3 HOWTO do Linux kernel development
4 =================================
6 This is the be-all, end-all document on this topic. It contains
7 instructions on how to become a Linux kernel developer and how to learn
8 to work with the Linux kernel development community. It tries to not
9 contain anything related to the technical aspects of kernel programming,
10 but will help point you in the right direction for that.
12 If anything in this document becomes out of date, please send in patches
13 to the maintainer of this file, who is listed at the bottom of the
20 So, you want to learn how to become a Linux kernel developer? Or you
21 have been told by your manager, "Go write a Linux driver for this
22 device." This document's goal is to teach you everything you need to
23 know to achieve this by describing the process you need to go through,
24 and hints on how to work with the community. It will also try to
25 explain some of the reasons why the community works like it does.
27 The kernel is written mostly in C, with some architecture-dependent
28 parts written in assembly. A good understanding of C is required for
29 kernel development. Assembly (any architecture) is not required unless
30 you plan to do low-level development for that architecture. Though they
31 are not a good substitute for a solid C education and/or years of
32 experience, the following books are good for, if anything, reference:
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]
38 The kernel is written using GNU C and the GNU toolchain. While it
39 adheres to the ISO C89 standard, it uses a number of extensions that are
40 not featured in the standard. The kernel is a freestanding C
41 environment, with no reliance on the standard C library, so some
42 portions of the C standard are not supported. Arbitrary long long
43 divisions and floating point are not allowed. It can sometimes be
44 difficult to understand the assumptions the kernel has on the toolchain
45 and the extensions that it uses, and unfortunately there is no
46 definitive reference for them. Please check the gcc info pages (`info
47 gcc`) for some information on them.
49 Please remember that you are trying to learn how to work with the
50 existing development community. It is a diverse group of people, with
51 high standards for coding, style and procedure. These standards have
52 been created over time based on what they have found to work best for
53 such a large and geographically dispersed team. Try to learn as much as
54 possible about these standards ahead of time, as they are well
55 documented; do not expect people to adapt to you or your company's way
62 The Linux kernel source code is released under the GPL. Please see the file
63 COPYING in the main directory of the source tree. The Linux kernel licensing
64 rules and how to use `SPDX <https://spdx.org/>`_ identifiers in source code are
65 descibed in :ref:`Documentation/process/license-rules.rst <kernel_licensing>`.
66 If you have further questions about the license, please contact a lawyer, and do
67 not ask on the Linux kernel mailing list. The people on the mailing lists are
68 not lawyers, and you should not rely on their statements on legal matters.
70 For common questions and answers about the GPL, please see:
72 https://www.gnu.org/licenses/gpl-faq.html
78 The Linux kernel source tree has a large range of documents that are
79 invaluable for learning how to interact with the kernel community. When
80 new features are added to the kernel, it is recommended that new
81 documentation files are also added which explain how to use the feature.
82 When a kernel change causes the interface that the kernel exposes to
83 userspace to change, it is recommended that you send the information or
84 a patch to the manual pages explaining the change to the manual pages
85 maintainer at mtk.manpages@gmail.com, and CC the list
86 linux-api@vger.kernel.org.
88 Here is a list of files that are in the kernel source tree that are
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.
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
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.
108 :ref:`Documentation/process/submitting-patches.rst <submittingpatches>` and :ref:`Documentation/process/submitting-drivers.rst <submittingdrivers>`
109 These files describe in explicit detail how to successfully create
110 and send a patch, including (but not limited to):
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.
120 Other excellent descriptions of how to create patches properly are:
123 https://www.ozlabs.org/~akpm/stuff/tpp.txt
125 "Linux kernel patch submission format"
126 http://linux.yyz.us/patch-format.html
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:
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)
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.
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.
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.
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
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.
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.
167 The kernel also has a large number of documents that can be
168 automatically generated from the source code itself or from
169 ReStructuredText markups (ReST), like this one. This includes a
170 full description of the in-kernel API, and rules on how to handle
173 All such documents can be generated as PDF or HTML by running::
178 respectively from the main kernel source directory.
