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1.. _process_howto:
2
3HOWTO do Linux kernel development
4=================================
5
6This is the be-all, end-all document on this topic. It contains
7instructions on how to become a Linux kernel developer and how to learn
8to work with the Linux kernel development community. It tries to not
9contain anything related to the technical aspects of kernel programming,
10but will help point you in the right direction for that.
11
12If anything in this document becomes out of date, please send in patches
13to the maintainer of this file, who is listed at the bottom of the
14document.
15
16
17Introduction
18------------
19
20So, you want to learn how to become a Linux kernel developer? Or you
21have been told by your manager, "Go write a Linux driver for this
22device." This document's goal is to teach you everything you need to
23know to achieve this by describing the process you need to go through,
24and hints on how to work with the community. It will also try to
25explain some of the reasons why the community works like it does.
26
27The kernel is written mostly in C, with some architecture-dependent
28parts written in assembly. A good understanding of C is required for
29kernel development. Assembly (any architecture) is not required unless
30you plan to do low-level development for that architecture. Though they
31are not a good substitute for a solid C education and/or years of
32experience, the following books are good for, if anything, reference:
33
34 - "The C Programming Language" by Kernighan and Ritchie [Prentice Hall]
35 - "Practical C Programming" by Steve Oualline [O'Reilly]
36 - "C: A Reference Manual" by Harbison and Steele [Prentice Hall]
37
38The kernel is written using GNU C and the GNU toolchain. While it
39adheres to the ISO C11 standard, it uses a number of extensions that are
40not featured in the standard. The kernel is a freestanding C
41environment, with no reliance on the standard C library, so some
42portions of the C standard are not supported. Arbitrary long long
43divisions and floating point are not allowed. It can sometimes be
44difficult to understand the assumptions the kernel has on the toolchain
45and the extensions that it uses, and unfortunately there is no
46definitive reference for them. Please check the gcc info pages (`info
47gcc`) for some information on them.
48
49Please remember that you are trying to learn how to work with the
50existing development community. It is a diverse group of people, with
51high standards for coding, style and procedure. These standards have
52been created over time based on what they have found to work best for
53such a large and geographically dispersed team. Try to learn as much as
54possible about these standards ahead of time, as they are well
55documented; do not expect people to adapt to you or your company's way
56of doing things.
57
58
59Legal Issues
60------------
61
62The Linux kernel source code is released under the GPL. Please see the file
63COPYING in the main directory of the source tree. The Linux kernel licensing
64rules and how to use `SPDX <https://spdx.org/>`_ identifiers in source code are
65described in :ref:`Documentation/process/license-rules.rst <kernel_licensing>`.
66If you have further questions about the license, please contact a lawyer, and do
67not ask on the Linux kernel mailing list. The people on the mailing lists are
68not lawyers, and you should not rely on their statements on legal matters.
69
70For common questions and answers about the GPL, please see:
71
72 https://www.gnu.org/licenses/gpl-faq.html
73
74
75Documentation
76-------------
77
78The Linux kernel source tree has a large range of documents that are
79invaluable for learning how to interact with the kernel community. When
80new features are added to the kernel, it is recommended that new
81documentation files are also added which explain how to use the feature.
82When a kernel change causes the interface that the kernel exposes to
83userspace to change, it is recommended that you send the information or
84a patch to the manual pages explaining the change to the manual pages
85maintainer at mtk.manpages@gmail.com, and CC the list
86linux-api@vger.kernel.org.
87
88Here is a list of files that are in the kernel source tree that are
89required reading:
90
91 :ref:`Documentation/admin-guide/README.rst <readme>`
92 This file gives a short background on the Linux kernel and describes
93 what is necessary to do to configure and build the kernel. People
94 who are new to the kernel should start here.
95
96 :ref:`Documentation/process/changes.rst <changes>`
97 This file gives a list of the minimum levels of various software
98 packages that are necessary to build and run the kernel
99 successfully.
100
101 :ref:`Documentation/process/coding-style.rst <codingstyle>`
102 This describes the Linux kernel coding style, and some of the
103 rationale behind it. All new code is expected to follow the
104 guidelines in this document. Most maintainers will only accept
105 patches if these rules are followed, and many people will only
106 review code if it is in the proper style.
107
108 :ref:`Documentation/process/submitting-patches.rst <submittingpatches>`
109 This file describes in explicit detail how to successfully create
110 and send a patch, including (but not limited to):
111
112 - Email contents
113 - Email format
114 - Who to send it to
115
116 Following these rules will not guarantee success (as all patches are
117 subject to scrutiny for content and style), but not following them
118 will almost always prevent it.
119
120 Other excellent descriptions of how to create patches properly are:
121
122 "The Perfect Patch"
123 https://www.ozlabs.org/~akpm/stuff/tpp.txt
124
125 "Linux kernel patch submission format"
126 https://web.archive.org/web/20180829112450/http://linux.yyz.us/patch-format.html
127
128 :ref:`Documentation/process/stable-api-nonsense.rst <stable_api_nonsense>`
129 This file describes the rationale behind the conscious decision to
130 not have a stable API within the kernel, including things like:
131
132 - Subsystem shim-layers (for compatibility?)
133 - Driver portability between Operating Systems.
134 - Mitigating rapid change within the kernel source tree (or
135 preventing rapid change)
136
137 This document is crucial for understanding the Linux development
138 philosophy and is very important for people moving to Linux from
139 development on other Operating Systems.
140
141 :ref:`Documentation/admin-guide/security-bugs.rst <securitybugs>`
142 If you feel you have found a security problem in the Linux kernel,
143 please follow the steps in this document to help notify the kernel
144 developers, and help solve the issue.
