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        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.  For earlier kernel versions, you can get it

from .

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://www.zip.com.au/~akpm/linux/patches/

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."

If your description starts to get long, that's a sign that you probably

need to split up your patch.  See #3, next.

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.

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 #4, 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 managed by Adrian Bunk; which collects "trivial"

patches. 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)

URL: 

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.

WARNING: Some mailers like Mozilla send your messages with

---- message header ----

Content-Type: text/plain; charset=us-ascii; format=flowed

---- message header ----

The problem is that "format=flowed" makes some of the mailers

on receiving side to replace TABs with spaces and do similar

changes. Thus the patches from you can look corrupted.

To fix this just make your mozilla defaults/pref/mailnews.js file to look like:

pref("mailnews.send_plaintext_flowed", false); // RFC 2646=======

pref("mailnews.display.disable_format_flowed_support", true);

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 40 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 a 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.

13) When to use Acked-by:

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.

14) 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.

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.

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.

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.

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

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 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:

--