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Based on kernel version 4.8. Page generated on 2016-10-06 23:16 EST.

1	HOWTO do Linux kernel development
2	---------------------------------
3	
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.
9	
10	If anything in this document becomes out of date, please send in patches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.
13	
14	
15	Introduction
16	------------
17	
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.
24	
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:
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]
34	
35	The kernel is written using GNU C and the GNU toolchain.  While it
36	adheres to the ISO C89 standard, it uses a number of extensions that are
37	not featured in the standard.  The kernel is a freestanding C
38	environment, with no reliance on the standard C library, so some
39	portions of the C standard are not supported.  Arbitrary long long
40	divisions and floating point are not allowed.  It can sometimes be
41	difficult to understand the assumptions the kernel has on the toolchain
42	and the extensions that it uses, and unfortunately there is no
43	definitive reference for them.  Please check the gcc info pages (`info
44	gcc`) for some information on them.
45	
46	Please remember that you are trying to learn how to work with the
47	existing development community.  It is a diverse group of people, with
48	high standards for coding, style and procedure.  These standards have
49	been created over time based on what they have found to work best for
50	such a large and geographically dispersed team.  Try to learn as much as
51	possible about these standards ahead of time, as they are well
52	documented; do not expect people to adapt to you or your company's way
53	of doing things.
54	
55	
56	Legal Issues
57	------------
58	
59	The Linux kernel source code is released under the GPL.  Please see the
60	file, COPYING, in the main directory of the source tree, for details on
61	the license.  If you have further questions about the license, please
62	contact a lawyer, and do not ask on the Linux kernel mailing list.  The
63	people on the mailing lists are not lawyers, and you should not rely on
64	their statements on legal matters.
65	
66	For common questions and answers about the GPL, please see:
67		http://www.gnu.org/licenses/gpl-faq.html
68	
69	
70	Documentation
71	-------------
72	
73	The Linux kernel source tree has a large range of documents that are
74	invaluable for learning how to interact with the kernel community.  When
75	new features are added to the kernel, it is recommended that new
76	documentation files are also added which explain how to use the feature.
77	When a kernel change causes the interface that the kernel exposes to
78	userspace to change, it is recommended that you send the information or
79	a patch to the manual pages explaining the change to the manual pages
80	maintainer at mtk.manpages@gmail.com, and CC the list
81	linux-api@vger.kernel.org.
82	
83	Here is a list of files that are in the kernel source tree that are
84	required reading:
85	  README
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.
89	
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.
94	
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.
101	
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.
112	
113	    Other excellent descriptions of how to create patches properly are:
114		"The Perfect Patch"
115			http://www.ozlabs.org/~akpm/stuff/tpp.txt
116		"Linux kernel patch submission format"
117			http://linux.yyz.us/patch-format.html
118	
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.
129	
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.
134	
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.
141	
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.
146	
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.
151	
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.
155	
156	The kernel also has a large number of documents that can be
157	automatically generated from the source code itself.  This includes a
158	full description of the in-kernel API, and rules on how to handle
159	locking properly.  The documents will be created in the
160	Documentation/DocBook/ directory and can be generated as PDF,
161	Postscript, HTML, and man pages by running:
162		make pdfdocs
163		make psdocs
164		make htmldocs
165		make mandocs
166	respectively from the main kernel source directory.
167	
168	
169	Becoming A Kernel Developer
170	---------------------------
171	
172	If you do not know anything about Linux kernel development, you should
173	look at the Linux KernelNewbies project:
174		http://kernelnewbies.org
175	It consists of a helpful mailing list where you can ask almost any type
176	of basic kernel development question (make sure to search the archives
177	first, before asking something that has already been answered in the
178	past.)  It also has an IRC channel that you can use to ask questions in
179	real-time, and a lot of helpful documentation that is useful for
180	learning about Linux kernel development.
181	
182	The website has basic information about code organization, subsystems,
183	and current projects (both in-tree and out-of-tree). It also describes
184	some basic logistical information, like how to compile a kernel and
185	apply a patch.
