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 If anything in this document becomes out of date, please send in patchesto the maintainer of this file, who is listed at the bottom of thedocument.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] - "C:  A Reference Manual" by Harbison and Steele [Prentice Hall] 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: 	https://www.gnu.org/licenses/gpl-faq.html  Documentation------------- 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@gmail.com, and CC the listlinux-api@vger.kernel.org. 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.   :ref:`Documentation/Changes <changes>`    This file gives a list of the minimum levels of various software    packages that are necessary to build and run the kernel    successfully.   :ref:`Documentation/CodingStyle <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.   :ref:`Documentation/SubmittingPatches <submittingpatches>` and :ref:`Documentation/SubmittingDrivers <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"		https://www.ozlabs.org/~akpm/stuff/tpp.txt 	"Linux kernel patch submission format"		http://linux.yyz.us/patch-format.html   :ref:`Documentation/stable_api_nonsense.txt <stable_api_nonsense>`    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 (or	preventing 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.   :ref:`Documentation/SecurityBugs <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.   :ref:`Documentation/ManagementStyle <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.   :ref:`Documentation/stable_kernel_rules.txt <stable_kernel_rules>`    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.   :ref:`Documentation/kernel-docs.txt <kernel_docs>`    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.   :ref:`Documentation/applying-patches.txt <applying_patches>`    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 or fromReStructuredText markups (ReST), like this one. This includes afull description of the in-kernel API, and rules on how to handlelocking properly. All such documents can be generated as PDF or HTML by running:: 	make pdfdocs	make htmldocs respectively from the main kernel source directory. The documents that uses ReST markup will be generated at Documentation/output.They can also be generated on LaTeX and ePub formats with:: 	make latexdocs	make epubdocs Currently, there are some documents written on DocBook that are inthe process of conversion to ReST. Such documents will be created in theDocumentation/DocBook/ directory and can be generated also asPostscript or man pages by running:: 	make psdocs	make mandocs Becoming A Kernel Developer--------------------------- If you do not know anything about Linux kernel development, you shouldlook at the Linux KernelNewbies project: 	https://kernelnewbies.org It 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: 	https://kernelnewbies.org/KernelJanitors 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: 	https://selenic.com/mailman/listinfo/kernel-mentors Before 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://lxr.free-electrons.com/  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 4.x kernel tree  - 4.x.y -stable kernel tree  - 4.x -git kernel patches  - subsystem specific kernel trees and patches  - the 4.x -next kernel tree for integration tests 4.x kernel tree-----------------4.x kernels are maintained by Linus Torvalds, and can be found onhttps://kernel.org in the pub/linux/kernel/v4.x/ 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    -next 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 https://git-scm.com/) 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's	released according to perceived bug status, not according to a	preconceived timeline."* 4.x.y -stable kernel tree-------------------------Kernels with 3-part versions are -stable kernels. They containrelatively small and critical fixes for security problems or significantregressions discovered in a given 4.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 4.x.y kernel is available, then the highest numbered 4.xkernel is the current stable kernel. 4.x.y are maintained by the "stable" team <stable@vger.kernel.org>, andare released as needs dictate.  The normal release period is approximatelytwo weeks, but it can be longer if there are no pressing problems.  Asecurity-related problem, instead, can cause a release to happen almostinstantly. 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. 4.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. Subsystem Specific kernel trees and patches-------------------------------------------The maintainers of the various kernel subsystems --- and also manykernel subsystem developers --- expose their current state ofdevelopment in source repositories.  That way, others can see what ishappening in the different areas of the kernel.  In areas wheredevelopment is rapid, a developer may be asked to base his submissionsonto such a subsystem kernel tree so that conflicts between thesubmission and other already ongoing work are avoided. Most of these repositories are git trees, but there are also other SCMsin use, or patch queues being published as quilt series.  Addresses ofthese subsystem repositories are listed in the MAINTAINERS file.  Manyof them can be browsed at https://git.kernel.org/. Before a proposed patch is committed to such a subsystem tree, it issubject to review which primarily happens on mailing lists (see therespective section below).  For several kernel subsystems, this reviewprocess is tracked with the tool patchwork.  Patchwork offers a webinterface which shows patch postings, any comments on a patch orrevisions to it, and maintainers can mark patches as under review,accepted, or rejected.  Most of these patchwork sites are listed athttps://patchwork.kernel.org/. 4.x -next kernel tree for integration tests-------------------------------------------Before updates from subsystem trees are merged into the mainline 4.xtree, they need to be integration-tested.  For this purpose, a specialtesting repository exists into which virtually all subsystem trees arepulled on an almost daily basis: 	https://git.kernel.org/?p=linux/kernel/git/next/linux-next.git This way, the -next kernel gives a summary outlook onto what will beexpected to go into the mainline kernel at the next merge period.Adventurous testers are very welcome to runtime-test the -next kernel.  Bug Reporting------------- https://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: 	https://bugzilla.kernel.org/page.cgi?id=faq.html The 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.  Managing bug reports-------------------- One of the best ways to put into practice your hacking skills is by fixingbugs reported by other people. Not only you will help to make the kernelmore stable, you'll learn to fix real world problems and you will improveyour skills, and other developers will be aware of your presence. Fixingbugs is one of the best ways to get merits among other developers, becausenot many people like wasting time fixing other people's bugs. To work in the already reported bug reports, go to https://bugzilla.kernel.org.If you want to be advised of the future bug reports, you can subscribe to thebugme-new mailing list (only new bug reports are mailed here) or to thebugme-janitor mailing list (every change in the bugzilla is mailed here) 	https://lists.linux-foundation.org/mailman/listinfo/bugme-new 	https://lists.linux-foundation.org/mailman/listinfo/bugme-janitors   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-kernel There 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.kernel It 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.html Please 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't want to deal withattachments or compressed patches; they may want to comment onindividual lines of your patch, which works only that way. Make sure youuse a mail program that does not mangle spaces and tab characters. Agood first test is to send the mail to yourself and try to apply yourown patch by yourself. If that doesn't work, get your mail program fixedor 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  - silence Remember, 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 changes In 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 corporate

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