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+HOWTO do Linux kernel development
+This is the be-all, end-all document on this topic. It contains
+instructions on how to become a Linux kernel developer and how to learn
+to work with the Linux kernel development community. It tries to not
+contain 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+If anything in this document becomes out of date, please send in patches
+to the maintainer of this file, who is listed at the bottom of the
+So, you want to learn how to become a Linux kernel developer? Or you
+have been told by your manager, "Go write a Linux driver for this
+device." This document's goal is to teach you everything you need to
+know 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 to
+explain some of the reasons why the community works like it does.
+The kernel is written mostly in C, with some architecture-dependent
+parts written in assembly. A good understanding of C is required for
+kernel development. Assembly (any architecture) is not required unless
+you plan to do low-level development for that architecture. Though they
+are not a good substitute for a solid C education and/or years of
+experience, 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 it
+adheres to the ISO C89 standard, it uses a number of extensions that are
+not featured in the standard. The kernel is a freestanding C
+environment, with no reliance on the standard C library, so some
+portions of the C standard are not supported. Arbitrary long long
+divisions and floating point are not allowed. It can sometimes be
+difficult to understand the assumptions the kernel has on the toolchain
+and the extensions that it uses, and unfortunately there is no
+definitive reference for them. Please check the gcc info pages (`info
+gcc`) for some information on them.
+Please remember that you are trying to learn how to work with the
+existing development community. It is a diverse group of people, with
+high standards for coding, style and procedure. These standards have
+been created over time based on what they have found to work best for
+such a large and geographically dispersed team. Try to learn as much as
+possible about these standards ahead of time, as they are well
+documented; do not expect people to adapt to you or your company's way
+of doing things.
+Legal Issues
+The Linux kernel source code is released under the GPL. Please see the
+file, COPYING, in the main directory of the source tree, for details on
+the license. If you have further questions about the license, please
+contact a lawyer, and do not ask on the Linux kernel mailing list. The
+people on the mailing lists are not lawyers, and you should not rely on
+their statements on legal matters.
+For common questions and answers about the GPL, please see:
+ http://www.gnu.org/licenses/gpl-faq.html
+The Linux kernel source tree has a large range of documents that are
+invaluable for learning how to interact with the kernel community. When
+new features are added to the kernel, it is recommended that new
+documentation files are also added which explain how to use the feature.
+When a kernel change causes the interface that the kernel exposes to
+userspace to change, it is recommended that you send the information or
+a patch to the manual pages explaining the change to the manual pages
+maintainer at mtk.manpages@gmail.com, and CC the list
+Here is a list of files that are in the kernel source tree that are
+required reading:
+ 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://userweb.kernel.org/~akpm/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 (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.
+ 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 be
+automatically generated from the source code itself. This includes a
+full description of the in-kernel API, and rules on how to handle
+locking properly. The documents will be created in the
+Documentation/DocBook/ directory and can be generated as PDF,
+Postscript, HTML, and man pages by running:
+ make pdfdocs
+ make psdocs
+ make htmldocs
+ make mandocs
+respectively from the main kernel source directory.
+Becoming A Kernel Developer
+If you do not know anything about Linux kernel development, you should
+look at the Linux KernelNewbies project:
+ http://kernelnewbies.org
+It consists of a helpful mailing list where you can ask almost any type
+of basic kernel development question (make sure to search the archives
+first, before asking something that has already been answered in the
+past.) It also has an IRC channel that you can use to ask questions in
+real-time, and a lot of helpful documentation that is useful for
+learning 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 describes
+some basic logistical information, like how to compile a kernel and
+apply a patch.
+If you do not know where you want to start, but you want to look for
+some task to start doing to join into the kernel development community,
+go to the Linux Kernel Janitor's project:
+ http://kernelnewbies.org/KernelJanitors
+It is a great place to start. It describes a list of relatively simple
+problems that need to be cleaned up and fixed within the Linux kernel
+source tree. Working with the developers in charge of this project, you
+will 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, if
+you do not already have an idea.
+If you already have a chunk of code that you want to put into the kernel
+tree, but need some help getting it in the proper form, the
+kernel-mentors project was created to help you out with this. It is a
+mailing list, and can be found at:
+ http://selenic.com/mailman/listinfo/kernel-mentors
+Before making any actual modifications to the Linux kernel code, it is
+imperative to understand how the code in question works. For this
+purpose, nothing is better than reading through it directly (most tricky
+bits are commented well), perhaps even with the help of specialized
+tools. One such tool that is particularly recommended is the Linux
+Cross-Reference project, which is able to present source code in a
+self-referential, indexed webpage format. An excellent up-to-date
+repository of the kernel code may be found at:
+ http://lxr.linux.no/+trees
+The development process
+Linux kernel development process currently consists of a few different
+main kernel "branches" and lots of different subsystem-specific kernel
+branches. These different branches are:
+ - main 3.x kernel tree
+ - 3.x.y -stable kernel tree
+ - 3.x -git kernel patches
+ - subsystem specific kernel trees and patches
+ - the 3.x -next kernel tree for integration tests
+3.x kernel tree
+3.x kernels are maintained by Linus Torvalds, and can be found on
+kernel.org in the pub/linux/kernel/v3.x/ directory. Its development
+process 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 http://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.