145
146 :ref:`Documentation/process/management-style.rst <managementstyle>`
147 This document describes how Linux kernel maintainers operate and the
148 shared ethos behind their methodologies. This is important reading
149 for anyone new to kernel development (or anyone simply curious about
150 it), as it resolves a lot of common misconceptions and confusion
151 about the unique behavior of kernel maintainers.
152
153 :ref:`Documentation/process/stable-kernel-rules.rst <stable_kernel_rules>`
154 This file describes the rules on how the stable kernel releases
155 happen, and what to do if you want to get a change into one of these
156 releases.
157
158 :ref:`Documentation/process/kernel-docs.rst <kernel_docs>`
159 A list of external documentation that pertains to kernel
160 development. Please consult this list if you do not find what you
161 are looking for within the in-kernel documentation.
162
163 :ref:`Documentation/process/applying-patches.rst <applying_patches>`
164 A good introduction describing exactly what a patch is and how to
165 apply it to the different development branches of the kernel.
166
167The kernel also has a large number of documents that can be
168automatically generated from the source code itself or from
169ReStructuredText markups (ReST), like this one. This includes a
170full description of the in-kernel API, and rules on how to handle
171locking properly.
172
173All such documents can be generated as PDF or HTML by running::
174
175 make pdfdocs
176 make htmldocs
177
178respectively from the main kernel source directory.
179
180The documents that uses ReST markup will be generated at Documentation/output.
181They can also be generated on LaTeX and ePub formats with::
182
183 make latexdocs
184 make epubdocs
185
186Becoming A Kernel Developer
187---------------------------
188
189If you do not know anything about Linux kernel development, you should
190look at the Linux KernelNewbies project:
191
192 https://kernelnewbies.org
193
194It consists of a helpful mailing list where you can ask almost any type
195of basic kernel development question (make sure to search the archives
196first, before asking something that has already been answered in the
197past.) It also has an IRC channel that you can use to ask questions in
198real-time, and a lot of helpful documentation that is useful for
199learning about Linux kernel development.
200
201The website has basic information about code organization, subsystems,
202and current projects (both in-tree and out-of-tree). It also describes
203some basic logistical information, like how to compile a kernel and
204apply a patch.
205
206If you do not know where you want to start, but you want to look for
207some task to start doing to join into the kernel development community,
208go to the Linux Kernel Janitor's project:
209
210 https://kernelnewbies.org/KernelJanitors
211
212It is a great place to start. It describes a list of relatively simple
213problems that need to be cleaned up and fixed within the Linux kernel
214source tree. Working with the developers in charge of this project, you
215will learn the basics of getting your patch into the Linux kernel tree,
216and possibly be pointed in the direction of what to go work on next, if
217you do not already have an idea.
218
219Before making any actual modifications to the Linux kernel code, it is
220imperative to understand how the code in question works. For this
221purpose, nothing is better than reading through it directly (most tricky
222bits are commented well), perhaps even with the help of specialized
223tools. One such tool that is particularly recommended is the Linux
224Cross-Reference project, which is able to present source code in a
225self-referential, indexed webpage format. An excellent up-to-date
226repository of the kernel code may be found at:
227
228 https://elixir.bootlin.com/
229
230
231The development process
232-----------------------
233
234Linux kernel development process currently consists of a few different
235main kernel "branches" and lots of different subsystem-specific kernel
236branches. These different branches are:
237
238 - Linus's mainline tree
239 - Various stable trees with multiple major numbers
240 - Subsystem-specific trees
241 - linux-next integration testing tree
242
243Mainline tree
244~~~~~~~~~~~~~
245
246The mainline tree is maintained by Linus Torvalds, and can be found at
247https://kernel.org or in the repo. Its development process is as follows:
248
249 - As soon as a new kernel is released a two week window is open,
250 during this period of time maintainers can submit big diffs to
251 Linus, usually the patches that have already been included in the
252 linux-next for a few weeks. The preferred way to submit big changes
253 is using git (the kernel's source management tool, more information
254 can be found at https://git-scm.com/) but plain patches are also just
255 fine.
256 - After two weeks a -rc1 kernel is released and the focus is on making the
257 new kernel as rock solid as possible. Most of the patches at this point
258 should fix a regression. Bugs that have always existed are not
259 regressions, so only push these kinds of fixes if they are important.
260 Please note that a whole new driver (or filesystem) might be accepted
261 after -rc1 because there is no risk of causing regressions with such a
262 change as long as the change is self-contained and does not affect areas
263 outside of the code that is being added. git can be used to send
264 patches to Linus after -rc1 is released, but the patches need to also be
265 sent to a public mailing list for review.
266 - A new -rc is released whenever Linus deems the current git tree to
267 be in a reasonably sane state adequate for testing. The goal is to
268 release a new -rc kernel every week.
269 - Process continues until the kernel is considered "ready", the
270 process should last around 6 weeks.
271
272It is worth mentioning what Andrew Morton wrote on the linux-kernel
273mailing list about kernel releases:
274
275 *"Nobody knows when a kernel will be released, because it's
276 released according to perceived bug status, not according to a
277 preconceived timeline."*
278
279Various stable trees with multiple major numbers
280~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
281
282Kernels with 3-part versions are -stable kernels. They contain
283relatively small and critical fixes for security problems or significant
284regressions discovered in a given major mainline release. Each release
285in a major stable series increments the third part of the version
286number, keeping the first two parts the same.
287
288This is the recommended branch for users who want the most recent stable
289kernel and are not interested in helping test development/experimental
290versions.