186	
187	If you do not know where you want to start, but you want to look for
188	some task to start doing to join into the kernel development community,
189	go to the Linux Kernel Janitor's project:
190		http://kernelnewbies.org/KernelJanitors
191	It is a great place to start.  It describes a list of relatively simple
192	problems that need to be cleaned up and fixed within the Linux kernel
193	source tree.  Working with the developers in charge of this project, you
194	will learn the basics of getting your patch into the Linux kernel tree,
195	and possibly be pointed in the direction of what to go work on next, if
196	you do not already have an idea.
197	
198	If you already have a chunk of code that you want to put into the kernel
199	tree, but need some help getting it in the proper form, the
200	kernel-mentors project was created to help you out with this.  It is a
201	mailing list, and can be found at:
202		http://selenic.com/mailman/listinfo/kernel-mentors
203	
204	Before making any actual modifications to the Linux kernel code, it is
205	imperative to understand how the code in question works.  For this
206	purpose, nothing is better than reading through it directly (most tricky
207	bits are commented well), perhaps even with the help of specialized
208	tools.  One such tool that is particularly recommended is the Linux
209	Cross-Reference project, which is able to present source code in a
210	self-referential, indexed webpage format. An excellent up-to-date
211	repository of the kernel code may be found at:
212		http://lxr.free-electrons.com/
213	
214	
215	The development process
216	-----------------------
217	
218	Linux kernel development process currently consists of a few different
219	main kernel "branches" and lots of different subsystem-specific kernel
220	branches.  These different branches are:
221	  - main 4.x kernel tree
222	  - 4.x.y -stable kernel tree
223	  - 4.x -git kernel patches
224	  - subsystem specific kernel trees and patches
225	  - the 4.x -next kernel tree for integration tests
226	
227	4.x kernel tree
228	-----------------
229	4.x kernels are maintained by Linus Torvalds, and can be found on
230	kernel.org in the pub/linux/kernel/v4.x/ directory.  Its development
231	process 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 http://git-scm.com/) 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	
254	It is worth mentioning what Andrew Morton wrote on the linux-kernel
255	mailing list about kernel releases:
256		"Nobody knows when a kernel will be released, because it's
257		released according to perceived bug status, not according to a
258		preconceived timeline."
259	
260	4.x.y -stable kernel tree
261	-------------------------
262	Kernels with 3-part versions are -stable kernels. They contain
263	relatively small and critical fixes for security problems or significant
264	regressions discovered in a given 4.x kernel.
265	
266	This is the recommended branch for users who want the most recent stable
267	kernel and are not interested in helping test development/experimental
268	versions.
269	
270	If no 4.x.y kernel is available, then the highest numbered 4.x
271	kernel is the current stable kernel.
272	
273	4.x.y are maintained by the "stable" team <stable@vger.kernel.org>, and
274	are released as needs dictate.  The normal release period is approximately
275	two weeks, but it can be longer if there are no pressing problems.  A
276	security-related problem, instead, can cause a release to happen almost
277	instantly.
278	
279	The file Documentation/stable_kernel_rules.txt in the kernel tree
280	documents what kinds of changes are acceptable for the -stable tree, and
281	how the release process works.
282	
283	4.x -git patches
284	----------------
285	These are daily snapshots of Linus' kernel tree which are managed in a
286	git repository (hence the name.) These patches are usually released
287	daily and represent the current state of Linus' tree.  They are more
288	experimental than -rc kernels since they are generated automatically
289	without even a cursory glance to see if they are sane.
290	
291	Subsystem Specific kernel trees and patches
292	-------------------------------------------
293	The maintainers of the various kernel subsystems --- and also many
294	kernel subsystem developers --- expose their current state of
295	development in source repositories.  That way, others can see what is
296	happening in the different areas of the kernel.  In areas where
297	development is rapid, a developer may be asked to base his submissions
298	onto such a subsystem kernel tree so that conflicts between the
299	submission and other already ongoing work are avoided.
300	
301	Most of these repositories are git trees, but there are also other SCMs
302	in use, or patch queues being published as quilt series.  Addresses of
303	these subsystem repositories are listed in the MAINTAINERS file.  Many
304	of them can be browsed at http://git.kernel.org/.
