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Based on kernel version 4.9. Page generated on 2016-12-21 14:34 EST.

1	HOWTO do Linux kernel development
2	=================================
4	This is the be-all, end-all document on this topic.  It contains
5	instructions on how to become a Linux kernel developer and how to learn
6	to work with the Linux kernel development community.  It tries to not
7	contain anything related to the technical aspects of kernel programming,
8	but will help point you in the right direction for that.
10	If anything in this document becomes out of date, please send in patches
11	to the maintainer of this file, who is listed at the bottom of the
12	document.
15	Introduction
16	------------
18	So, you want to learn how to become a Linux kernel developer?  Or you
19	have been told by your manager, "Go write a Linux driver for this
20	device."  This document's goal is to teach you everything you need to
21	know to achieve this by describing the process you need to go through,
22	and hints on how to work with the community.  It will also try to
23	explain some of the reasons why the community works like it does.
25	The kernel is written mostly in C, with some architecture-dependent
26	parts written in assembly. A good understanding of C is required for
27	kernel development.  Assembly (any architecture) is not required unless
28	you plan to do low-level development for that architecture.  Though they
29	are not a good substitute for a solid C education and/or years of
30	experience, the following books are good for, if anything, reference:
32	 - "The C Programming Language" by Kernighan and Ritchie [Prentice Hall]
33	 - "Practical C Programming" by Steve Oualline [O'Reilly]
34	 - "C:  A Reference Manual" by Harbison and Steele [Prentice Hall]
36	The kernel is written using GNU C and the GNU toolchain.  While it
37	adheres to the ISO C89 standard, it uses a number of extensions that are
38	not featured in the standard.  The kernel is a freestanding C
39	environment, with no reliance on the standard C library, so some
40	portions of the C standard are not supported.  Arbitrary long long
41	divisions and floating point are not allowed.  It can sometimes be
42	difficult to understand the assumptions the kernel has on the toolchain
43	and the extensions that it uses, and unfortunately there is no
44	definitive reference for them.  Please check the gcc info pages (`info
45	gcc`) for some information on them.
47	Please remember that you are trying to learn how to work with the
48	existing development community.  It is a diverse group of people, with
49	high standards for coding, style and procedure.  These standards have
50	been created over time based on what they have found to work best for
51	such a large and geographically dispersed team.  Try to learn as much as
52	possible about these standards ahead of time, as they are well
53	documented; do not expect people to adapt to you or your company's way
54	of doing things.
57	Legal Issues
58	------------
60	The Linux kernel source code is released under the GPL.  Please see the
61	file, COPYING, in the main directory of the source tree, for details on
62	the license.  If you have further questions about the license, please
63	contact a lawyer, and do not ask on the Linux kernel mailing list.  The
64	people on the mailing lists are not lawyers, and you should not rely on
65	their statements on legal matters.
67	For common questions and answers about the GPL, please see:
69		https://www.gnu.org/licenses/gpl-faq.html
72	Documentation
73	-------------
75	The Linux kernel source tree has a large range of documents that are
76	invaluable for learning how to interact with the kernel community.  When
77	new features are added to the kernel, it is recommended that new
78	documentation files are also added which explain how to use the feature.
79	When a kernel change causes the interface that the kernel exposes to
80	userspace to change, it is recommended that you send the information or
81	a patch to the manual pages explaining the change to the manual pages
82	maintainer at mtk.manpages@gmail.com, and CC the list
83	linux-api@vger.kernel.org.
85	Here is a list of files that are in the kernel source tree that are
86	required reading:
89	    This file gives a short background on the Linux kernel and describes
90	    what is necessary to do to configure and build the kernel.  People
91	    who are new to the kernel should start here.
93	  :ref:`Documentation/Changes <changes>`
94	    This file gives a list of the minimum levels of various software
95	    packages that are necessary to build and run the kernel
96	    successfully.
98	  :ref:`Documentation/CodingStyle <codingstyle>`
99	    This describes the Linux kernel coding style, and some of the
100	    rationale behind it. All new code is expected to follow the
101	    guidelines in this document. Most maintainers will only accept
102	    patches if these rules are followed, and many people will only
103	    review code if it is in the proper style.
