1HOWTO do Linux kernel development
4This is the be-all, end-all document on this topic. It contains
5instructions on how to become a Linux kernel developer and how to learn
6to work with the Linux kernel development community. It tries to not
7contain anything related to the technical aspects of kernel programming,
8but will help point you in the right direction for that.
10If anything in this document becomes out of date, please send in patches
11to the maintainer of this file, who is listed at the bottom of the
18So, you want to learn how to become a Linux kernel developer? Or you
19have been told by your manager, "Go write a Linux driver for this
20device." This document's goal is to teach you everything you need to
21know to achieve this by describing the process you need to go through,
22and hints on how to work with the community. It will also try to
23explain some of the reasons why the community works like it does.
25The kernel is written mostly in C, with some architecture-dependent
26parts written in assembly. A good understanding of C is required for
27kernel development. Assembly (any architecture) is not required unless
28you plan to do low-level development for that architecture. Though they
29are not a good substitute for a solid C education and/or years of
30experience, the following books are good for, if anything, reference:
31 - "The C Programming Language" by Kernighan and Ritchie [Prentice Hall]
32 - "Practical C Programming" by Steve Oualline [O'Reilly]
33 - "C: A Reference Manual" by Harbison and Steele [Prentice Hall]
35The kernel is written using GNU C and the GNU toolchain. While it
36adheres to the ISO C89 standard, it uses a number of extensions that are
37not featured in the standard. The kernel is a freestanding C
38environment, with no reliance on the standard C library, so some
39portions of the C standard are not supported. Arbitrary long long
40divisions and floating point are not allowed. It can sometimes be
41difficult to understand the assumptions the kernel has on the toolchain
42and the extensions that it uses, and unfortunately there is no
43definitive reference for them. Please check the gcc info pages (`info
44gcc`) for some information on them.
46Please remember that you are trying to learn how to work with the
47existing development community. It is a diverse group of people, with
48high standards for coding, style and procedure. These standards have
49been created over time based on what they have found to work best for
50such a large and geographically dispersed team. Try to learn as much as
51possible about these standards ahead of time, as they are well
52documented; do not expect people to adapt to you or your company's way
53of doing things.
56Legal Issues
59The Linux kernel source code is released under the GPL. Please see the
60file, COPYING, in the main directory of the source tree, for details on
61the license. If you have further questions about the license, please
62contact a lawyer, and do not ask on the Linux kernel mailing list. The
63people on the mailing lists are not lawyers, and you should not rely on
64their statements on legal matters.
66For common questions and answers about the GPL, please see:
73The Linux kernel source tree has a large range of documents that are
74invaluable for learning how to interact with the kernel community. When
75new features are added to the kernel, it is recommended that new
76documentation files are also added which explain how to use the feature.
77When a kernel change causes the interface that the kernel exposes to
78userspace to change, it is recommended that you send the information or
79a patch to the manual pages explaining the change to the manual pages
80maintainer at, and CC the list
83Here is a list of files that are in the kernel source tree that are
84required reading:
86    This file gives a short background on the Linux kernel and describes
87    what is necessary to do to configure and build the kernel. People
88    who are new to the kernel should start here.
90  Documentation/Changes
91    This file gives a list of the minimum levels of various software
92    packages that are necessary to build and run the kernel
93    successfully.
95  Documentation/CodingStyle
96    This describes the Linux kernel coding style, and some of the
97    rationale behind it. All new code is expected to follow the
98    guidelines in this document. Most maintainers will only accept
99    patches if these rules are followed, and many people will only
100    review code if it is in the proper style.
102  Documentation/SubmittingPatches
103  Documentation/SubmittingDrivers
104    These files describe in explicit detail how to successfully create
105    and send a patch, including (but not limited to):
106       - Email contents
107       - Email format
108       - Who to send it to
109    Following these rules will not guarantee success (as all patches are
110    subject to scrutiny for content and style), but not following them
111    will almost always prevent it.
113    Other excellent descriptions of how to create patches properly are:
114    "The Perfect Patch"
116    "Linux kernel patch submission format"
119  Documentation/stable_api_nonsense.txt
120    This file describes the rationale behind the conscious decision to
121    not have a stable API within the kernel, including things like:
122      - Subsystem shim-layers (for compatibility?)
123      - Driver portability between Operating Systems.
