path: root/chromium/docs/website/site/chromium-os/chromiumos-design-docs/chromium-os-kernel/submitting-patches/index.md

---
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- - /chromium-os
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page_name: submitting-patches
title: Submitting Patches
---

[TOC]

## 	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. Read Documentation/SubmitChecklist for a list of
items to check before submitting code. If you are submitting a driver, also read
Documentation/SubmittingDrivers. --------------------------------------------

### SECTION 1 - CREATING AND SENDING YOUR CHANGE

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

#### 1) "diff -up"

------------ Use "diff -up" or "diff -uprN" to create patches. All changes to
the Linux kernel occur in the form of patches, as generated by diff(1). When
creating your patch, make sure to create it in "unified diff" format, as
supplied by the '-u' argument to 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-2.6      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-2.6         tar xvfz linux-2.6.12.tar.gz    mv linux-2.6.12
linux-2.6.12-vanilla    diff -uprN -X
linux-2.6.12-vanilla/Documentation/dontdiff \\            linux-2.6.12-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 diff(1)-generated
patch. The "dontdiff" file is included in the kernel tree in 2.6.12 and later.
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- generated it with
diff(1), to ensure accuracy. If your changes produce a lot of deltas, you may
want to look into splitting them into individual patches which modify things in
logical stages. This will facilitate easier reviewing by other kernel
developers, very important if you want your patch accepted. There are a number
of scripts which can aid in this: Quilt:
http://savannah.nongnu.org/projects/quilt Andrew Morton's patch scripts:
http://userweb.kernel.org/~akpm/stuff/patch-scripts.tar.gz Instead of these
scripts, quilt is the recommended patch management tool (see above).

#### 2) Describe your changes.

Describe the technical detail of the change(s) your patch includes. Be as
specific as possible. The WORST descriptions possible include things like
"update driver X", "bug fix for driver X", or "this patch includes updates for
subsystem X. Please apply." 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 #15, below. If your description
starts to get long, that's a sign that you probably need to split up your patch.
See #3, next. 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 patch merger 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 patch merger(s) and reviewers. Some
reviewers probably didn't even receive earlier versions of the patch. If the
patch fixes a logged bug entry, refer to that bug entry by number and URL. 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.

#### 3) Separate your changes.

Separate _logical changes_ into a single patch file. 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. 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. 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
Documentation/CodingStyle. Failure to do so simply wastes the reviewers time and
will get your patch rejected, probably without even being read. At a minimum you
should check your patches with the patch style checker prior to submission
(scripts/checkpatch.pl). You should be able to justify all violations that
remain in your patch.

#### 5) Select e-mail destination.

Look through the MAINTAINERS file and the source code, and determine if your
change applies to a specific subsystem of the kernel, with an assigned
maintainer. If so, e-mail that person. The script scripts/get_maintainer.pl can
be very useful at this step. If no maintainer is listed, or the maintainer does
not respond, send your patch to the primary Linux kernel developer's mailing
list, linux-kernel@vger.kernel.org. Most kernel developers monitor this e-mail
list, and can comment on your changes. 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 . He gets a lot of e-mail,
so typically you should do your best to -avoid- sending him e-mail. Patches
which are bug fixes, are "obvious" changes, or similarly require little
discussion should be sent or CC'd to Linus. Patches which require discussion or
do not have a clear advantage should usually be sent first to linux-kernel. Only
after the patch is discussed should the patch then be submitted to Linus.

#### 6) Select your CC (e-mail carbon copy) list.

Unless you have a reason NOT to do so, CC linux-kernel@vger.kernel.org. Other
kernel developers besides Linus need to be aware of your change, so that they
may comment on it and offer code review and suggestions. linux-kernel is the
primary Linux kernel developer mailing list. Other mailing lists are available
for specific subsystems, such as USB, framebuffer devices, the VFS, the SCSI
subsystem, etc. See the MAINTAINERS file for a mailing list that relates
specifically to your change. Majordomo lists of VGER.KERNEL.ORG at:        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. Even if the maintainer did not respond in step #5, make sure to
ALWAYS copy the maintainer when you change their code. 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 which could break 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 (eg. check_region) 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)

#### 7) 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
submitting 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 Documentation/email-clients.txt for
hints about configuring your e-mail client so that it sends your patches
untouched.

#### 8) E-mail size.

When sending patches to Linus, always follow step #7. 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.

#### 9) Name your kernel version.

It is important to note, either in the subject line or in the patch description,
the kernel version to which this patch applies. If the patch does not apply
cleanly to the latest kernel version, Linus will not apply it.

#### 10) Don't get discouraged. Re-submit.

After you have submitted your change, be patient and wait. If Linus likes your
change and applies it, it will appear in the next version of the kernel that he
releases. However, if your change doesn't appear in the next version of the
kernel, there could be any number of reasons. It's YOUR job to narrow down those
reasons, correct what was wrong, and submit your updated change. It is quite
common for Linus to "drop" your patch without comment. That's the nature of the
system. If he drops your patch, it could be due to \* Your patch did not apply
cleanly to the latest kernel version. \* Your patch was not sufficiently
discussed on linux-kernel. \* A style issue (see section 2). \* An e-mail
formatting issue (re-read this section). \* A technical problem with your
change. \* He gets tons of e-mail, and yours got lost in the shuffle. \* You are
being annoying. When in doubt, solicit comments on linux-kernel mailing list.