105	  :ref:`Documentation/SubmittingPatches <submittingpatches>` and :ref:`Documentation/SubmittingDrivers <submittingdrivers>`
106	    These files describe in explicit detail how to successfully create
107	    and send a patch, including (but not limited to):
109	       - Email contents
110	       - Email format
111	       - Who to send it to
113	    Following these rules will not guarantee success (as all patches are
114	    subject to scrutiny for content and style), but not following them
115	    will almost always prevent it.
117	    Other excellent descriptions of how to create patches properly are:
119		"The Perfect Patch"
120			https://www.ozlabs.org/~akpm/stuff/tpp.txt
122		"Linux kernel patch submission format"
123			http://linux.yyz.us/patch-format.html
125	  :ref:`Documentation/stable_api_nonsense.txt <stable_api_nonsense>`
126	    This file describes the rationale behind the conscious decision to
127	    not have a stable API within the kernel, including things like:
129	      - Subsystem shim-layers (for compatibility?)
130	      - Driver portability between Operating Systems.
131	      - Mitigating rapid change within the kernel source tree (or
132		preventing rapid change)
134	    This document is crucial for understanding the Linux development
135	    philosophy and is very important for people moving to Linux from
136	    development on other Operating Systems.
138	  :ref:`Documentation/SecurityBugs <securitybugs>`
139	    If you feel you have found a security problem in the Linux kernel,
140	    please follow the steps in this document to help notify the kernel
141	    developers, and help solve the issue.
143	  :ref:`Documentation/ManagementStyle <managementstyle>`
144	    This document describes how Linux kernel maintainers operate and the
145	    shared ethos behind their methodologies.  This is important reading
146	    for anyone new to kernel development (or anyone simply curious about
147	    it), as it resolves a lot of common misconceptions and confusion
148	    about the unique behavior of kernel maintainers.
150	  :ref:`Documentation/stable_kernel_rules.txt <stable_kernel_rules>`
151	    This file describes the rules on how the stable kernel releases
152	    happen, and what to do if you want to get a change into one of these
153	    releases.
155	  :ref:`Documentation/kernel-docs.txt <kernel_docs>`
156	    A list of external documentation that pertains to kernel
157	    development.  Please consult this list if you do not find what you
158	    are looking for within the in-kernel documentation.
160	  :ref:`Documentation/applying-patches.txt <applying_patches>`
161	    A good introduction describing exactly what a patch is and how to
162	    apply it to the different development branches of the kernel.
164	The kernel also has a large number of documents that can be
165	automatically generated from the source code itself or from
166	ReStructuredText markups (ReST), like this one. This includes a
167	full description of the in-kernel API, and rules on how to handle
168	locking properly.
170	All such documents can be generated as PDF or HTML by running::
172		make pdfdocs
173		make htmldocs
175	respectively from the main kernel source directory.
177	The documents that uses ReST markup will be generated at Documentation/output.
178	They can also be generated on LaTeX and ePub formats with::
180		make latexdocs
181		make epubdocs
183	Currently, there are some documents written on DocBook that are in
184	the process of conversion to ReST. Such documents will be created in the
185	Documentation/DocBook/ directory and can be generated also as
186	Postscript or man pages by running::
188		make psdocs
189		make mandocs
191	Becoming A Kernel Developer
192	---------------------------
194	If you do not know anything about Linux kernel development, you should
195	look at the Linux KernelNewbies project:
197		https://kernelnewbies.org
199	It consists of a helpful mailing list where you can ask almost any type
200	of basic kernel development question (make sure to search the archives
201	first, before asking something that has already been answered in the
202	past.)  It also has an IRC channel that you can use to ask questions in
203	real-time, and a lot of helpful documentation that is useful for
204	learning about Linux kernel development.
206	The website has basic information about code organization, subsystems,
207	and current projects (both in-tree and out-of-tree). It also describes
208	some basic logistical information, like how to compile a kernel and
209	apply a patch.
211	If you do not know where you want to start, but you want to look for
212	some task to start doing to join into the kernel development community,
213	go to the Linux Kernel Janitor's project:
215		https://kernelnewbies.org/KernelJanitors
217	It is a great place to start.  It describes a list of relatively simple
218	problems that need to be cleaned up and fixed within the Linux kernel
219	source tree.  Working with the developers in charge of this project, you
220	will learn the basics of getting your patch into the Linux kernel tree,
221	and possibly be pointed in the direction of what to go work on next, if
222	you do not already have an idea.