291
292Stable trees are maintained by the "stable" team <stable@vger.kernel.org>, and
293are released as needs dictate. The normal release period is approximately
294two weeks, but it can be longer if there are no pressing problems. A
295security-related problem, instead, can cause a release to happen almost
296instantly.
297
298The file :ref:`Documentation/process/stable-kernel-rules.rst <stable_kernel_rules>`
299in the kernel tree documents what kinds of changes are acceptable for
300the -stable tree, and how the release process works.
301
302Subsystem-specific trees
303~~~~~~~~~~~~~~~~~~~~~~~~
304
305The maintainers of the various kernel subsystems --- and also many
306kernel subsystem developers --- expose their current state of
307development in source repositories. That way, others can see what is
308happening in the different areas of the kernel. In areas where
309development is rapid, a developer may be asked to base his submissions
310onto such a subsystem kernel tree so that conflicts between the
311submission and other already ongoing work are avoided.
312
313Most of these repositories are git trees, but there are also other SCMs
314in use, or patch queues being published as quilt series. Addresses of
315these subsystem repositories are listed in the MAINTAINERS file. Many
316of them can be browsed at https://git.kernel.org/.
317
318Before a proposed patch is committed to such a subsystem tree, it is
319subject to review which primarily happens on mailing lists (see the
320respective section below). For several kernel subsystems, this review
321process is tracked with the tool patchwork. Patchwork offers a web
322interface which shows patch postings, any comments on a patch or
323revisions to it, and maintainers can mark patches as under review,
324accepted, or rejected. Most of these patchwork sites are listed at
325https://patchwork.kernel.org/.
326
327linux-next integration testing tree
328~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
329
330Before updates from subsystem trees are merged into the mainline tree,
331they need to be integration-tested. For this purpose, a special
332testing repository exists into which virtually all subsystem trees are
333pulled on an almost daily basis:
334
335 https://git.kernel.org/?p=linux/kernel/git/next/linux-next.git
336
337This way, the linux-next gives a summary outlook onto what will be
338expected to go into the mainline kernel at the next merge period.
339Adventurous testers are very welcome to runtime-test the linux-next.
340
341
342Bug Reporting
343-------------
344
345The file 'Documentation/admin-guide/reporting-issues.rst' in the main kernel
346source directory describes how to report a possible kernel bug, and details
347what kind of information is needed by the kernel developers to help track
348down the problem.
349
350
351Managing bug reports
352--------------------
353
354One of the best ways to put into practice your hacking skills is by fixing
355bugs reported by other people. Not only you will help to make the kernel
356more stable, but you'll also learn to fix real world problems and you will
357improve your skills, and other developers will be aware of your presence.
358Fixing bugs is one of the best ways to get merits among other developers,
359because not many people like wasting time fixing other people's bugs.
360
361To work on already reported bug reports, find a subsystem you are interested in.
362Check the MAINTAINERS file where bugs for that subsystem get reported to; often
363it will be a mailing list, rarely a bugtracker. Search the archives of said
364place for recent reports and help where you see fit. You may also want to check
365https://bugzilla.kernel.org for bug reports; only a handful of kernel subsystems
366use it actively for reporting or tracking, nevertheless bugs for the whole
367kernel get filed there.
368
369
370Mailing lists
371-------------
372
373As some of the above documents describe, the majority of the core kernel
374developers participate on the Linux Kernel Mailing list. Details on how
375to subscribe and unsubscribe from the list can be found at:
376
377 http://vger.kernel.org/vger-lists.html#linux-kernel
378
379There are archives of the mailing list on the web in many different
380places. Use a search engine to find these archives. For example:
381
382 https://lore.kernel.org/lkml/
383
384It is highly recommended that you search the archives about the topic
385you want to bring up, before you post it to the list. A lot of things
386already discussed in detail are only recorded at the mailing list
387archives.
388
389Most of the individual kernel subsystems also have their own separate
390mailing list where they do their development efforts. See the
391MAINTAINERS file for a list of what these lists are for the different
392groups.
393
394Many of the lists are hosted on kernel.org. Information on them can be
395found at:
396
397 http://vger.kernel.org/vger-lists.html
398
399Please remember to follow good behavioral habits when using the lists.
400Though a bit cheesy, the following URL has some simple guidelines for
401interacting with the list (or any list):
402
403 http://www.albion.com/netiquette/
404
405If multiple people respond to your mail, the CC: list of recipients may
406get pretty large. Don't remove anybody from the CC: list without a good
407reason, or don't reply only to the list address. Get used to receiving the
408mail twice, one from the sender and the one from the list, and don't try
409to tune that by adding fancy mail-headers, people will not like it.
410
411Remember to keep the context and the attribution of your replies intact,
412keep the "John Kernelhacker wrote ...:" lines at the top of your reply, and
413add your statements between the individual quoted sections instead of
414writing at the top of the mail.
415
416If you add patches to your mail, make sure they are plain readable text
417as stated in :ref:`Documentation/process/submitting-patches.rst <submittingpatches>`.
418Kernel developers don't want to deal with
419attachments or compressed patches; they may want to comment on
420individual lines of your patch, which works only that way. Make sure you
421use a mail program that does not mangle spaces and tab characters. A
422good first test is to send the mail to yourself and try to apply your
423own patch by yourself. If that doesn't work, get your mail program fixed
424or change it until it works.
425
426Above all, please remember to show respect to other subscribers.
427
428
429Working with the community
430--------------------------
431
432The goal of the kernel community is to provide the best possible kernel
433there is. When you submit a patch for acceptance, it will be reviewed
434on its technical merits and those alone. So, what should you be
435expecting?