305	
306	Before a proposed patch is committed to such a subsystem tree, it is
307	subject to review which primarily happens on mailing lists (see the
308	respective section below).  For several kernel subsystems, this review
309	process is tracked with the tool patchwork.  Patchwork offers a web
310	interface which shows patch postings, any comments on a patch or
311	revisions to it, and maintainers can mark patches as under review,
312	accepted, or rejected.  Most of these patchwork sites are listed at
313	http://patchwork.kernel.org/.
314	
315	4.x -next kernel tree for integration tests
316	-------------------------------------------
317	Before updates from subsystem trees are merged into the mainline 4.x
318	tree, they need to be integration-tested.  For this purpose, a special
319	testing repository exists into which virtually all subsystem trees are
320	pulled on an almost daily basis:
321		http://git.kernel.org/?p=linux/kernel/git/next/linux-next.git
322	
323	This way, the -next kernel gives a summary outlook onto what will be
324	expected to go into the mainline kernel at the next merge period.
325	Adventurous testers are very welcome to runtime-test the -next kernel.
326	
327	
328	Bug Reporting
329	-------------
330	
331	bugzilla.kernel.org is where the Linux kernel developers track kernel
332	bugs.  Users are encouraged to report all bugs that they find in this
333	tool.  For details on how to use the kernel bugzilla, please see:
334		http://bugzilla.kernel.org/page.cgi?id=faq.html
335	
336	The file REPORTING-BUGS in the main kernel source directory has a good
337	template for how to report a possible kernel bug, and details what kind
338	of information is needed by the kernel developers to help track down the
339	problem.
340	
341	
342	Managing bug reports
343	--------------------
344	
345	One of the best ways to put into practice your hacking skills is by fixing
346	bugs reported by other people. Not only you will help to make the kernel
347	more stable, you'll learn to fix real world problems and you will improve
348	your skills, and other developers will be aware of your presence. Fixing
349	bugs is one of the best ways to get merits among other developers, because
350	not many people like wasting time fixing other people's bugs.
351	
352	To work in the already reported bug reports, go to http://bugzilla.kernel.org.
353	If you want to be advised of the future bug reports, you can subscribe to the
354	bugme-new mailing list (only new bug reports are mailed here) or to the
355	bugme-janitor mailing list (every change in the bugzilla is mailed here)
356	
357		http://lists.linux-foundation.org/mailman/listinfo/bugme-new
358		http://lists.linux-foundation.org/mailman/listinfo/bugme-janitors
359	
360	
361	
362	Mailing lists
363	-------------
364	
365	As some of the above documents describe, the majority of the core kernel
366	developers participate on the Linux Kernel Mailing list.  Details on how
367	to subscribe and unsubscribe from the list can be found at:
368		http://vger.kernel.org/vger-lists.html#linux-kernel
369	There are archives of the mailing list on the web in many different
370	places.  Use a search engine to find these archives.  For example:
371		http://dir.gmane.org/gmane.linux.kernel
372	It is highly recommended that you search the archives about the topic
373	you want to bring up, before you post it to the list. A lot of things
374	already discussed in detail are only recorded at the mailing list
375	archives.
376	
377	Most of the individual kernel subsystems also have their own separate
378	mailing list where they do their development efforts.  See the
379	MAINTAINERS file for a list of what these lists are for the different
380	groups.
381	
382	Many of the lists are hosted on kernel.org. Information on them can be
383	found at:
384		http://vger.kernel.org/vger-lists.html
385	
386	Please remember to follow good behavioral habits when using the lists.
387	Though a bit cheesy, the following URL has some simple guidelines for
388	interacting with the list (or any list):
389		http://www.albion.com/netiquette/
390	
391	If multiple people respond to your mail, the CC: list of recipients may
392	get pretty large. Don't remove anybody from the CC: list without a good
393	reason, or don't reply only to the list address. Get used to receiving the
394	mail twice, one from the sender and the one from the list, and don't try
395	to tune that by adding fancy mail-headers, people will not like it.
396	
397	Remember to keep the context and the attribution of your replies intact,
398	keep the "John Kernelhacker wrote ...:" lines at the top of your reply, and
399	add your statements between the individual quoted sections instead of
400	writing at the top of the mail.