+ - Known regressions in each release are periodically posted to the
+ linux-kernel mailing list. The goal is to reduce the length of
+ that list to zero before declaring the kernel to be "ready," but, in
+ the real world, a small number of regressions often remain at
+ release time.
+It is worth mentioning what Andrew Morton wrote on the linux-kernel
+mailing 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."
+3.x.y -stable kernel tree
+Kernels with 3-part versions are -stable kernels. They contain
+relatively small and critical fixes for security problems or significant
+regressions discovered in a given 3.x kernel.
+This is the recommended branch for users who want the most recent stable
+kernel and are not interested in helping test development/experimental
+If no 3.x.y kernel is available, then the highest numbered 3.x
+kernel is the current stable kernel.
+3.x.y are maintained by the "stable" team <stable@vger.kernel.org>, and
+are released as needs dictate. The normal release period is approximately
+two weeks, but it can be longer if there are no pressing problems. A
+security-related problem, instead, can cause a release to happen almost
+The file Documentation/stable_kernel_rules.txt in the kernel tree
+documents what kinds of changes are acceptable for the -stable tree, and
+how the release process works.
+3.x -git patches
+These are daily snapshots of Linus' kernel tree which are managed in a
+git repository (hence the name.) These patches are usually released
+daily and represent the current state of Linus' tree. They are more
+experimental than -rc kernels since they are generated automatically
+without 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 many
+kernel subsystem developers --- expose their current state of
+development in source repositories. That way, others can see what is
+happening in the different areas of the kernel. In areas where
+development is rapid, a developer may be asked to base his submissions
+onto such a subsystem kernel tree so that conflicts between the
+submission and other already ongoing work are avoided.
+Most of these repositories are git trees, but there are also other SCMs
+in use, or patch queues being published as quilt series. Addresses of
+these subsystem repositories are listed in the MAINTAINERS file. Many
+of them can be browsed at http://git.kernel.org/.
+Before a proposed patch is committed to such a subsystem tree, it is
+subject to review which primarily happens on mailing lists (see the
+respective section below). For several kernel subsystems, this review
+process is tracked with the tool patchwork. Patchwork offers a web
+interface which shows patch postings, any comments on a patch or
+revisions to it, and maintainers can mark patches as under review,
+accepted, or rejected. Most of these patchwork sites are listed at
+3.x -next kernel tree for integration tests
+Before updates from subsystem trees are merged into the mainline 3.x
+tree, they need to be integration-tested. For this purpose, a special
+testing repository exists into which virtually all subsystem trees are
+pulled on an almost daily basis:
+ http://git.kernel.org/?p=linux/kernel/git/next/linux-next.git
+ http://linux.f-seidel.de/linux-next/pmwiki/
+This way, the -next kernel gives a summary outlook onto what will be
+expected to go into the mainline kernel at the next merge period.
+Adventurous testers are very welcome to runtime-test the -next kernel.
+Bug Reporting
+bugzilla.kernel.org is where the Linux kernel developers track kernel
+bugs. Users are encouraged to report all bugs that they find in this
+tool. For details on how to use the kernel bugzilla, please see:
+ http://bugzilla.kernel.org/page.cgi?id=faq.html
+The file REPORTING-BUGS in the main kernel source directory has a good
+template for how to report a possible kernel bug, and details what kind
+of information is needed by the kernel developers to help track down the
+Managing bug reports
+One of the best ways to put into practice your hacking skills is by fixing
+bugs reported by other people. Not only you will help to make the kernel
+more stable, you'll learn to fix real world problems and you will improve
+your skills, and other developers will be aware of your presence. Fixing
+bugs is one of the best ways to get merits among other developers, because
+not many people like wasting time fixing other people's bugs.
+To work in the already reported bug reports, go to http://bugzilla.kernel.org.
+If you want to be advised of the future bug reports, you can subscribe to the
+bugme-new mailing list (only new bug reports are mailed here) or to the
+bugme-janitor mailing list (every change in the bugzilla is mailed here)
+ http://lists.linux-foundation.org/mailman/listinfo/bugme-new
+ http://lists.linux-foundation.org/mailman/listinfo/bugme-janitors
+Mailing lists
+As some of the above documents describe, the majority of the core kernel
+developers participate on the Linux Kernel Mailing list. Details on how
+to 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 different
+places. 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 topic
+you want to bring up, before you post it to the list. A lot of things
+already discussed in detail are only recorded at the mailing list
+Most of the individual kernel subsystems also have their own separate
+mailing list where they do their development efforts. See the
+MAINTAINERS file for a list of what these lists are for the different
+Many of the lists are hosted on kernel.org. Information on them can be
+found 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 for
+interacting with the list (or any list):
+ http://www.albion.com/netiquette/
+If multiple people respond to your mail, the CC: list of recipients may
+get pretty large. Don't remove anybody from the CC: list without a good
+reason, or don't reply only to the list address. Get used to receiving the
+mail twice, one from the sender and the one from the list, and don't try
+to 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, and
+add your statements between the individual quoted sections instead of
+writing at the top of the mail.