436
437 - criticism
438 - comments
439 - requests for change
440 - requests for justification
441 - silence
442
443Remember, this is part of getting your patch into the kernel. You have
444to be able to take criticism and comments about your patches, evaluate
445them at a technical level and either rework your patches or provide
446clear and concise reasoning as to why those changes should not be made.
447If there are no responses to your posting, wait a few days and try
448again, sometimes things get lost in the huge volume.
449
450What should you not do?
451
452 - expect your patch to be accepted without question
453 - become defensive
454 - ignore comments
455 - resubmit the patch without making any of the requested changes
456
457In a community that is looking for the best technical solution possible,
458there will always be differing opinions on how beneficial a patch is.
459You have to be cooperative, and willing to adapt your idea to fit within
460the kernel. Or at least be willing to prove your idea is worth it.
461Remember, being wrong is acceptable as long as you are willing to work
462toward a solution that is right.
463
464It is normal that the answers to your first patch might simply be a list
465of a dozen things you should correct. This does **not** imply that your
466patch will not be accepted, and it is **not** meant against you
467personally. Simply correct all issues raised against your patch and
468resend it.
469
470
471Differences between the kernel community and corporate structures
472-----------------------------------------------------------------
473
474The kernel community works differently than most traditional corporate
475development environments. Here are a list of things that you can try to
476do to avoid problems:
477
478 Good things to say regarding your proposed changes:
479
480 - "This solves multiple problems."
481 - "This deletes 2000 lines of code."
482 - "Here is a patch that explains what I am trying to describe."
483 - "I tested it on 5 different architectures..."
484 - "Here is a series of small patches that..."
485 - "This increases performance on typical machines..."
486
487 Bad things you should avoid saying:
488
489 - "We did it this way in AIX/ptx/Solaris, so therefore it must be
490 good..."
491 - "I've being doing this for 20 years, so..."
492 - "This is required for my company to make money"
493 - "This is for our Enterprise product line."
494 - "Here is my 1000 page design document that describes my idea"
495 - "I've been working on this for 6 months..."
496 - "Here's a 5000 line patch that..."
497 - "I rewrote all of the current mess, and here it is..."
498 - "I have a deadline, and this patch needs to be applied now."
499
500Another way the kernel community is different than most traditional
501software engineering work environments is the faceless nature of
502interaction. One benefit of using email and irc as the primary forms of
503communication is the lack of discrimination based on gender or race.
504The Linux kernel work environment is accepting of women and minorities
505because all you are is an email address. The international aspect also
506helps to level the playing field because you can't guess gender based on
507a person's name. A man may be named Andrea and a woman may be named Pat.
508Most women who have worked in the Linux kernel and have expressed an
509opinion have had positive experiences.
510
511The language barrier can cause problems for some people who are not
512comfortable with English. A good grasp of the language can be needed in
513order to get ideas across properly on mailing lists, so it is
514recommended that you check your emails to make sure they make sense in
515English before sending them.
516
517
518Break up your changes
519---------------------
520
521The Linux kernel community does not gladly accept large chunks of code
522dropped on it all at once. The changes need to be properly introduced,
523discussed, and broken up into tiny, individual portions. This is almost
524the exact opposite of what companies are used to doing. Your proposal
525should also be introduced very early in the development process, so that
526you can receive feedback on what you are doing. It also lets the
527community feel that you are working with them, and not simply using them
528as a dumping ground for your feature. However, don't send 50 emails at
529one time to a mailing list, your patch series should be smaller than
530that almost all of the time.
531
532The reasons for breaking things up are the following:
533
5341) Small patches increase the likelihood that your patches will be
535 applied, since they don't take much time or effort to verify for
536 correctness. A 5 line patch can be applied by a maintainer with
537 barely a second glance. However, a 500 line patch may take hours to
538 review for correctness (the time it takes is exponentially
539 proportional to the size of the patch, or something).
540
541 Small patches also make it very easy to debug when something goes
542 wrong. It's much easier to back out patches one by one than it is
543 to dissect a very large patch after it's been applied (and broken
544 something).
545
5462) It's important not only to send small patches, but also to rewrite
547 and simplify (or simply re-order) patches before submitting them.
548
549Here is an analogy from kernel developer Al Viro:
550
551 *"Think of a teacher grading homework from a math student. The
552 teacher does not want to see the student's trials and errors
553 before they came up with the solution. They want to see the
554 cleanest, most elegant answer. A good student knows this, and
555 would never submit her intermediate work before the final
556 solution.*
557
558 *The same is true of kernel development. The maintainers and
559 reviewers do not want to see the thought process behind the
560 solution to the problem one is solving. They want to see a
561 simple and elegant solution."*
562
563It may be challenging to keep the balance between presenting an elegant
564solution and working together with the community and discussing your
565unfinished work. Therefore it is good to get early in the process to
566get feedback to improve your work, but also keep your changes in small
567chunks that they may get already accepted, even when your whole task is
568not ready for inclusion now.
569
570Also realize that it is not acceptable to send patches for inclusion
571that are unfinished and will be "fixed up later."
572
573
574Justify your change
575-------------------
576
577Along with breaking up your patches, it is very important for you to let
578the Linux community know why they should add this change. New features
579must be justified as being needed and useful.
580
581
582Document your change
583--------------------
584
585When sending in your patches, pay special attention to what you say in
586the text in your email. This information will become the ChangeLog
587information for the patch, and will be preserved for everyone to see for
588all time. It should describe the patch completely, containing:
589
590 - why the change is necessary
591 - the overall design approach in the patch
592 - implementation details
593 - testing results
594
595For more details on what this should all look like, please see the
596ChangeLog section of the document:
597
598 "The Perfect Patch"
599 https://www.ozlabs.org/~akpm/stuff/tpp.txt
600
601
602All of these things are sometimes very hard to do. It can take years to
603perfect these practices (if at all). It's a continuous process of
604improvement that requires a lot of patience and determination. But
605don't give up, it's possible. Many have done it before, and each had to
606start exactly where you are now.