124      - Mitigating rapid change within the kernel source tree (or
125    preventing rapid change)
126    This document is crucial for understanding the Linux development
127    philosophy and is very important for people moving to Linux from
128    development on other Operating Systems.
130  Documentation/SecurityBugs
131    If you feel you have found a security problem in the Linux kernel,
132    please follow the steps in this document to help notify the kernel
133    developers, and help solve the issue.
135  Documentation/ManagementStyle
136    This document describes how Linux kernel maintainers operate and the
137    shared ethos behind their methodologies. This is important reading
138    for anyone new to kernel development (or anyone simply curious about
139    it), as it resolves a lot of common misconceptions and confusion
140    about the unique behavior of kernel maintainers.
142  Documentation/stable_kernel_rules.txt
143    This file describes the rules on how the stable kernel releases
144    happen, and what to do if you want to get a change into one of these
145    releases.
147  Documentation/kernel-docs.txt
148    A list of external documentation that pertains to kernel
149    development. Please consult this list if you do not find what you
150    are looking for within the in-kernel documentation.
152  Documentation/applying-patches.txt
153    A good introduction describing exactly what a patch is and how to
154    apply it to the different development branches of the kernel.
156The kernel also has a large number of documents that can be
157automatically generated from the source code itself. This includes a
158full description of the in-kernel API, and rules on how to handle
159locking properly. The documents will be created in the
160Documentation/DocBook/ directory and can be generated as PDF,
161Postscript, HTML, and man pages by running:
162    make pdfdocs
163    make psdocs
164    make htmldocs
165    make mandocs
166respectively from the main kernel source directory.
169Becoming A Kernel Developer
172If you do not know anything about Linux kernel development, you should
173look at the Linux KernelNewbies project:
175It consists of a helpful mailing list where you can ask almost any type
176of basic kernel development question (make sure to search the archives
177first, before asking something that has already been answered in the
178past.) It also has an IRC channel that you can use to ask questions in
179real-time, and a lot of helpful documentation that is useful for
180learning about Linux kernel development.
182The website has basic information about code organization, subsystems,
183and current projects (both in-tree and out-of-tree). It also describes
184some basic logistical information, like how to compile a kernel and
185apply a patch.
187If you do not know where you want to start, but you want to look for
188some task to start doing to join into the kernel development community,
189go to the Linux Kernel Janitor's project:
191It is a great place to start. It describes a list of relatively simple
192problems that need to be cleaned up and fixed within the Linux kernel
193source tree. Working with the developers in charge of this project, you
194will learn the basics of getting your patch into the Linux kernel tree,
195and possibly be pointed in the direction of what to go work on next, if
196you do not already have an idea.
198If you already have a chunk of code that you want to put into the kernel
199tree, but need some help getting it in the proper form, the
200kernel-mentors project was created to help you out with this. It is a
201mailing list, and can be found at:
204Before making any actual modifications to the Linux kernel code, it is
205imperative to understand how the code in question works. For this
206purpose, nothing is better than reading through it directly (most tricky
207bits are commented well), perhaps even with the help of specialized
208tools. One such tool that is particularly recommended is the Linux
209Cross-Reference project, which is able to present source code in a
210self-referential, indexed webpage format. An excellent up-to-date
211repository of the kernel code may be found at:
215The development process
218Linux kernel development process currently consists of a few different
219main kernel "branches" and lots of different subsystem-specific kernel
220branches. These different branches are:
221  - main 2.6.x kernel tree
222  - 2.6.x.y -stable kernel tree
223  - 2.6.x -git kernel patches
224  - subsystem specific kernel trees and patches
225  - the 2.6.x -next kernel tree for integration tests
2272.6.x kernel tree
2292.6.x kernels are maintained by Linus Torvalds, and can be found on in the pub/linux/kernel/v2.6/ directory. Its development
231process is as follows:
232  - As soon as a new kernel is released a two weeks window is open,
233    during this period of time maintainers can submit big diffs to
234    Linus, usually the patches that have already been included in the
235    -next kernel for a few weeks. The preferred way to submit big changes
236    is using git (the kernel's source management tool, more information
237    can be found at but plain patches are also just
238    fine.