224	If you already have a chunk of code that you want to put into the kernel
225	tree, but need some help getting it in the proper form, the
226	kernel-mentors project was created to help you out with this.  It is a
227	mailing list, and can be found at:
229		https://selenic.com/mailman/listinfo/kernel-mentors
231	Before making any actual modifications to the Linux kernel code, it is
232	imperative to understand how the code in question works.  For this
233	purpose, nothing is better than reading through it directly (most tricky
234	bits are commented well), perhaps even with the help of specialized
235	tools.  One such tool that is particularly recommended is the Linux
236	Cross-Reference project, which is able to present source code in a
237	self-referential, indexed webpage format. An excellent up-to-date
238	repository of the kernel code may be found at:
240		http://lxr.free-electrons.com/
243	The development process
244	-----------------------
246	Linux kernel development process currently consists of a few different
247	main kernel "branches" and lots of different subsystem-specific kernel
248	branches.  These different branches are:
250	  - main 4.x kernel tree
251	  - 4.x.y -stable kernel tree
252	  - 4.x -git kernel patches
253	  - subsystem specific kernel trees and patches
254	  - the 4.x -next kernel tree for integration tests
256	4.x kernel tree
257	-----------------
258	4.x kernels are maintained by Linus Torvalds, and can be found on
259	https://kernel.org in the pub/linux/kernel/v4.x/ directory.  Its development
260	process is as follows:
262	  - As soon as a new kernel is released a two weeks window is open,
263	    during this period of time maintainers can submit big diffs to
264	    Linus, usually the patches that have already been included in the
265	    -next kernel for a few weeks.  The preferred way to submit big changes
266	    is using git (the kernel's source management tool, more information
267	    can be found at https://git-scm.com/) but plain patches are also just
268	    fine.
269	  - After two weeks a -rc1 kernel is released it is now possible to push
270	    only patches that do not include new features that could affect the
271	    stability of the whole kernel.  Please note that a whole new driver
272	    (or filesystem) might be accepted after -rc1 because there is no
273	    risk of causing regressions with such a change as long as the change
274	    is self-contained and does not affect areas outside of the code that
275	    is being added.  git can be used to send patches to Linus after -rc1
276	    is released, but the patches need to also be sent to a public
277	    mailing list for review.
278	  - A new -rc is released whenever Linus deems the current git tree to
279	    be in a reasonably sane state adequate for testing.  The goal is to
280	    release a new -rc kernel every week.
281	  - Process continues until the kernel is considered "ready", the
282	    process should last around 6 weeks.
284	It is worth mentioning what Andrew Morton wrote on the linux-kernel
285	mailing list about kernel releases:
287		*"Nobody knows when a kernel will be released, because it's
288		released according to perceived bug status, not according to a
289		preconceived timeline."*
291	4.x.y -stable kernel tree
292	-------------------------
293	Kernels with 3-part versions are -stable kernels. They contain
294	relatively small and critical fixes for security problems or significant
295	regressions discovered in a given 4.x kernel.
297	This is the recommended branch for users who want the most recent stable
298	kernel and are not interested in helping test development/experimental
299	versions.
301	If no 4.x.y kernel is available, then the highest numbered 4.x
302	kernel is the current stable kernel.
304	4.x.y are maintained by the "stable" team <stable@vger.kernel.org>, and
305	are released as needs dictate.  The normal release period is approximately
306	two weeks, but it can be longer if there are no pressing problems.  A
307	security-related problem, instead, can cause a release to happen almost
308	instantly.
310	The file Documentation/stable_kernel_rules.txt in the kernel tree
311	documents what kinds of changes are acceptable for the -stable tree, and
312	how the release process works.
314	4.x -git patches
315	----------------
316	These are daily snapshots of Linus' kernel tree which are managed in a
317	git repository (hence the name.) These patches are usually released
318	daily and represent the current state of Linus' tree.  They are more
319	experimental than -rc kernels since they are generated automatically
320	without even a cursory glance to see if they are sane.