607
608
609
610
611----------
612
613Thanks to Paolo Ciarrocchi who allowed the "Development Process"
614(https://lwn.net/Articles/94386/) section
615to be based on text he had written, and to Randy Dunlap and Gerrit
616Huizenga for some of the list of things you should and should not say.
617Also thanks to Pat Mochel, Hanna Linder, Randy Dunlap, Kay Sievers,
618Vojtech Pavlik, Jan Kara, Josh Boyer, Kees Cook, Andrew Morton, Andi
619Kleen, Vadim Lobanov, Jesper Juhl, Adrian Bunk, Keri Harris, Frans Pop,
620David A. Wheeler, Junio Hamano, Michael Kerrisk, and Alex Shepard for
621their review, comments, and contributions. Without their help, this
622document would not have been possible.
623
624
625
626Maintainer: Greg Kroah-Hartman <greg@kroah.com>
1HOWTO do Linux kernel development
2=================================
3
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.
9
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
12document.
13
14
15Introduction
16------------
17
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.
24
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
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]
35
36The kernel is written using GNU C and the GNU toolchain. While it
37adheres to the ISO C89 standard, it uses a number of extensions that are
38not featured in the standard. The kernel is a freestanding C
39environment, with no reliance on the standard C library, so some
40portions of the C standard are not supported. Arbitrary long long
41divisions and floating point are not allowed. It can sometimes be
42difficult to understand the assumptions the kernel has on the toolchain
43and the extensions that it uses, and unfortunately there is no
44definitive reference for them. Please check the gcc info pages (`info
45gcc`) for some information on them.
46
47Please remember that you are trying to learn how to work with the
48existing development community. It is a diverse group of people, with
49high standards for coding, style and procedure. These standards have
50been created over time based on what they have found to work best for
51such a large and geographically dispersed team. Try to learn as much as
52possible about these standards ahead of time, as they are well
53documented; do not expect people to adapt to you or your company's way
54of doing things.
55
56
57Legal Issues
58------------
59
60The Linux kernel source code is released under the GPL. Please see the
61file, COPYING, in the main directory of the source tree, for details on
62the license. If you have further questions about the license, please
63contact a lawyer, and do not ask on the Linux kernel mailing list. The
64people on the mailing lists are not lawyers, and you should not rely on
65their statements on legal matters.
66
67For common questions and answers about the GPL, please see:
68
69 https://www.gnu.org/licenses/gpl-faq.html
70
71
72Documentation
73-------------
74
75The Linux kernel source tree has a large range of documents that are
76invaluable for learning how to interact with the kernel community. When
77new features are added to the kernel, it is recommended that new
78documentation files are also added which explain how to use the feature.
79When a kernel change causes the interface that the kernel exposes to
80userspace to change, it is recommended that you send the information or
81a patch to the manual pages explaining the change to the manual pages
82maintainer at mtk.manpages@gmail.com, and CC the list
83linux-api@vger.kernel.org.
84
85Here is a list of files that are in the kernel source tree that are
86required reading:
87
88 README
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.
92
93 :ref:`Documentation/process/changes.rst <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.
97
98 :ref:`Documentation/process/coding-style.rst <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.
104
105 :ref:`Documentation/process/submitting-patches.rst <submittingpatches>` and :ref:`Documentation/process/submitting-drivers.rst <submittingdrivers>`
106 These files describe in explicit detail how to successfully create
107 and send a patch, including (but not limited to):
108
109 - Email contents
110 - Email format
111 - Who to send it to
112
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.
116
117 Other excellent descriptions of how to create patches properly are:
118
119 "The Perfect Patch"
120 https://www.ozlabs.org/~akpm/stuff/tpp.txt
121
122 "Linux kernel patch submission format"
123 http://linux.yyz.us/patch-format.html
124
125 :ref:`Documentation/process/stable-api-nonsense.rst <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:
128
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)
133
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.
137
138 :ref:`Documentation/admin-guide/security-bugs.rst <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.
142
143 :ref:`Documentation/process/management-style.rst <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.
149
150 :ref:`Documentation/process/stable-kernel-rules.rst <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.
154
155 :ref:`Documentation/process/kernel-docs.rst <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.
159
160 :ref:`Documentation/process/applying-patches.rst <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.
163
164The kernel also has a large number of documents that can be
165automatically generated from the source code itself or from
166ReStructuredText markups (ReST), like this one. This includes a
167full description of the in-kernel API, and rules on how to handle
168locking properly.
169
170All such documents can be generated as PDF or HTML by running::
171
172 make pdfdocs
173 make htmldocs
174
175respectively from the main kernel source directory.
176
177The documents that uses ReST markup will be generated at Documentation/output.
178They can also be generated on LaTeX and ePub formats with::
179
180 make latexdocs
181 make epubdocs
182
183Currently, there are some documents written on DocBook that are in
184the process of conversion to ReST. Such documents will be created in the
185Documentation/DocBook/ directory and can be generated also as
186Postscript or man pages by running::
187
188 make psdocs
189 make mandocs
190
191Becoming A Kernel Developer
192---------------------------
193
194If you do not know anything about Linux kernel development, you should
195look at the Linux KernelNewbies project:
196
197 https://kernelnewbies.org
198
199It consists of a helpful mailing list where you can ask almost any type
200of basic kernel development question (make sure to search the archives
201first, before asking something that has already been answered in the
202past.) It also has an IRC channel that you can use to ask questions in
203real-time, and a lot of helpful documentation that is useful for
204learning about Linux kernel development.
