howto do linux kernel development---take3

Thanks to everyone for their review comments on the last version of thedocument.  I'm starting to get the same corrections, so here's anupdated version.One big thing that I seem to not have gotten accross properly is, thisis not a document that will help you in the technical programming aspectof kernel development.  There are _loads_ of articles, books, and otherthings out there that will help with that.  This is for the social andproceedure aspects, which is woefully undocumented.I've change the very first paragraph to hopefully get that point accrossbetter.  If I still haven't, suggestions on how to do that are greatlyappreciated.I think the file is a lot better now, and is ready for inclusion.  Anyobjections?thanks,greg k-h------------------------------HOWTO do Linux kernel development---------------------------------This is the be-all, end-all document on this topic.  It containsinstructions on how to become a Linux kernel developer and how to learnto work with the Linux kernel development community.  It tries to notcontain anything related to the technical aspects of kernel programming,but will help point you in the right direction for that.If anything in this document becomes out of date, please send in patchesto the maintainer of this file, who is listed at the bottom of thedocument.Introduction------------So, you want to learn how to become a Linux kernel developer?  Or youhave been told by your manager, "Go write a Linux driver for thisdevice."  This document's goal is to teach you everything you need toknow to achieve this by describing the process you need to go through,and hints on how to work with the community.  It will also try toexplain some of the reasons why the community works like it does.The kernel is written mostly in C, with some architecture-dependentparts written in assembly. A good understanding of C is required forkernel development.  Assembly (any architecture) is not required unlessyou plan to do low-level development for that architecture.  Though theyare not a good substitute for a solid C education and/or years ofexperience, the following books are good for, if anything, reference: - "The C Programming Language" by Kernighan and Ritchie [Prentice Hall] - "Practical C Programming" by Steve Oualline [O'Reilly]The kernel is written using GNU C and the GNU toolchain.  While itadheres to the ISO C89 standard, it uses a number of extensions that arenot featured in the standard.  The kernel is a freestanding Cenvironment, with no reliance on the standard C library, so someportions of the C standard are not supported.  Arbitrary long longdivisions and floating point are not allowed.  It can sometimes bedifficult to understand the assumptions the kernel has on the toolchainand the extensions that it uses, and unfortunately there is nodefinitive reference for them.  Please check the gcc info pages (`infogcc`) for some information on them.Please remember that you are trying to learn how to work with theexisting development community.  It is a diverse group of people, withhigh standards for coding, style and procedure.  These standards havebeen created over time based on what they have found to work best forsuch a large and geographically dispersed team.  Try to learn as much aspossible about these standards ahead of time, as they are welldocumented; do not expect people to adapt to you or your company's wayof doing things.Legal Issues------------The Linux kernel source code is released under the GPL.  Please see thefile, COPYING, in the main directory of the source tree, for details onthe license.  If you have further questions about the license, pleasecontact a lawyer, and do not ask on the Linux kernel mailing list.  Thepeople on the mailing lists are not lawyers, and you should not rely ontheir statements on legal matters.For common questions and answers about the GPL, please see:http://www.gnu.org/licenses/gpl-faq.htmlDocumentation------------The Linux kernel source tree has a large range of documents that areinvaluable for learning how to interact with the kernel community.  Whennew features are added to the kernel, it is recommended that newdocumentation files are also added which explain how to use the feature.When a kernel change causes the interface that the kernel exposes touserspace to change, it is recommended that you send the information ora patch to the manual pages explaining the change to the manual pagesmaintainer at mtk-manpages <at> gmx.net.Here is a list of files that are in the kernel source tree that arerequired reading:  README    This file gives a short background on the Linux kernel and describes    what is necessary to do to configure and build the kernel.  People    who are new to the kernel should start here.  Documentation/Changes    This file gives a list of the minimum levels of various software    packages that are necessary to build and run the kernel    successfully.  Documentation/CodingStyle    This describes the Linux kernel coding style, and some of the    rationale behind it. All new code is expected to follow the    guidelines in this document. Most maintainers will only accept    patches if these rules are followed, and many people will only    review code if it is in the proper style.  Documentation/SubmittingPatches  Documentation/SubmittingDrivers    These files describe in explicit detail how to successfully create    and send a patch, including (but not limited to):       - Email contents       - Email format       - Who to send it to    Following these rules will not guarantee success (as all patches are    subject to scrutiny for content and style), but not following them    will almost always prevent it.    