239  - After two weeks a -rc1 kernel is released it is now possible to push
240    only patches that do not include new features that could affect the
241    stability of the whole kernel. Please note that a whole new driver
242    (or filesystem) might be accepted after -rc1 because there is no
243    risk of causing regressions with such a change as long as the change
244    is self-contained and does not affect areas outside of the code that
245    is being added. git can be used to send patches to Linus after -rc1
246    is released, but the patches need to also be sent to a public
247    mailing list for review.
248  - A new -rc is released whenever Linus deems the current git tree to
249    be in a reasonably sane state adequate for testing. The goal is to
250    release a new -rc kernel every week.
251  - Process continues until the kernel is considered "ready", the
252    process should last around 6 weeks.
253  - Known regressions in each release are periodically posted to the
254    linux-kernel mailing list. The goal is to reduce the length of
255    that list to zero before declaring the kernel to be "ready," but, in
256    the real world, a small number of regressions often remain at
257    release time.
259It is worth mentioning what Andrew Morton wrote on the linux-kernel
260mailing list about kernel releases:
261    "Nobody knows when a kernel will be released, because it's
262    released according to perceived bug status, not according to a
263    preconceived timeline."
2652.6.x.y -stable kernel tree
267Kernels with 4-part versions are -stable kernels. They contain
268relatively small and critical fixes for security problems or significant
269regressions discovered in a given 2.6.x kernel.
271This is the recommended branch for users who want the most recent stable
272kernel and are not interested in helping test development/experimental
275If no 2.6.x.y kernel is available, then the highest numbered 2.6.x
276kernel is the current stable kernel.
2782.6.x.y are maintained by the "stable" team <>, and are
279released as needs dictate. The normal release period is approximately
280two weeks, but it can be longer if there are no pressing problems. A
281security-related problem, instead, can cause a release to happen almost
284The file Documentation/stable_kernel_rules.txt in the kernel tree
285documents what kinds of changes are acceptable for the -stable tree, and
286how the release process works.
2882.6.x -git patches
290These are daily snapshots of Linus' kernel tree which are managed in a
291git repository (hence the name.) These patches are usually released
292daily and represent the current state of Linus' tree. They are more
293experimental than -rc kernels since they are generated automatically
294without even a cursory glance to see if they are sane.
296Subsystem Specific kernel trees and patches
298The maintainers of the various kernel subsystems --- and also many
299kernel subsystem developers --- expose their current state of
300development in source repositories. That way, others can see what is
301happening in the different areas of the kernel. In areas where
302development is rapid, a developer may be asked to base his submissions
303onto such a subsystem kernel tree so that conflicts between the
304submission and other already ongoing work are avoided.
306Most of these repositories are git trees, but there are also other SCMs
307in use, or patch queues being published as quilt series. Addresses of
308these subsystem repositories are listed in the MAINTAINERS file. Many
309of them can be browsed at
311Before a proposed patch is committed to such a subsystem tree, it is
312subject to review which primarily happens on mailing lists (see the
313respective section below). For several kernel subsystems, this review
314process is tracked with the tool patchwork. Patchwork offers a web
315interface which shows patch postings, any comments on a patch or
316revisions to it, and maintainers can mark patches as under review,
317accepted, or rejected. Most of these patchwork sites are listed at
318 or
3202.6.x -next kernel tree for integration tests
322Before updates from subsystem trees are merged into the mainline 2.6.x
323tree, they need to be integration-tested. For this purpose, a special
324testing repository exists into which virtually all subsystem trees are
325pulled on an almost daily basis:
329This way, the -next kernel gives a summary outlook onto what will be
330expected to go into the mainline kernel at the next merge period.
331Adventurous testers are very welcome to runtime-test the -next kernel.
334Bug Reporting
336 is where the Linux kernel developers track kernel
338bugs. Users are encouraged to report all bugs that they find in this
339tool. For details on how to use the kernel bugzilla, please see:
342The file REPORTING-BUGS in the main kernel source directory has a good
343template for how to report a possible kernel bug, and details what kind
344of information is needed by the kernel developers to help track down the
348Managing bug reports
351One of the best ways to put into practice your hacking skills is by fixing
352bugs reported by other people. Not only you will help to make the kernel
353more stable, you'll learn to fix real world problems and you will improve
354your skills, and other developers will be aware of your presence. Fixing
355bugs is one of the best ways to get merits among other developers, because
356not many people like wasting time fixing other people's bugs.