205
206The website has basic information about code organization, subsystems,
207and current projects (both in-tree and out-of-tree). It also describes
208some basic logistical information, like how to compile a kernel and
209apply a patch.
210
211If you do not know where you want to start, but you want to look for
212some task to start doing to join into the kernel development community,
213go to the Linux Kernel Janitor's project:
214
215 https://kernelnewbies.org/KernelJanitors
216
217It is a great place to start. It describes a list of relatively simple
218problems that need to be cleaned up and fixed within the Linux kernel
219source tree. Working with the developers in charge of this project, you
220will learn the basics of getting your patch into the Linux kernel tree,
221and possibly be pointed in the direction of what to go work on next, if
222you do not already have an idea.
223
224If you already have a chunk of code that you want to put into the kernel
225tree, but need some help getting it in the proper form, the
226kernel-mentors project was created to help you out with this. It is a
227mailing list, and can be found at:
228
229 https://selenic.com/mailman/listinfo/kernel-mentors
230
231Before making any actual modifications to the Linux kernel code, it is
232imperative to understand how the code in question works. For this
233purpose, nothing is better than reading through it directly (most tricky
234bits are commented well), perhaps even with the help of specialized
235tools. One such tool that is particularly recommended is the Linux
236Cross-Reference project, which is able to present source code in a
237self-referential, indexed webpage format. An excellent up-to-date
238repository of the kernel code may be found at:
239
240 http://lxr.free-electrons.com/
241
242
243The development process
244-----------------------
245
246Linux kernel development process currently consists of a few different
247main kernel "branches" and lots of different subsystem-specific kernel
248branches. These different branches are:
249
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
255
2564.x kernel tree
257~~~~~~~~~~~~~~~
258
2594.x kernels are maintained by Linus Torvalds, and can be found on
260https://kernel.org in the pub/linux/kernel/v4.x/ directory. Its development
261process is as follows:
262
263 - As soon as a new kernel is released a two weeks window is open,
264 during this period of time maintainers can submit big diffs to
265 Linus, usually the patches that have already been included in the
266 -next kernel for a few weeks. The preferred way to submit big changes
267 is using git (the kernel's source management tool, more information
268 can be found at https://git-scm.com/) but plain patches are also just
269 fine.
270 - After two weeks a -rc1 kernel is released and the focus is on making the
271 new kernel as rock solid as possible. Most of the patches at this point
272 should fix a regression. Bugs that have always existed are not
273 regressions, so only push these kinds of fixes if they are important.
274 Please note that a whole new driver (or filesystem) might be accepted
275 after -rc1 because there is no risk of causing regressions with such a
276 change as long as the change is self-contained and does not affect areas
277 outside of the code that is being added. git can be used to send
278 patches to Linus after -rc1 is released, but the patches need to also be
279 sent to a public mailing list for review.
280 - A new -rc is released whenever Linus deems the current git tree to
281 be in a reasonably sane state adequate for testing. The goal is to
282 release a new -rc kernel every week.
283 - Process continues until the kernel is considered "ready", the
284 process should last around 6 weeks.
285
286It is worth mentioning what Andrew Morton wrote on the linux-kernel
287mailing list about kernel releases:
288
289 *"Nobody knows when a kernel will be released, because it's
290 released according to perceived bug status, not according to a
291 preconceived timeline."*
292
2934.x.y -stable kernel tree
294~~~~~~~~~~~~~~~~~~~~~~~~~
295
296Kernels with 3-part versions are -stable kernels. They contain
297relatively small and critical fixes for security problems or significant
298regressions discovered in a given 4.x kernel.
299
300This is the recommended branch for users who want the most recent stable
301kernel and are not interested in helping test development/experimental
302versions.
303
304If no 4.x.y kernel is available, then the highest numbered 4.x
305kernel is the current stable kernel.
306
3074.x.y are maintained by the "stable" team <stable@vger.kernel.org>, and
308are released as needs dictate. The normal release period is approximately
309two weeks, but it can be longer if there are no pressing problems. A
310security-related problem, instead, can cause a release to happen almost
311instantly.
312
313The file Documentation/process/stable-kernel-rules.rst in the kernel tree
314documents what kinds of changes are acceptable for the -stable tree, and
315how the release process works.
316
3174.x -git patches
318~~~~~~~~~~~~~~~~
319
320These are daily snapshots of Linus' kernel tree which are managed in a
321git repository (hence the name.) These patches are usually released
322daily and represent the current state of Linus' tree. They are more
323experimental than -rc kernels since they are generated automatically
324without even a cursory glance to see if they are sane.
325
326Subsystem Specific kernel trees and patches
327~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
328
329The maintainers of the various kernel subsystems --- and also many
330kernel subsystem developers --- expose their current state of
331development in source repositories. That way, others can see what is
332happening in the different areas of the kernel. In areas where
333development is rapid, a developer may be asked to base his submissions
334onto such a subsystem kernel tree so that conflicts between the
335submission and other already ongoing work are avoided.
336
337Most of these repositories are git trees, but there are also other SCMs
338in use, or patch queues being published as quilt series. Addresses of
339these subsystem repositories are listed in the MAINTAINERS file. Many
340of them can be browsed at https://git.kernel.org/.
341
342Before a proposed patch is committed to such a subsystem tree, it is
343subject to review which primarily happens on mailing lists (see the
344respective section below). For several kernel subsystems, this review
345process is tracked with the tool patchwork. Patchwork offers a web
346interface which shows patch postings, any comments on a patch or
347revisions to it, and maintainers can mark patches as under review,
348accepted, or rejected. Most of these patchwork sites are listed at
349https://patchwork.kernel.org/.
