ozlabs.org/~akpm/stuff/tpp.txt>

  Jeff Garzik, "Linux kernel patch submission format".

    <http://linux.yyz.us/patch-format.html>

  Greg Kroah-Hartman, "How to piss off a kernel subsystem maintainer".

    <http://www.kroah.com/log/linux/maintainer.html>

    <http://www.kroah.com/log/linux/maintainer-02.html>

    <http://www.kroah.com/log/linux/maintainer-03.html>

    <http://www.kroah.com/log/linux/maintainer-04.html>

    <http://www.kroah.com/log/linux/maintainer-05.html>

    <http://www.kroah.com/log/linux/maintainer-06.html>

  NO!!!! No more huge patch bombs to linux-kernel@vger.kernel.org people!

    <https://lkml.org/lkml/2005/7/11/336>

  Kernel Documentation/CodingStyle:

    :ref:`Documentation/CodingStyle <codingstyle>`

  Linus Torvalds's mail on the canonical patch format:

    <http://lkml.org/lkml/2005/4/7/183>

  
  Andi Kleen, "On submitting kernel patches"

    Some strategies to get difficult or controversial changes in.

    http://halobates.de/on-submitting-patches.pdf