Other excellent descriptions of how to create patches properly are:"The Perfect Patch"http://www.zip.com.au/~akpm/linux/patches/stuff/tpp.txt"Linux kernel patch submission format"http://linux.yyz.us/patch-format.html  Documentation/stable_api_nonsense.txt    This file describes the rationale behind the conscious decision to    not have a stable API within the kernel, including things like:      - Subsystem shim-layers (for compatibility?)      - Driver portability between Operating Systems.      - Mitigating rapid change within the kernel source tree (orpreventing rapid change)    This document is crucial for understanding the Linux development    philosophy and is very important for people moving to Linux from    development on other Operating Systems.  Documentation/SecurityBugs    If you feel you have found a security problem in the Linux kernel,    please follow the steps in this document to help notify the kernel    developers, and help solve the issue.  Documentation/ManagementStyle    This document describes how Linux kernel maintainers operate and the    shared ethos behind their methodologies.  This is important reading    for anyone new to kernel development (or anyone simply curious about    it), as it resolves a lot of common misconceptions and confusion    about the unique behavior of kernel maintainers.  Documentation/stable_kernel_rules.txt    This file describes the rules on how the stable kernel releases    happen, and what to do if you want to get a change into one of these    releases.  Documentation/kernel-docs.txt    A list of external documentation that pertains to kernel    development.  Please consult this list if you do not find what you    are looking for within the in-kernel documentation.  Documentation/applying-patches.txt    A good introduction describing exactly what a patch is and how to    apply it to the different development branches of the kernel.The kernel also has a large number of documents that can beautomatically generated from the source code itself.  This includes afull description of the in-kernel API, and rules on how to handlelocking properly.  The documents will be created in theDocumentation/DocBook/ directory and can be generated as PDF,Postscript, HTML, and man pages by running:make pdfdocsmake psdocsmake htmldocsmake mandocsrespectively from the main kernel source directory.Becoming A Kernel Developer---------------------------If you do not know anything about Linux kernel development, you shouldlook at the Linux KernelNewbies project:http://kernelnewbies.orgIt consists of a helpful mailing list where you can ask almost any typeof basic kernel development question (make sure to search the archivesfirst, before asking something that has already been answered in thepast.)  It also has an IRC channel that you can use to ask questions inreal-time, and a lot of helpful documentation that is useful forlearning about Linux kernel development.The website has basic information about code organization, subsystems,and current projects (both in-tree and out-of-tree). It also describessome basic logistical information, like how to compile a kernel andapply a patch.If you do not know where you want to start, but you want to look forsome task to start doing to join into the kernel development community,go to the Linux Kernel Janitor's project:http://janitor.kernelnewbies.org/It is a great place to start.  It describes a list of relatively simpleproblems that need to be cleaned up and fixed within the Linux kernelsource tree.  Working with the developers in charge of this project, youwill learn the basics of getting your patch into the Linux kernel tree,and possibly be pointed in the direction of what to go work on next, ifyou do not already have an idea.If you already have a chunk of code that you want to put into the kerneltree, but need some help getting it in the proper form, thekernel-mentors project was created to help you out with this.  It is amailing list, and can be found at:http://selenic.com/mailman/listinfo/kernel-mentorsBefore making any actual modifications to the Linux kernel code, it isimperative to understand how the code in question works.  For thispurpose, nothing is better than reading through it directly (most trickybits are commented well), perhaps even with the help of specializedtools.  One such tool that is particularly recommended is the LinuxCross-Reference project, which is able to present source code in aself-referential, indexed webpage format. An excellent up-to-daterepository of the kernel code may be found at:http://sosdg.org/~coywolf/lxr/The development process-----------------------Linux kernel development process currently consists of a few differentmain kernel "branches" and lots of different subsystem-specific kernelbranches.  These different branches are:  - main 2.6.x kernel tree  - 2.6.x.y -stable kernel tree  - 2.6.x -git kernel patches  - 2.6.x -mm kernel patches  - subsystem specific kernel trees and patches2.6.x kernel tree-----------------2.6.x kernels are maintained by Linus Torvalds, and can be found onkernel.org in the pub/linux/kernel/v2.6/ directory.  Its developmentprocess is as follows:  - As soon as a new kernel is released a two weeks window is open,    during this period of time maintainers can submit big diffs to    Linus, usually the patches that have already been included in the    -mm kernel for a few weeks.  