350
3514.x -next kernel tree for integration tests
352~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
353
354Before updates from subsystem trees are merged into the mainline 4.x
355tree, they need to be integration-tested. For this purpose, a special
356testing repository exists into which virtually all subsystem trees are
357pulled on an almost daily basis:
358
359 https://git.kernel.org/?p=linux/kernel/git/next/linux-next.git
360
361This way, the -next kernel gives a summary outlook onto what will be
362expected to go into the mainline kernel at the next merge period.
363Adventurous testers are very welcome to runtime-test the -next kernel.
364
365
366Bug Reporting
367-------------
368
369https://bugzilla.kernel.org is where the Linux kernel developers track kernel
370bugs. Users are encouraged to report all bugs that they find in this
371tool. For details on how to use the kernel bugzilla, please see:
372
373 https://bugzilla.kernel.org/page.cgi?id=faq.html
374
375The file admin-guide/reporting-bugs.rst in the main kernel source directory has a good
376template for how to report a possible kernel bug, and details what kind
377of information is needed by the kernel developers to help track down the
378problem.
379
380
381Managing bug reports
382--------------------
383
384One of the best ways to put into practice your hacking skills is by fixing
385bugs reported by other people. Not only you will help to make the kernel
386more stable, you'll learn to fix real world problems and you will improve
387your skills, and other developers will be aware of your presence. Fixing
388bugs is one of the best ways to get merits among other developers, because
389not many people like wasting time fixing other people's bugs.
390
391To work in the already reported bug reports, go to https://bugzilla.kernel.org.
392If you want to be advised of the future bug reports, you can subscribe to the
393bugme-new mailing list (only new bug reports are mailed here) or to the
394bugme-janitor mailing list (every change in the bugzilla is mailed here)
395
396 https://lists.linux-foundation.org/mailman/listinfo/bugme-new
397
398 https://lists.linux-foundation.org/mailman/listinfo/bugme-janitors
399
400
401
402Mailing lists
403-------------
404
405As some of the above documents describe, the majority of the core kernel
406developers participate on the Linux Kernel Mailing list. Details on how
407to subscribe and unsubscribe from the list can be found at:
408
409 http://vger.kernel.org/vger-lists.html#linux-kernel
410
411There are archives of the mailing list on the web in many different
412places. Use a search engine to find these archives. For example:
413
414 http://dir.gmane.org/gmane.linux.kernel
415
416It is highly recommended that you search the archives about the topic
417you want to bring up, before you post it to the list. A lot of things
418already discussed in detail are only recorded at the mailing list
419archives.
420
421Most of the individual kernel subsystems also have their own separate
422mailing list where they do their development efforts. See the
423MAINTAINERS file for a list of what these lists are for the different
424groups.
425
426Many of the lists are hosted on kernel.org. Information on them can be
427found at:
428
429 http://vger.kernel.org/vger-lists.html
430
431Please remember to follow good behavioral habits when using the lists.
432Though a bit cheesy, the following URL has some simple guidelines for
433interacting with the list (or any list):
434
435 http://www.albion.com/netiquette/
436
437If multiple people respond to your mail, the CC: list of recipients may
438get pretty large. Don't remove anybody from the CC: list without a good
439reason, or don't reply only to the list address. Get used to receiving the
440mail twice, one from the sender and the one from the list, and don't try
441to tune that by adding fancy mail-headers, people will not like it.
442
443Remember to keep the context and the attribution of your replies intact,
444keep the "John Kernelhacker wrote ...:" lines at the top of your reply, and
445add your statements between the individual quoted sections instead of
446writing at the top of the mail.
447
448If you add patches to your mail, make sure they are plain readable text
449as stated in Documentation/process/submitting-patches.rst.
450Kernel developers don't want to deal with
451attachments or compressed patches; they may want to comment on
452individual lines of your patch, which works only that way. Make sure you
453use a mail program that does not mangle spaces and tab characters. A
454good first test is to send the mail to yourself and try to apply your
455own patch by yourself. If that doesn't work, get your mail program fixed
456or change it until it works.
457
458Above all, please remember to show respect to other subscribers.
459
460
461Working with the community
462--------------------------
463
464The goal of the kernel community is to provide the best possible kernel
465there is. When you submit a patch for acceptance, it will be reviewed
466on its technical merits and those alone. So, what should you be
467expecting?
468
469 - criticism
470 - comments
471 - requests for change
472 - requests for justification
473 - silence
474
475Remember, this is part of getting your patch into the kernel. You have
476to be able to take criticism and comments about your patches, evaluate
477them at a technical level and either rework your patches or provide
478clear and concise reasoning as to why those changes should not be made.
479If there are no responses to your posting, wait a few days and try
480again, sometimes things get lost in the huge volume.
481
482What should you not do?
483
484 - expect your patch to be accepted without question
485 - become defensive
486 - ignore comments
487 - resubmit the patch without making any of the requested changes
488
489In a community that is looking for the best technical solution possible,
490there will always be differing opinions on how beneficial a patch is.
491You have to be cooperative, and willing to adapt your idea to fit within
492the kernel. Or at least be willing to prove your idea is worth it.
493Remember, being wrong is acceptable as long as you are willing to work
494toward a solution that is right.
495
496It is normal that the answers to your first patch might simply be a list
497of a dozen things you should correct. This does **not** imply that your
498patch will not be accepted, and it is **not** meant against you
499personally. Simply correct all issues raised against your patch and
500resend it.