401	
402	If you add patches to your mail, make sure they are plain readable text
403	as stated in Documentation/SubmittingPatches. Kernel developers don't
404	want to deal with attachments or compressed patches; they may want
405	to comment on individual lines of your patch, which works only that way.
406	Make sure you use a mail program that does not mangle spaces and tab
407	characters. A good first test is to send the mail to yourself and try
408	to apply your own patch by yourself. If that doesn't work, get your
409	mail program fixed or change it until it works.
410	
411	Above all, please remember to show respect to other subscribers.
412	
413	
414	Working with the community
415	--------------------------
416	
417	The goal of the kernel community is to provide the best possible kernel
418	there is.  When you submit a patch for acceptance, it will be reviewed
419	on its technical merits and those alone.  So, what should you be
420	expecting?
421	  - criticism
422	  - comments
423	  - requests for change
424	  - requests for justification
425	  - silence
426	
427	Remember, this is part of getting your patch into the kernel.  You have
428	to be able to take criticism and comments about your patches, evaluate
429	them at a technical level and either rework your patches or provide
430	clear and concise reasoning as to why those changes should not be made.
431	If there are no responses to your posting, wait a few days and try
432	again, sometimes things get lost in the huge volume.
433	
434	What should you not do?
435	  - expect your patch to be accepted without question
436	  - become defensive
437	  - ignore comments
438	  - resubmit the patch without making any of the requested changes
439	
440	In a community that is looking for the best technical solution possible,
441	there will always be differing opinions on how beneficial a patch is.
442	You have to be cooperative, and willing to adapt your idea to fit within
443	the kernel.  Or at least be willing to prove your idea is worth it.
444	Remember, being wrong is acceptable as long as you are willing to work
445	toward a solution that is right.
446	
447	It is normal that the answers to your first patch might simply be a list
448	of a dozen things you should correct.  This does _not_ imply that your
449	patch will not be accepted, and it is _not_ meant against you
450	personally.  Simply correct all issues raised against your patch and
451	resend it.
452	
453	
454	Differences between the kernel community and corporate structures
455	-----------------------------------------------------------------
456	
457	The kernel community works differently than most traditional corporate
458	development environments.  Here are a list of things that you can try to
459	do to avoid problems:
460	  Good things to say regarding your proposed changes:
461	    - "This solves multiple problems."
462	    - "This deletes 2000 lines of code."
463	    - "Here is a patch that explains what I am trying to describe."
464	    - "I tested it on 5 different architectures..."
465	    - "Here is a series of small patches that..."
466	    - "This increases performance on typical machines..."
467	
468	  Bad things you should avoid saying:
469	    - "We did it this way in AIX/ptx/Solaris, so therefore it must be
470	      good..."
471	    - "I've being doing this for 20 years, so..."
472	    - "This is required for my company to make money"
473	    - "This is for our Enterprise product line."
474	    - "Here is my 1000 page design document that describes my idea"
475	    - "I've been working on this for 6 months..."
476	    - "Here's a 5000 line patch that..."
477	    - "I rewrote all of the current mess, and here it is..."
478	    - "I have a deadline, and this patch needs to be applied now."
479	
480	Another way the kernel community is different than most traditional
481	software engineering work environments is the faceless nature of
482	interaction.  One benefit of using email and irc as the primary forms of
483	communication is the lack of discrimination based on gender or race.
484	The Linux kernel work environment is accepting of women and minorities
485	because all you are is an email address.  The international aspect also
486	helps to level the playing field because you can't guess gender based on
487	a person's name. A man may be named Andrea and a woman may be named Pat.
488	Most women who have worked in the Linux kernel and have expressed an
489	opinion have had positive experiences.