322	Subsystem Specific kernel trees and patches
323	-------------------------------------------
324	The maintainers of the various kernel subsystems --- and also many
325	kernel subsystem developers --- expose their current state of
326	development in source repositories.  That way, others can see what is
327	happening in the different areas of the kernel.  In areas where
328	development is rapid, a developer may be asked to base his submissions
329	onto such a subsystem kernel tree so that conflicts between the
330	submission and other already ongoing work are avoided.
332	Most of these repositories are git trees, but there are also other SCMs
333	in use, or patch queues being published as quilt series.  Addresses of
334	these subsystem repositories are listed in the MAINTAINERS file.  Many
335	of them can be browsed at https://git.kernel.org/.
337	Before a proposed patch is committed to such a subsystem tree, it is
338	subject to review which primarily happens on mailing lists (see the
339	respective section below).  For several kernel subsystems, this review
340	process is tracked with the tool patchwork.  Patchwork offers a web
341	interface which shows patch postings, any comments on a patch or
342	revisions to it, and maintainers can mark patches as under review,
343	accepted, or rejected.  Most of these patchwork sites are listed at
344	https://patchwork.kernel.org/.
346	4.x -next kernel tree for integration tests
347	-------------------------------------------
348	Before updates from subsystem trees are merged into the mainline 4.x
349	tree, they need to be integration-tested.  For this purpose, a special
350	testing repository exists into which virtually all subsystem trees are
351	pulled on an almost daily basis:
353		https://git.kernel.org/?p=linux/kernel/git/next/linux-next.git
355	This way, the -next kernel gives a summary outlook onto what will be
356	expected to go into the mainline kernel at the next merge period.
357	Adventurous testers are very welcome to runtime-test the -next kernel.
360	Bug Reporting
361	-------------
363	https://bugzilla.kernel.org is where the Linux kernel developers track kernel
364	bugs.  Users are encouraged to report all bugs that they find in this
365	tool.  For details on how to use the kernel bugzilla, please see:
367		https://bugzilla.kernel.org/page.cgi?id=faq.html
369	The file REPORTING-BUGS in the main kernel source directory has a good
370	template for how to report a possible kernel bug, and details what kind
371	of information is needed by the kernel developers to help track down the
372	problem.
375	Managing bug reports
376	--------------------
378	One of the best ways to put into practice your hacking skills is by fixing
379	bugs reported by other people. Not only you will help to make the kernel
380	more stable, you'll learn to fix real world problems and you will improve
381	your skills, and other developers will be aware of your presence. Fixing
382	bugs is one of the best ways to get merits among other developers, because
383	not many people like wasting time fixing other people's bugs.
385	To work in the already reported bug reports, go to https://bugzilla.kernel.org.
386	If you want to be advised of the future bug reports, you can subscribe to the
387	bugme-new mailing list (only new bug reports are mailed here) or to the
388	bugme-janitor mailing list (every change in the bugzilla is mailed here)
390		https://lists.linux-foundation.org/mailman/listinfo/bugme-new
392		https://lists.linux-foundation.org/mailman/listinfo/bugme-janitors
396	Mailing lists
397	-------------
399	As some of the above documents describe, the majority of the core kernel
400	developers participate on the Linux Kernel Mailing list.  Details on how
401	to subscribe and unsubscribe from the list can be found at:
403		http://vger.kernel.org/vger-lists.html#linux-kernel
405	There are archives of the mailing list on the web in many different
406	places.  Use a search engine to find these archives.  For example:
408		http://dir.gmane.org/gmane.linux.kernel
410	It is highly recommended that you search the archives about the topic
411	you want to bring up, before you post it to the list. A lot of things
412	already discussed in detail are only recorded at the mailing list
413	archives.
415	Most of the individual kernel subsystems also have their own separate
416	mailing list where they do their development efforts.  See the
417	MAINTAINERS file for a list of what these lists are for the different
418	groups.