The preferred way to submit big changes    is using git (the kernel's source management tool, more information    can be found at http://git.or.cz/) but plain patches are also just    fine.  - After two weeks a -rc1 kernel is released it is now possible to push    only patches that do not include new features that could affect the    stability of the whole kernel.  Please note that a whole new driver    (or filesystem) might be accepted after -rc1 because there is no    risk of causing regressions with such a change as long as the change    is self-contained and does not affect areas outside of the code that    is being added.  git can be used to send patches to Linus after -rc1    is released, but the patches need to also be sent to a public    mailing list for review.  - A new -rc is released whenever Linus deems the current git tree to    be in a reasonably sane state adequate for testing.  The goal is to    release a new -rc kernel every week.  - Process continues until the kernel is considered "ready", the    process should last around 6 weeks.It is worth mentioning what Andrew Morton wrote on the linux-kernelmailing list about kernel releases:"Nobody knows when a kernel will be released, because it'sreleased according to perceived bug status, not according to apreconceived timeline."2.6.x.y -stable kernel tree---------------------------Kernels with 4 digit versions are -stable kernels. They containrelatively small and critical fixes for security problems or significantregressions discovered in a given 2.6.x kernel.This is the recommended branch for users who want the most recent stablekernel and are not interested in helping test development/experimentalversions.If no 2.6.x.y kernel is available, then the highest numbered 2.6.xkernel is the current stable kernel.2.6.x.y are maintained by the "stable" team <stable <at> kernel.org>, and arereleased almost every other week.The file Documentation/stable_kernel_rules.txt in the kernel treedocuments what kinds of changes are acceptable for the -stable tree, andhow the release process works.2.6.x -git patches------------------These are daily snapshots of Linus' kernel tree which are managed in agit repository (hence the name.) These patches are usually releaseddaily and represent the current state of Linus' tree.  They are moreexperimental than -rc kernels since they are generated automaticallywithout even a cursory glance to see if they are sane.2.6.x -mm kernel patches------------------------These are experimental kernel patches released by Andrew Morton.  Andrewtakes all of the different subsystem kernel trees and patches and mushesthem together, along with a lot of patches that have been plucked fromthe linux-kernel mailing list.  This tree serves as a proving ground fornew features and patches.  Once a patch has proved its worth in -mm fora while Andrew or the subsystem maintainer pushes it on to Linus forinclusion in mainline.It is heavily encouraged that all new patches get tested in the -mm treebefore they are sent to Linus for inclusion in the main kernel tree.These kernels are not appropriate for use on systems that are supposedto be stable and they are more risky to run than any of the otherbranches.If you wish to help out with the kernel development process, please testand use these kernel releases and provide feedback to the linux-kernelmailing list if you have any problems, and if everything works properly.In addition to all the other experimental patches, these kernels usuallyalso contain any changes in the mainline -git kernels available at thetime of release.The -mm kernels are not released on a fixed schedule, but usually a few-mm kernels are released in between each -rc kernel (1 to 3 is common).Subsystem Specific kernel trees and patches-------------------------------------------A number of the different kernel subsystem developers expose theirdevelopment trees so that others can see what is happening in thedifferent areas of the kernel.  These trees are pulled into the -mmkernel releases as described above.Here is a list of some of the different kernel trees available:  git trees:    - Kbuild development tree, Sam Ravnborg <sam <at> ravnborg.org>kernel.org:/pub/scm/linux/kernel/git/sam/kbuild.git    - ACPI development tree, Len Brown <len.brown <at> intel.com>kernel.org:/pub/scm/linux/kernel/git/lenb/linux-acpi-2.6.git    - Block development tree, Jens Axboe <axboe <at> suse.de>kernel.org:/pub/scm/linux/kernel/git/axboe/linux-2.6-block.git    - DRM development tree, Dave Airlie <airlied <at> linux.ie>kernel.org:/pub/scm/linux/kernel/git/airlied/drm-2.6.git    - ia64 development tree, Tony Luck <tony.luck <at> intel.com>kernel.org:/pub/scm/linux/kernel/git/aegl/linux-2.6.git    - ieee1394 development tree, Jody McIntyre <scjody <at> modernduck.com>kernel.org:/pub/scm/linux/kernel/git/scjody/ieee1394.git    - infiniband, Roland Dreier <rolandd <at> cisco.com>kernel.org:/pub/scm/linux/kernel/git/roland/infiniband.git    - libata, Jeff Garzik <jgarzik <at> pobox.com>kernel.org:/pub/scm/linux/kernel/git/jgarzik/libata-dev.git    - network drivers, Jeff Garzik <jgarzik <at> pobox.com>kernel.org:/pub/scm/linux/kernel/git/jgarzik/netdev-2.6.git    - pcmcia, Dominik Brodowski <linux <at> dominikbrodowski.net>kernel.org:/pub/scm/linux/kernel/git/brodo/pcmcia-2.6.git    - SCSI, James Bottomley <James.Bottomley <at> SteelEye.com>kernel.org:/pub/scm/linux/kernel/git/jejb/scsi-misc-2.6.git  Other git kernel trees can be found listed at http://kernel.org/git  quilt trees:    - USB, PCI, Driver Core, and I2C, Greg Kroah-Hartman <gregkh <at> suse.de>kernel.org/pub/linux/kernel/people/gregkh/gregkh-2.6/Bug Reporting-------------bugzilla.kernel.org is where the Linux kernel developers track kernelbugs.  