358To work in the already reported bug reports, go to
359If you want to be advised of the future bug reports, you can subscribe to the
360bugme-new mailing list (only new bug reports are mailed here) or to the
361bugme-janitor mailing list (every change in the bugzilla is mailed here)
368Mailing lists
371As some of the above documents describe, the majority of the core kernel
372developers participate on the Linux Kernel Mailing list. Details on how
373to subscribe and unsubscribe from the list can be found at:
375There are archives of the mailing list on the web in many different
376places. Use a search engine to find these archives. For example:
378It is highly recommended that you search the archives about the topic
379you want to bring up, before you post it to the list. A lot of things
380already discussed in detail are only recorded at the mailing list
383Most of the individual kernel subsystems also have their own separate
384mailing list where they do their development efforts. See the
385MAINTAINERS file for a list of what these lists are for the different
388Many of the lists are hosted on Information on them can be
389found at:
392Please remember to follow good behavioral habits when using the lists.
393Though a bit cheesy, the following URL has some simple guidelines for
394interacting with the list (or any list):
397If multiple people respond to your mail, the CC: list of recipients may
398get pretty large. Don't remove anybody from the CC: list without a good
399reason, or don't reply only to the list address. Get used to receiving the
400mail twice, one from the sender and the one from the list, and don't try
401to tune that by adding fancy mail-headers, people will not like it.
403Remember to keep the context and the attribution of your replies intact,
404keep the "John Kernelhacker wrote ...:" lines at the top of your reply, and
405add your statements between the individual quoted sections instead of
406writing at the top of the mail.
408If you add patches to your mail, make sure they are plain readable text
409as stated in Documentation/SubmittingPatches. Kernel developers don't
410want to deal with attachments or compressed patches; they may want
411to comment on individual lines of your patch, which works only that way.
412Make sure you use a mail program that does not mangle spaces and tab
413characters. A good first test is to send the mail to yourself and try
414to apply your own patch by yourself. If that doesn't work, get your
415mail program fixed or change it until it works.
417Above all, please remember to show respect to other subscribers.
420Working with the community
423The goal of the kernel community is to provide the best possible kernel
424there is. When you submit a patch for acceptance, it will be reviewed
425on its technical merits and those alone. So, what should you be
427  - criticism
428  - comments
429  - requests for change
430  - requests for justification
431  - silence
433Remember, this is part of getting your patch into the kernel. You have
434to be able to take criticism and comments about your patches, evaluate
435them at a technical level and either rework your patches or provide
436clear and concise reasoning as to why those changes should not be made.
437If there are no responses to your posting, wait a few days and try
438again, sometimes things get lost in the huge volume.
440What should you not do?
441  - expect your patch to be accepted without question
442  - become defensive
443  - ignore comments
444  - resubmit the patch without making any of the requested changes
446In a community that is looking for the best technical solution possible,
447there will always be differing opinions on how beneficial a patch is.
448You have to be cooperative, and willing to adapt your idea to fit within
449the kernel. Or at least be willing to prove your idea is worth it.
450Remember, being wrong is acceptable as long as you are willing to work
451toward a solution that is right.
453It is normal that the answers to your first patch might simply be a list
454of a dozen things you should correct. This does _not_ imply that your
455patch will not be accepted, and it is _not_ meant against you
456personally. Simply correct all issues raised against your patch and
457resend it.
460Differences between the kernel community and corporate structures
463The kernel community works differently than most traditional corporate
464development environments. Here are a list of things that you can try to
465do to try to avoid problems:
466  Good things to say regarding your proposed changes:
467    - "This solves multiple problems."
468    - "This deletes 2000 lines of code."
469    - "Here is a patch that explains what I am trying to describe."
470    - "I tested it on 5 different architectures..."
471    - "Here is a series of small patches that..."
472    - "This increases performance on typical machines..."
474  Bad things you should avoid saying:
475    - "We did it this way in AIX/ptx/Solaris, so therefore it must be
476      good..."
477    - "I've being doing this for 20 years, so..."
478    - "This is required for my company to make money"
479    - "This is for our Enterprise product line."
480    - "Here is my 1000 page design document that describes my idea"
481    - "I've been working on this for 6 months..."
482    - "Here's a 5000 line patch that..."
483    - "I rewrote all of the current mess, and here it is..."
484    - "I have a deadline, and this patch needs to be applied now."
