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.

If you are submitting a driver, also read Documentation/SubmittingDrivers.



--------------------------------------------
SECTION 1 - CREATING AND SENDING YOUR CHANGE
--------------------------------------------



1) "diff -u"
------------

Use "diff -u" or "diff -urN" 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).
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= /devel/linux-2.4
        MYFILE=  drivers/net/mydriver.c

        cd $SRCTREE
        cp $MYFILE $MYFILE.orig
        vi $MYFILE      # make your change
        diff -u $MYFILE.orig $MYFILE > /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-2.4

        tar xvfz linux-2.4.0-test11.tar.gz
        mv linux linux-vanilla
        wget http://www.moses.uklinux.net/patches/dontdiff
        diff -urN -X dontdiff linux-vanilla $MYSRC > /tmp/patch
        rm -f dontdiff

"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.  dontdiff is maintained by Tigran Aivazian <tigran@veritas.com>

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.


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 each logical change into its own 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.

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.


4) 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.

Linus Torvalds is the final arbiter of all changes accepted into the
Linux kernel.  His e-mail address is <torvalds@osdl.org>.  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.



5) 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.

Even if the maintainer did not respond in step #4, make sure to ALWAYS
copy the maintainer when you change their code.



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 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.



7) E-mail size.

When sending patches to Linus, always follow step #6.

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.



8) 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.



9) 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 (See Figure 1)

When in doubt, solicit comments on linux-kernel mailing list.



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.



-----------------------------------
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.



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 = init_etherdev (NULL, 0);
        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 = init_etherdev (NULL, 0);
        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"