420	Many of the lists are hosted on kernel.org. Information on them can be
421	found at:
423		http://vger.kernel.org/vger-lists.html
425	Please remember to follow good behavioral habits when using the lists.
426	Though a bit cheesy, the following URL has some simple guidelines for
427	interacting with the list (or any list):
429		http://www.albion.com/netiquette/
431	If multiple people respond to your mail, the CC: list of recipients may
432	get pretty large. Don't remove anybody from the CC: list without a good
433	reason, or don't reply only to the list address. Get used to receiving the
434	mail twice, one from the sender and the one from the list, and don't try
435	to tune that by adding fancy mail-headers, people will not like it.
437	Remember to keep the context and the attribution of your replies intact,
438	keep the "John Kernelhacker wrote ...:" lines at the top of your reply, and
439	add your statements between the individual quoted sections instead of
440	writing at the top of the mail.
442	If you add patches to your mail, make sure they are plain readable text
443	as stated in Documentation/SubmittingPatches.
444	Kernel developers don't want to deal with
445	attachments or compressed patches; they may want to comment on
446	individual lines of your patch, which works only that way. Make sure you
447	use a mail program that does not mangle spaces and tab characters. A
448	good first test is to send the mail to yourself and try to apply your
449	own patch by yourself. If that doesn't work, get your mail program fixed
450	or change it until it works.
452	Above all, please remember to show respect to other subscribers.
455	Working with the community
456	--------------------------
458	The goal of the kernel community is to provide the best possible kernel
459	there is.  When you submit a patch for acceptance, it will be reviewed
460	on its technical merits and those alone.  So, what should you be
461	expecting?
463	  - criticism
464	  - comments
465	  - requests for change
466	  - requests for justification
467	  - silence
469	Remember, this is part of getting your patch into the kernel.  You have
470	to be able to take criticism and comments about your patches, evaluate
471	them at a technical level and either rework your patches or provide
472	clear and concise reasoning as to why those changes should not be made.
473	If there are no responses to your posting, wait a few days and try
474	again, sometimes things get lost in the huge volume.
476	What should you not do?
478	  - expect your patch to be accepted without question
479	  - become defensive
480	  - ignore comments
481	  - resubmit the patch without making any of the requested changes
483	In a community that is looking for the best technical solution possible,
484	there will always be differing opinions on how beneficial a patch is.
485	You have to be cooperative, and willing to adapt your idea to fit within
486	the kernel.  Or at least be willing to prove your idea is worth it.
487	Remember, being wrong is acceptable as long as you are willing to work
488	toward a solution that is right.
490	It is normal that the answers to your first patch might simply be a list
491	of a dozen things you should correct.  This does **not** imply that your
492	patch will not be accepted, and it is **not** meant against you
493	personally.  Simply correct all issues raised against your patch and
494	resend it.
497	Differences between the kernel community and corporate structures
498	-----------------------------------------------------------------
500	The kernel community works differently than most traditional corporate
501	development environments.  Here are a list of things that you can try to
502	do to avoid problems:
504	  Good things to say regarding your proposed changes:
506	    - "This solves multiple problems."
507	    - "This deletes 2000 lines of code."
508	    - "Here is a patch that explains what I am trying to describe."
509	    - "I tested it on 5 different architectures..."
510	    - "Here is a series of small patches that..."
511	    - "This increases performance on typical machines..."
513	  Bad things you should avoid saying:
515	    - "We did it this way in AIX/ptx/Solaris, so therefore it must be
516	      good..."
517	    - "I've being doing this for 20 years, so..."
518	    - "This is required for my company to make money"
519	    - "This is for our Enterprise product line."
520	    - "Here is my 1000 page design document that describes my idea"
521	    - "I've been working on this for 6 months..."
522	    - "Here's a 5000 line patch that..."
523	    - "I rewrote all of the current mess, and here it is..."
524	    - "I have a deadline, and this patch needs to be applied now."
526	Another way the kernel community is different than most traditional
527	software engineering work environments is the faceless nature of
528	interaction.  One benefit of using email and irc as the primary forms of
529	communication is the lack of discrimination based on gender or race.
530	The Linux kernel work environment is accepting of women and minorities
531	because all you are is an email address.  The international aspect also
532	helps to level the playing field because you can't guess gender based on
533	a person's name. A man may be named Andrea and a woman may be named Pat.
534	Most women who have worked in the Linux kernel and have expressed an
535	opinion have had positive experiences.
