How to Get Your Change Into the Linux Kernel		or	Care And Operation Of Your Linus TorvaldsFor a person or company who wishes to submit a change to the Linuxkernel, the process can sometimes be daunting if you're not familiarwith "the system."  This text is a collection of suggestions whichcan 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, asgenerated by diff(1).  When creating your patch, make sure to create itin "unified diff" format, as supplied by the '-u' argument to diff(1).Patches should be based in the root kernel source directory, not inany 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/patchTo create a patch for multiple files, you should unpack a "vanilla",or unmodified kernel source tree, and generate a diff against yourown 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 duringthe build process, and should be ignored in any diff(1)-generatedpatch.  dontdiff is maintained by Tigran Aivazian <tigran@veritas.com>Make sure your patch does not include any extra files which do notbelong 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 includethings like "update driver X", "bug fix for driver X", or "this patchincludes updates for subsystem X.  Please apply."If your description starts to get long, that's a sign that you probablyneed 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 performanceenhancements for a single driver, separate those changes into twoor more patches.  If your changes include an API update, and a newdriver 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 changeis contained within a single patch.If one patch depends on another patch in order for a change to becomplete, 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 determineif your change applies to a specific subsystem of the kernel, withan assigned maintainer.  If so, e-mail that person.If no maintainer is listed, or the maintainer does not respond, sendyour patch to the primary Linux kernel developer's mailing list,linux-kernel@vger.kernel.org.  Most kernel developers monitor thise-mail list, and can comment on your changes.Linus Torvalds is the final arbiter of all changes accepted into theLinux kernel.  His e-mail address is torvalds@transmeta.com.  He getsa lot of e-mail, so typically you should do your best to -avoid- sendinghim e-mail.Patches which are bug fixes, are "obvious" changes, or similarlyrequire little discussion should be sent or CC'd to Linus.  Patcheswhich require discussion or do not have a clear advantage shouldusually be sent first to linux-kernel.  Only after the patch isdiscussed 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 asUSB, framebuffer devices, the VFS, the SCSI subsystem, etc.  See theMAINTAINERS file for a mailing list that relates specifically toyour change.Even if the maintainer did not respond in step #4, make sure to ALWAYScopy 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 commenton the changes you are submitting.  It is important for a kerneldeveloper to be able to "quote" your changes, using standard e-mailtools, 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 MIMEattachment as plain text, making it impossible to comment on yourcode.  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 askyou 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 somemaintainers.  If your patch, uncompressed, exceeds 40Kb in size,it is preferred that you store your patch on an Internet-accessibleserver, 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 patchdescription, 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 Linuslikes your change and applies it, it will appear in the next versionof the kernel that he releases.However, if your change doesn't appear in the next version of thekernel, there could be any number of reasons.  It's YOUR job tonarrow down those reasons, correct what was wrong, and submit yourupdated 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 bedue 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 subjectDue to high e-mail traffic to Linus, and to linux-kernel, it is commonconvention to prefix your subject line with [PATCH].  This lets Linusand other kernel developers more easily distinguish patches from othere-mail discussions.-----------------------------------SECTION 2 - HINTS, TIPS, AND TRICKS-----------------------------------This section lists many of the common "rules" associated with codesubmitted to the kernel.  There are always exceptions... but you musthave a really good reason for doing so.  You could probably call thissection Linus Computer Science 101.1) Read Documentation/CodingStyleNuff said.  If your code deviates too much from this, it is likelyto be rejected without further review, and without comment.2) #ifdefs are uglyCode cluttered with ifdefs is difficult to read and maintain.  Don't doit.  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);	#endifCleaned-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 macroStatic inline functions are greatly preferred over macros.They provide type safety, have no length limitations, no formattinglimitations, and under gcc they are as cheap as macros.Macros should only be used for cases where a static inline is clearlysuboptimal [there a few, isolated cases of this in fast paths],or where it is impossible to use a static inline function [such asstring-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 notbe useful:  "Make it as simple as you can, and no simpler"