501
502
503Differences between the kernel community and corporate structures
504-----------------------------------------------------------------
505
506The kernel community works differently than most traditional corporate
507development environments. Here are a list of things that you can try to
508do to avoid problems:
509
510 Good things to say regarding your proposed changes:
511
512 - "This solves multiple problems."
513 - "This deletes 2000 lines of code."
514 - "Here is a patch that explains what I am trying to describe."
515 - "I tested it on 5 different architectures..."
516 - "Here is a series of small patches that..."
517 - "This increases performance on typical machines..."
518
519 Bad things you should avoid saying:
520
521 - "We did it this way in AIX/ptx/Solaris, so therefore it must be
522 good..."
523 - "I've being doing this for 20 years, so..."
524 - "This is required for my company to make money"
525 - "This is for our Enterprise product line."
526 - "Here is my 1000 page design document that describes my idea"
527 - "I've been working on this for 6 months..."
528 - "Here's a 5000 line patch that..."
529 - "I rewrote all of the current mess, and here it is..."
530 - "I have a deadline, and this patch needs to be applied now."
531
532Another way the kernel community is different than most traditional
533software engineering work environments is the faceless nature of
534interaction. One benefit of using email and irc as the primary forms of
535communication is the lack of discrimination based on gender or race.
536The Linux kernel work environment is accepting of women and minorities
537because all you are is an email address. The international aspect also
538helps to level the playing field because you can't guess gender based on
539a person's name. A man may be named Andrea and a woman may be named Pat.
540Most women who have worked in the Linux kernel and have expressed an
541opinion have had positive experiences.
542
543The language barrier can cause problems for some people who are not
544comfortable with English. A good grasp of the language can be needed in
545order to get ideas across properly on mailing lists, so it is
546recommended that you check your emails to make sure they make sense in
547English before sending them.
548
549
550Break up your changes
551---------------------
552
553The Linux kernel community does not gladly accept large chunks of code
554dropped on it all at once. The changes need to be properly introduced,
555discussed, and broken up into tiny, individual portions. This is almost
556the exact opposite of what companies are used to doing. Your proposal
557should also be introduced very early in the development process, so that
558you can receive feedback on what you are doing. It also lets the
559community feel that you are working with them, and not simply using them
560as a dumping ground for your feature. However, don't send 50 emails at
561one time to a mailing list, your patch series should be smaller than
562that almost all of the time.
563
564The reasons for breaking things up are the following:
565
5661) Small patches increase the likelihood that your patches will be
567 applied, since they don't take much time or effort to verify for
568 correctness. A 5 line patch can be applied by a maintainer with
569 barely a second glance. However, a 500 line patch may take hours to
570 review for correctness (the time it takes is exponentially
571 proportional to the size of the patch, or something).
572
573 Small patches also make it very easy to debug when something goes
574 wrong. It's much easier to back out patches one by one than it is
575 to dissect a very large patch after it's been applied (and broken
576 something).
577
5782) It's important not only to send small patches, but also to rewrite
579 and simplify (or simply re-order) patches before submitting them.
580
581Here is an analogy from kernel developer Al Viro:
582
583 *"Think of a teacher grading homework from a math student. The
584 teacher does not want to see the student's trials and errors
585 before they came up with the solution. They want to see the
586 cleanest, most elegant answer. A good student knows this, and
587 would never submit her intermediate work before the final
588 solution.*
589
590 *The same is true of kernel development. The maintainers and
591 reviewers do not want to see the thought process behind the
592 solution to the problem one is solving. They want to see a
593 simple and elegant solution."*
594
595It may be challenging to keep the balance between presenting an elegant
596solution and working together with the community and discussing your
597unfinished work. Therefore it is good to get early in the process to
598get feedback to improve your work, but also keep your changes in small
599chunks that they may get already accepted, even when your whole task is
600not ready for inclusion now.
601
602Also realize that it is not acceptable to send patches for inclusion
603that are unfinished and will be "fixed up later."
604
605
606Justify your change
607-------------------
608
609Along with breaking up your patches, it is very important for you to let
610the Linux community know why they should add this change. New features
611must be justified as being needed and useful.
612
613
614Document your change
615--------------------
616
617When sending in your patches, pay special attention to what you say in
618the text in your email. This information will become the ChangeLog
619information for the patch, and will be preserved for everyone to see for
620all time. It should describe the patch completely, containing:
621
622 - why the change is necessary
623 - the overall design approach in the patch
624 - implementation details
625 - testing results
626
627For more details on what this should all look like, please see the
628ChangeLog section of the document:
629
630 "The Perfect Patch"
631 http://www.ozlabs.org/~akpm/stuff/tpp.txt
632
633
634All of these things are sometimes very hard to do. It can take years to
635perfect these practices (if at all). It's a continuous process of
636improvement that requires a lot of patience and determination. But
637don't give up, it's possible. Many have done it before, and each had to
638start exactly where you are now.
639
640
641
642
643----------
644
645Thanks to Paolo Ciarrocchi who allowed the "Development Process"
646(https://lwn.net/Articles/94386/) section
647to be based on text he had written, and to Randy Dunlap and Gerrit
648Huizenga for some of the list of things you should and should not say.
649Also thanks to Pat Mochel, Hanna Linder, Randy Dunlap, Kay Sievers,
650Vojtech Pavlik, Jan Kara, Josh Boyer, Kees Cook, Andrew Morton, Andi
651Kleen, Vadim Lobanov, Jesper Juhl, Adrian Bunk, Keri Harris, Frans Pop,
652David A. Wheeler, Junio Hamano, Michael Kerrisk, and Alex Shepard for
653their review, comments, and contributions. Without their help, this
654document would not have been possible.
655
656
657
658Maintainer: Greg Kroah-Hartman <greg@kroah.com>