Users are encouraged to report all bugs that they find in thistool.  For details on how to use the kernel bugzilla, please see:http://test.kernel.org/bugzilla/faq.htmlThe file REPORTING-BUGS in the main kernel source directory has a goodtemplate for how to report a possible kernel bug, and details what kindof information is needed by the kernel developers to help track down theproblem.Mailing lists-------------As some of the above documents describe, the majority of the core kerneldevelopers participate on the Linux Kernel Mailing list.  Details on howto subscribe and unsubscribe from the list can be found at:http://vger.kernel.org/vger-lists.html#linux-kernelThere are archives of the mailing list on the web in many differentplaces.  Use a search engine to find these archives.  For example:http://dir.gmane.org/gmane.linux.kernelIt is highly recommended that you search the archives about the topicyou want to bring up, before you post it to the list. A lot of thingsalready discussed in detail are only recorded at the mailing listarchives.Most of the individual kernel subsystems also have their own separatemailing list where they do their development efforts.  See theMAINTAINERS file for a list of what these lists are for the differentgroups.Many of the lists are hosted on kernel.org. Information on them can befound at:http://vger.kernel.org/vger-lists.htmlPlease remember to follow good behavioral habits when using the lists.Though a bit cheesy, the following URL has some simple guidelines forinteracting with the list (or any list):http://www.albion.com/netiquette/If multiple people respond to your mail, the CC: list of recipients mayget pretty large. Don't remove anybody from the CC: list without a goodreason, or don't reply only to the list address. Get used to receiving themail twice, one from the sender and the one from the list, and don't tryto tune that by adding fancy mail-headers, people will not like it.Remember to keep the context and the attribution of your replies intact,keep the "John Kernelhacker wrote ...:" lines at the top of your reply, andadd your statements between the individual quoted sections instead ofwriting at the top of the mail.If you add patches to your mail, make sure they are plain readable textas stated in Documentation/SubmittingPatches. Kernel developers don'twant to deal with attachments or compressed patches; they may wantto comment on individual lines of your patch, which works only that way.Make sure you use a mail program that does not mangle spaces and tabcharacters. A good first test is to send the mail to yourself and tryto apply your own patch by yourself. If that doesn't work, get yourmail program fixed or change it until it works.Above all, please remember to show respect to other subscribers.Working with the community--------------------------The goal of the kernel community is to provide the best possible kernelthere is.  When you submit a patch for acceptance, it will be reviewedon its technical merits and those alone.  So, what should you beexpecting?  - criticism  - comments  - requests for change  - requests for justification  - silenceRemember, this is part of getting your patch into the kernel.  You haveto be able to take criticism and comments about your patches, evaluatethem at a technical level and either rework your patches or provideclear and concise reasoning as to why those changes should not be made.If there are no responses to your posting, wait a few days and tryagain, sometimes things get lost in the huge volume.What should you not do?  - expect your patch to be accepted without question  - become defensive  - ignore comments  - resubmit the patch without making any of the requested changesIn a community that is looking for the best technical solution possible,there will always be differing opinions on how beneficial a patch is.You have to be cooperative, and willing to adapt your idea to fit withinthe kernel.  Or at least be willing to prove your idea is worth it.Remember, being wrong is acceptable as long as you are willing to worktoward a solution that is right.It is normal that the answers to your first patch might simply be a listof a dozen things you should correct.  This does _not_ imply that yourpatch will not be accepted, and it is _not_ meant against youpersonally.  Simply correct all issues raised against your patch andresend it.Differences between the kernel community and corporate structures-----------------------------------------------------------------The kernel community works differently than most traditional corporatedevelopment environments.  Here are a list of things that you can try todo to try to avoid problems:  Good things to say regarding your proposed changes:    - "This solves multiple problems."    - "This deletes 2000 lines of code."    - "Here is a patch that explains what I am trying to describe."    - "I tested it on 5 different architectures..."    - "Here is a series of small patches that..."    - "This increases performance on typical machines..."  Bad things you should avoid saying:    - "We did it this way in AIX/ptx/Solaris, so therefore it must be      good..."    - "I've being doing this for 20 years, so..."    - "This is required for my company to make money"    - "This is for our Enterprise product line."    - "Here is my 1000 page design document that describes my idea"    - "I've been working on this for 6 months..."    - "Here's a 5000 line patch that..."    - "I rewrote all of the current mess, and here it is..."    - "I have a deadline, and this patch needs to be applied now."Another way the kernel community is different than most traditionalsoftware engineering work environments is the faceless nature ofinteraction.  One benefit of using email and irc as the primary forms ofcommunication is the lack of discrimination based on gender or race.The Linux kernel work environment is accepting of women and minoritiesbecause all you are is an email address.  The international aspect alsohelps to level the playing field because you can't guess gender based ona person's name. A man may be named Andrea and a woman may be named Pat.Most women who have worked in the Linux kernel and have expressed anopinion have had positive experiences.The language barrier can cause problems for some people who are notcomfortable with English.  A good grasp of the language can be needed inorder to get ideas across properly on mailing lists, so it isrecommended that you check your emails to make sure they make sense inEnglish before sending them.Break up your changes---------------------The Linux kernel community does not gladly accept large chunks of codedropped on it all at once.  The changes need to be properly introduced,discussed, and broken up into tiny, individual portions.  This is almostthe exact opposite of what companies are used to doing.  Your proposalshould also be introduced very early in the development process, so thatyou can receive feedback on what you are doing.  It also lets thecommunity feel that you are working with them, and not simply using themas a dumping ground for your feature.  However, don't send 50 emails atone time to a mailing list, your patch series should be smaller thanthat almost all of the time.The reasons for breaking things up are the following:1) Small patches increase the likelihood that your patches will be   applied, since they don't take much time or effort to verify for   correctness.  A 5 line patch can be applied by a maintainer with   barely a second glance. However, a 500 line patch may take hours to   review for correctness (the time it takes is exponentially   proportional to the size of the patch, or something).   Small patches also make it very easy to debug when something goes   wrong.  It's much easier to back out patches one by one than it is   to dissect a very large patch after it's been applied (and broken   something).2) It's important not only to send small patches, but also to rewrite   and simplify (or simply re-order) patches before submitting them.Here is an analogy from kernel developer Al Viro:"Think of a teacher grading homework from a math student.  Theteacher does not want to see the student's trials and errorsbefore they came up with the solution. They want to see thecleanest, most elegant answer.  A good student knows this, andwould never submit her intermediate work before the finalsolution."The same is true of kernel development. The maintainers andreviewers do not want to see the thought process behind thesolution to the problem one is solving. They want to see asimple and elegant solution."It may be challenging to keep the balance between presenting an elegantsolution and working together with the community and discussing yourunfinished work. Therefore it is good to get early in the process toget feedback to improve your work, but also keep your changes in smallchunks that they may get already accepted, even when your whole task isnot ready for inclusion now.Also realize that it is not acceptable to send patches for inclusionthat are unfinished and will be "fixed up later."Justify your change-------------------Along with breaking up your patches, it is very important for you to letthe Linux community know why they should add this change.  New featuresmust be justified as being needed and useful.Document your change--------------------When sending in your patches, pay special attention to what you say inthe text in your email.  This information will become the ChangeLoginformation for the patch, and will be preserved for everyone to see forall time.  It should describe the patch completely, containing:  - why the change is necessary  - the overall design approach in the patch  - implementation details  - testing resultsFor more details on what this should all look like, please see theChangeLog section of the document:  "The Perfect Patch"      http://www.zip.com.au/~akpm/linux/patches/stuff/tpp.txtAll of these things are sometimes very hard to do. It can take years toperfect these practices (if at all). It's a continuous process ofimprovement that requires a lot of patience and determination. Butdon't give up, it's possible. Many have done it before, and each had tostart exactly where you are now.----------Thanks to Paolo Ciarrocchi who allowed the "Development Process" sectionto be based on text he had written, and to Randy Dunlap and GerritHuizenga for some of the list of things you should and should not say.Also thanks to Pat Mochel, Hanna Linder, Randy Dunlap, Kay Sievers,Vojtech Pavlik, Jan Kara, Josh Boyer, Kees Cook, Andrew Morton, AndiKleen, Vadim Lobanov, Jesper Juhl, Adrian Bunk, Keri Harris, Frans Pop,David A. Wheeler, Junio Hamano, Michael Kerrisk, and Alex Shepard fortheir review and comments on early drafts of this document.Maintainer: Greg Kroah-Hartman <greg <at> kroah.com>