486Another way the kernel community is different than most traditional
487software engineering work environments is the faceless nature of
488interaction. One benefit of using email and irc as the primary forms of
489communication is the lack of discrimination based on gender or race.
490The Linux kernel work environment is accepting of women and minorities
491because all you are is an email address. The international aspect also
492helps to level the playing field because you can't guess gender based on
493a person's name. A man may be named Andrea and a woman may be named Pat.
494Most women who have worked in the Linux kernel and have expressed an
495opinion have had positive experiences.
497The language barrier can cause problems for some people who are not
498comfortable with English. A good grasp of the language can be needed in
499order to get ideas across properly on mailing lists, so it is
500recommended that you check your emails to make sure they make sense in
501English before sending them.
504Break up your changes
507The Linux kernel community does not gladly accept large chunks of code
508dropped on it all at once. The changes need to be properly introduced,
509discussed, and broken up into tiny, individual portions. This is almost
510the exact opposite of what companies are used to doing. Your proposal
511should also be introduced very early in the development process, so that
512you can receive feedback on what you are doing. It also lets the
513community feel that you are working with them, and not simply using them
514as a dumping ground for your feature. However, don't send 50 emails at
515one time to a mailing list, your patch series should be smaller than
516that almost all of the time.
518The reasons for breaking things up are the following:
5201) Small patches increase the likelihood that your patches will be
521   applied, since they don't take much time or effort to verify for
522   correctness. A 5 line patch can be applied by a maintainer with
523   barely a second glance. However, a 500 line patch may take hours to
524   review for correctness (the time it takes is exponentially
525   proportional to the size of the patch, or something).
527   Small patches also make it very easy to debug when something goes
528   wrong. It's much easier to back out patches one by one than it is
529   to dissect a very large patch after it's been applied (and broken
530   something).
5322) It's important not only to send small patches, but also to rewrite
533   and simplify (or simply re-order) patches before submitting them.
535Here is an analogy from kernel developer Al Viro:
536    "Think of a teacher grading homework from a math student. The
537    teacher does not want to see the student's trials and errors
538    before they came up with the solution. They want to see the
539    cleanest, most elegant answer. A good student knows this, and
540    would never submit her intermediate work before the final
541    solution."
543    The same is true of kernel development. The maintainers and
544    reviewers do not want to see the thought process behind the
545    solution to the problem one is solving. They want to see a
546    simple and elegant solution."
548It may be challenging to keep the balance between presenting an elegant
549solution and working together with the community and discussing your
550unfinished work. Therefore it is good to get early in the process to
551get feedback to improve your work, but also keep your changes in small
552chunks that they may get already accepted, even when your whole task is
553not ready for inclusion now.
555Also realize that it is not acceptable to send patches for inclusion
556that are unfinished and will be "fixed up later."
559Justify your change
562Along with breaking up your patches, it is very important for you to let
563the Linux community know why they should add this change. New features
564must be justified as being needed and useful.
567Document your change
570When sending in your patches, pay special attention to what you say in
571the text in your email. This information will become the ChangeLog
572information for the patch, and will be preserved for everyone to see for
573all time. It should describe the patch completely, containing:
574  - why the change is necessary
575  - the overall design approach in the patch
576  - implementation details
577  - testing results
579For more details on what this should all look like, please see the
580ChangeLog section of the document:
581  "The Perfect Patch"
587All of these things are sometimes very hard to do. It can take years to
588perfect these practices (if at all). It's a continuous process of
589improvement that requires a lot of patience and determination. But
590don't give up, it's possible. Many have done it before, and each had to
591start exactly where you are now.
597Thanks to Paolo Ciarrocchi who allowed the "Development Process"
598( section
599to be based on text he had written, and to Randy Dunlap and Gerrit
600Huizenga for some of the list of things you should and should not say.
601Also thanks to Pat Mochel, Hanna Linder, Randy Dunlap, Kay Sievers,
602Vojtech Pavlik, Jan Kara, Josh Boyer, Kees Cook, Andrew Morton, Andi
603Kleen, Vadim Lobanov, Jesper Juhl, Adrian Bunk, Keri Harris, Frans Pop,
604David A. Wheeler, Junio Hamano, Michael Kerrisk, and Alex Shepard for
605their review, comments, and contributions. Without their help, this
606document would not have been possible.
610Maintainer: Greg Kroah-Hartman <>

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