537	The language barrier can cause problems for some people who are not
538	comfortable with English.  A good grasp of the language can be needed in
539	order to get ideas across properly on mailing lists, so it is
540	recommended that you check your emails to make sure they make sense in
541	English before sending them.
544	Break up your changes
545	---------------------
547	The Linux kernel community does not gladly accept large chunks of code
548	dropped on it all at once.  The changes need to be properly introduced,
549	discussed, and broken up into tiny, individual portions.  This is almost
550	the exact opposite of what companies are used to doing.  Your proposal
551	should also be introduced very early in the development process, so that
552	you can receive feedback on what you are doing.  It also lets the
553	community feel that you are working with them, and not simply using them
554	as a dumping ground for your feature.  However, don't send 50 emails at
555	one time to a mailing list, your patch series should be smaller than
556	that almost all of the time.
558	The reasons for breaking things up are the following:
560	1) Small patches increase the likelihood that your patches will be
561	   applied, since they don't take much time or effort to verify for
562	   correctness.  A 5 line patch can be applied by a maintainer with
563	   barely a second glance. However, a 500 line patch may take hours to
564	   review for correctness (the time it takes is exponentially
565	   proportional to the size of the patch, or something).
567	   Small patches also make it very easy to debug when something goes
568	   wrong.  It's much easier to back out patches one by one than it is
569	   to dissect a very large patch after it's been applied (and broken
570	   something).
572	2) It's important not only to send small patches, but also to rewrite
573	   and simplify (or simply re-order) patches before submitting them.
575	Here is an analogy from kernel developer Al Viro:
577		*"Think of a teacher grading homework from a math student.  The
578		teacher does not want to see the student's trials and errors
579		before they came up with the solution. They want to see the
580		cleanest, most elegant answer.  A good student knows this, and
581		would never submit her intermediate work before the final
582		solution.*
584		*The same is true of kernel development. The maintainers and
585		reviewers do not want to see the thought process behind the
586		solution to the problem one is solving. They want to see a
587		simple and elegant solution."*
589	It may be challenging to keep the balance between presenting an elegant
590	solution and working together with the community and discussing your
591	unfinished work. Therefore it is good to get early in the process to
592	get feedback to improve your work, but also keep your changes in small
593	chunks that they may get already accepted, even when your whole task is
594	not ready for inclusion now.
596	Also realize that it is not acceptable to send patches for inclusion
597	that are unfinished and will be "fixed up later."
600	Justify your change
601	-------------------
603	Along with breaking up your patches, it is very important for you to let
604	the Linux community know why they should add this change.  New features
605	must be justified as being needed and useful.
608	Document your change
609	--------------------
611	When sending in your patches, pay special attention to what you say in
612	the text in your email.  This information will become the ChangeLog
613	information for the patch, and will be preserved for everyone to see for
614	all time.  It should describe the patch completely, containing:
616	  - why the change is necessary
617	  - the overall design approach in the patch
618	  - implementation details
619	  - testing results
621	For more details on what this should all look like, please see the
622	ChangeLog section of the document:
624	  "The Perfect Patch"
625	      http://www.ozlabs.org/~akpm/stuff/tpp.txt
628	All of these things are sometimes very hard to do. It can take years to
629	perfect these practices (if at all). It's a continuous process of
630	improvement that requires a lot of patience and determination. But
631	don't give up, it's possible. Many have done it before, and each had to
632	start exactly where you are now.
637	----------
639	Thanks to Paolo Ciarrocchi who allowed the "Development Process"
640	(https://lwn.net/Articles/94386/) section
641	to be based on text he had written, and to Randy Dunlap and Gerrit
642	Huizenga for some of the list of things you should and should not say.
643	Also thanks to Pat Mochel, Hanna Linder, Randy Dunlap, Kay Sievers,
644	Vojtech Pavlik, Jan Kara, Josh Boyer, Kees Cook, Andrew Morton, Andi
645	Kleen, Vadim Lobanov, Jesper Juhl, Adrian Bunk, Keri Harris, Frans Pop,
646	David A. Wheeler, Junio Hamano, Michael Kerrisk, and Alex Shepard for
647	their review, comments, and contributions.  Without their help, this
648	document would not have been possible.
652	Maintainer: Greg Kroah-Hartman <greg@kroah.com>
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