+If you add patches to your mail, make sure they are plain readable text
+as stated in Documentation/SubmittingPatches. Kernel developers don't
+want to deal with attachments or compressed patches; they may want
+to 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 tab
+characters. A good first test is to send the mail to yourself and try
+to apply your own patch by yourself. If that doesn't work, get your
+mail 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 kernel
+there is. When you submit a patch for acceptance, it will be reviewed
+on its technical merits and those alone. So, what should you be
+ - criticism
+ - comments
+ - requests for change
+ - requests for justification
+ - silence
+Remember, this is part of getting your patch into the kernel. You have
+to be able to take criticism and comments about your patches, evaluate
+them at a technical level and either rework your patches or provide
+clear 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 try
+again, 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 within
+the 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 work
+toward a solution that is right.
+It is normal that the answers to your first patch might simply be a list
+of a dozen things you should correct. This does _not_ imply that your
+patch will not be accepted, and it is _not_ meant against you
+personally. Simply correct all issues raised against your patch and
+resend it.
+Differences between the kernel community and corporate structures
+The kernel community works differently than most traditional corporate
+development environments. Here are a list of things that you can try to
+do 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 traditional
+software engineering work environments is the faceless nature of
+interaction. One benefit of using email and irc as the primary forms of
+communication is the lack of discrimination based on gender or race.
+The Linux kernel work environment is accepting of women and minorities
+because all you are is an email address. The international aspect also
+helps to level the playing field because you can't guess gender based on
+a 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 an
+opinion have had positive experiences.
+The language barrier can cause problems for some people who are not
+comfortable with English. A good grasp of the language can be needed in
+order to get ideas across properly on mailing lists, so it is
+recommended that you check your emails to make sure they make sense in
+English before sending them.
+Break up your changes
+The Linux kernel community does not gladly accept large chunks of code
+dropped on it all at once. The changes need to be properly introduced,
+discussed, and broken up into tiny, individual portions. This is almost
+the exact opposite of what companies are used to doing. Your proposal
+should also be introduced very early in the development process, so that
+you can receive feedback on what you are doing. It also lets the
+community feel that you are working with them, and not simply using them
+as a dumping ground for your feature. However, don't send 50 emails at
+one time to a mailing list, your patch series should be smaller than
+that 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. The
+ teacher does not want to see the student's trials and errors
+ before they came up with the solution. They want to see the
+ cleanest, most elegant answer. A good student knows this, and
+ would never submit her intermediate work before the final
+ solution."
+ The same is true of kernel development. The maintainers and
+ reviewers do not want to see the thought process behind the
+ solution to the problem one is solving. They want to see a
+ simple and elegant solution."
+It may be challenging to keep the balance between presenting an elegant
+solution and working together with the community and discussing your
+unfinished work. Therefore it is good to get early in the process to
+get feedback to improve your work, but also keep your changes in small
+chunks that they may get already accepted, even when your whole task is
+not ready for inclusion now.
+Also realize that it is not acceptable to send patches for inclusion
+that are unfinished and will be "fixed up later."
+Justify your change
+Along with breaking up your patches, it is very important for you to let
+the Linux community know why they should add this change. New features
+must be justified as being needed and useful.
+Document your change
+When sending in your patches, pay special attention to what you say in
+the text in your email. This information will become the ChangeLog
+information for the patch, and will be preserved for everyone to see for
+all time. It should describe the patch completely, containing:
+ - why the change is necessary
+ - the overall design approach in the patch
+ - implementation details
+ - testing results
+For more details on what this should all look like, please see the
+ChangeLog section of the document:
+ "The Perfect Patch"
+ http://userweb.kernel.org/~akpm/stuff/tpp.txt
+All of these things are sometimes very hard to do. It can take years to
+perfect these practices (if at all). It's a continuous process of
+improvement that requires a lot of patience and determination. But
+don't give up, it's possible. Many have done it before, and each had to
+start exactly where you are now.
+Thanks to Paolo Ciarrocchi who allowed the "Development Process"
+(http://lwn.net/Articles/94386/) section
+to be based on text he had written, and to Randy Dunlap and Gerrit
+Huizenga 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, Andi
+Kleen, Vadim Lobanov, Jesper Juhl, Adrian Bunk, Keri Harris, Frans Pop,
+David A. Wheeler, Junio Hamano, Michael Kerrisk, and Alex Shepard for
+their review, comments, and contributions. Without their help, this
+document would not have been possible.
+Maintainer: Greg Kroah-Hartman <greg@kroah.com>