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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-If anything in this document becomes out of date, please send in patches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
-document.
-
-
-Introduction
-------------
-
-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
-
-
-Documentation
-------------
-
-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
-linux-api@vger.kernel.org.
-
-Here is a list of files that are in the kernel source tree that are
-required 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://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
-versions.
-
-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
-instantly.
-
-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
-http://patchwork.kernel.org/.
-
-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
-problem.
-
-
-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
-archives.
-
-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
-groups.
-
-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
-expecting?
- - 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 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 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>