490	
491	The language barrier can cause problems for some people who are not
492	comfortable with English.  A good grasp of the language can be needed in
493	order to get ideas across properly on mailing lists, so it is
494	recommended that you check your emails to make sure they make sense in
495	English before sending them.
496	
497	
498	Break up your changes
499	---------------------
500	
501	The Linux kernel community does not gladly accept large chunks of code
502	dropped on it all at once.  The changes need to be properly introduced,
503	discussed, and broken up into tiny, individual portions.  This is almost
504	the exact opposite of what companies are used to doing.  Your proposal
505	should also be introduced very early in the development process, so that
506	you can receive feedback on what you are doing.  It also lets the
507	community feel that you are working with them, and not simply using them
508	as a dumping ground for your feature.  However, don't send 50 emails at
509	one time to a mailing list, your patch series should be smaller than
510	that almost all of the time.
511	
512	The reasons for breaking things up are the following:
513	
514	1) Small patches increase the likelihood that your patches will be
515	   applied, since they don't take much time or effort to verify for
516	   correctness.  A 5 line patch can be applied by a maintainer with
517	   barely a second glance. However, a 500 line patch may take hours to
518	   review for correctness (the time it takes is exponentially
519	   proportional to the size of the patch, or something).
520	
521	   Small patches also make it very easy to debug when something goes
522	   wrong.  It's much easier to back out patches one by one than it is
523	   to dissect a very large patch after it's been applied (and broken
524	   something).
525	
526	2) It's important not only to send small patches, but also to rewrite
527	   and simplify (or simply re-order) patches before submitting them.
528	
529	Here is an analogy from kernel developer Al Viro:
530		"Think of a teacher grading homework from a math student.  The
531		teacher does not want to see the student's trials and errors
532		before they came up with the solution. They want to see the
533		cleanest, most elegant answer.  A good student knows this, and
534		would never submit her intermediate work before the final
535		solution."
536	
537		The same is true of kernel development. The maintainers and
538		reviewers do not want to see the thought process behind the
539		solution to the problem one is solving. They want to see a
540		simple and elegant solution."
541	
542	It may be challenging to keep the balance between presenting an elegant
543	solution and working together with the community and discussing your
544	unfinished work. Therefore it is good to get early in the process to
545	get feedback to improve your work, but also keep your changes in small
546	chunks that they may get already accepted, even when your whole task is
547	not ready for inclusion now.
548	
549	Also realize that it is not acceptable to send patches for inclusion
550	that are unfinished and will be "fixed up later."
551	
552	
553	Justify your change
554	-------------------
555	
556	Along with breaking up your patches, it is very important for you to let
557	the Linux community know why they should add this change.  New features
558	must be justified as being needed and useful.
559	
560	
561	Document your change
562	--------------------
563	
564	When sending in your patches, pay special attention to what you say in
565	the text in your email.  This information will become the ChangeLog
566	information for the patch, and will be preserved for everyone to see for
567	all time.  It should describe the patch completely, containing:
568	  - why the change is necessary
569	  - the overall design approach in the patch
570	  - implementation details
571	  - testing results
572	
573	For more details on what this should all look like, please see the
574	ChangeLog section of the document:
575	  "The Perfect Patch"
576	      http://www.ozlabs.org/~akpm/stuff/tpp.txt
577	
578	
579	
580	
581	All of these things are sometimes very hard to do. It can take years to
582	perfect these practices (if at all). It's a continuous process of
583	improvement that requires a lot of patience and determination. But
584	don't give up, it's possible. Many have done it before, and each had to
585	start exactly where you are now.
586	
587	
588	
589	
590	----------
591	Thanks to Paolo Ciarrocchi who allowed the "Development Process"
592	(http://lwn.net/Articles/94386/) section
593	to be based on text he had written, and to Randy Dunlap and Gerrit
594	Huizenga for some of the list of things you should and should not say.
595	Also thanks to Pat Mochel, Hanna Linder, Randy Dunlap, Kay Sievers,
596	Vojtech Pavlik, Jan Kara, Josh Boyer, Kees Cook, Andrew Morton, Andi
597	Kleen, Vadim Lobanov, Jesper Juhl, Adrian Bunk, Keri Harris, Frans Pop,
598	David A. Wheeler, Junio Hamano, Michael Kerrisk, and Alex Shepard for
599	their review, comments, and contributions.  Without their help, this
600	document would not have been possible.
601	
602	
603	
604	Maintainer: Greg Kroah-Hartman <greg@kroah.com>
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