#### 11) 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.

#### 12) 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
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       \[lucky@maintainer.example.org: struct foo moved from foo.c to
foo.h\]  Signed-off-by: Lucky K Maintainer This practise 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 practise 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
2.6-stable : Date: Tue May 13 19:10:30 2008 +0000 SCSI: libiscsi regression in
2.6.25: fix nop timer handling commit 4cf1043593db6a337f10e006c23c69e5fc93e722
upstream And here's what appears in 2.4 : 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
trouble-shoot bugs in your tree.

#### 13) 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 arrange 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:. 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. This tag documents that potentially
interested parties have been included in the discussion

#### 14) Using Reported-by:, Tested-by:, Reviewed-by: and Suggested-by:

If this patch fixes a problem reported by somebody else, consider adding a
Reported-by: tag to credit the reporter for their contribution. Please note that
this tag should not be added without the reporter's permission, especially if
the problem was not reported in a public forum. That said, if we diligently
credit our bug reporters, they will, hopefully, be inspired to help us again in
the future. 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.

#### 15) The canonical patch format

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. - An empty line. - The body of the
explanation, 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: \[PATCH tag\] ". 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: \[PATCHv2 001/207\]
x86: fix eflags tracking The "from" line must be the very first line in the
message body, and has the form: From: Original Author 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). See more details on the proper patch format in the following
references.

#### 16) Sending "git pull" requests (from Linus emails)

Please write the git repo address and branch name alone on the same line so that
I can't even by mistake pull from the wrong branch, and so that a triple-click
just selects the whole thing. So the proper format is something along the lines
of:     "Please pull from
git://jdelvare.pck.nerim.net/jdelvare-2.6 i2c-for-linus          to get these
changes:" so that I don't have to hunt-and-peck for the address and inevitably
get it wrong (actually, I've only gotten it wrong a few times, and checking
against the diffstat tells me when I get it wrong, but I'm just a lot more
comfortable when I don't have to "look for" the right thing to pull, and
double-check that I have the right branch-name). Please use "git diff -M --stat
--summary" to generate the diffstat: the -M enables rename detection, and the
summary enables a summary of new/deleted or renamed files. With rename
detection, the statistics are rather different \[...\] because git will notice
that a fair number of the changes are renames.
-----------------------------------

### SECTION 2 - HINTS, TIPS, AND TRICKS

----------------------------------- This section lists many of the common
"rules" associated with code submitted to the kernel. There are always
exceptions... but you must have a really good reason for doing so. You could
probably call this section Linus Computer Science 101.

#### 1) Read Documentation/CodingStyle

Nuff said. If your code deviates too much from this, it is likely to be rejected
without further review, and without comment. 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). The style checker should be viewed as a guide not as
the final word. 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.

#### 2) #ifdefs are ugly

Code cluttered with ifdefs is difficult to read and maintain. Don't do it.
Instead, put your ifdefs in a header, and conditionally define 'static inline'
functions, or macros, which are used in the code. Let the compiler optimize away
the "no-op" case. Simple example, of poor code:      dev = alloc_etherdev
(sizeof(struct funky_private));    if (!dev)               return -ENODEV;
#ifdef CONFIG_NET_FUNKINESS     init_funky_net(dev);    #endif Cleaned-up
example: (in header)  #ifndef CONFIG_NET_FUNKINESS    static inline void
init_funky_net (struct net_device \*d) {}    #endif (in the code itself)     dev
= alloc_etherdev (sizeof(struct funky_private));    if (!dev)
return -ENODEV;         init_funky_net(dev);

#### 3) 'static inline' is better than a macro

Static inline functions are greatly preferred over macros. They provide type
safety, have no length limitations, no formatting limitations, and under gcc
they are as cheap as macros. Macros should only be used for cases where a static
inline is clearly suboptimal \[there are a few, isolated cases of this in fast
paths\], or where it is impossible to use a static inline function \[such as
string-izing\]. 'static inline' is preferred over 'static __inline__', 'extern
inline', and 'extern __inline__'.

#### 4) Don't over-design.

Don't try to anticipate nebulous future cases which may or may not be useful:
"Make it as simple as you can, and no simpler." ----------------------

### SECTION 3 - REFERENCES

---------------------- Andrew Morton, "The perfect patch" (tpp). Jeff Garzik,
"Linux kernel patch submission format". Greg Kroah-Hartman, "How to piss off a
kernel subsystem maintainer". NO!!!! No more huge patch bombs to
linux-kernel@vger.kernel.org people! Kernel Documentation/CodingStyle: Linus
Torvalds's mail on the canonical patch format: Andi Kleen, "On submitting kernel
patches" Some strategies to get difficult or controversial changes in.
http://halobates.de/on-submitting-patches.pdf --