180 The documents that uses ReST markup will be generated at Documentation/output.
181 They can also be generated on LaTeX and ePub formats with::
186 Becoming A Kernel Developer
187 ---------------------------
189 If you do not know anything about Linux kernel development, you should
190 look at the Linux KernelNewbies project:
192 https://kernelnewbies.org
194 It consists of a helpful mailing list where you can ask almost any type
195 of basic kernel development question (make sure to search the archives
196 first, before asking something that has already been answered in the
197 past.) It also has an IRC channel that you can use to ask questions in
198 real-time, and a lot of helpful documentation that is useful for
199 learning about Linux kernel development.
201 The website has basic information about code organization, subsystems,
202 and current projects (both in-tree and out-of-tree). It also describes
203 some basic logistical information, like how to compile a kernel and
206 If you do not know where you want to start, but you want to look for
207 some task to start doing to join into the kernel development community,
208 go to the Linux Kernel Janitor's project:
210 https://kernelnewbies.org/KernelJanitors
212 It is a great place to start. It describes a list of relatively simple
213 problems that need to be cleaned up and fixed within the Linux kernel
214 source tree. Working with the developers in charge of this project, you
215 will learn the basics of getting your patch into the Linux kernel tree,
216 and possibly be pointed in the direction of what to go work on next, if
217 you do not already have an idea.
219 Before making any actual modifications to the Linux kernel code, it is
220 imperative to understand how the code in question works. For this
221 purpose, nothing is better than reading through it directly (most tricky
222 bits are commented well), perhaps even with the help of specialized
223 tools. One such tool that is particularly recommended is the Linux
224 Cross-Reference project, which is able to present source code in a
225 self-referential, indexed webpage format. An excellent up-to-date
226 repository of the kernel code may be found at:
228 http://lxr.free-electrons.com/
231 The development process
232 -----------------------
234 Linux kernel development process currently consists of a few different
235 main kernel "branches" and lots of different subsystem-specific kernel
236 branches. These different branches are:
238 - main 4.x kernel tree
239 - 4.x.y -stable kernel tree
240 - 4.x -git kernel patches
241 - subsystem specific kernel trees and patches
242 - the 4.x -next kernel tree for integration tests
247 4.x kernels are maintained by Linus Torvalds, and can be found on
248 https://kernel.org in the pub/linux/kernel/v4.x/ directory. Its development
249 process is as follows:
251 - As soon as a new kernel is released a two weeks window is open,
252 during this period of time maintainers can submit big diffs to
253 Linus, usually the patches that have already been included in the
254 -next kernel for a few weeks. The preferred way to submit big changes
255 is using git (the kernel's source management tool, more information
256 can be found at https://git-scm.com/) but plain patches are also just
258 - After two weeks a -rc1 kernel is released and the focus is on making the
259 new kernel as rock solid as possible. Most of the patches at this point
260 should fix a regression. Bugs that have always existed are not
261 regressions, so only push these kinds of fixes if they are important.
262 Please note that a whole new driver (or filesystem) might be accepted
263 after -rc1 because there is no risk of causing regressions with such a
264 change as long as the change is self-contained and does not affect areas
265 outside of the code that is being added. git can be used to send
266 patches to Linus after -rc1 is released, but the patches need to also be
267 sent to a public mailing list for review.
268 - A new -rc is released whenever Linus deems the current git tree to
269 be in a reasonably sane state adequate for testing. The goal is to
270 release a new -rc kernel every week.
271 - Process continues until the kernel is considered "ready", the
272 process should last around 6 weeks.
274 It is worth mentioning what Andrew Morton wrote on the linux-kernel
275 mailing list about kernel releases:
277 *"Nobody knows when a kernel will be released, because it's
278 released according to perceived bug status, not according to a
279 preconceived timeline."*
281 4.x.y -stable kernel tree
282 ~~~~~~~~~~~~~~~~~~~~~~~~~
284 Kernels with 3-part versions are -stable kernels. They contain
285 relatively small and critical fixes for security problems or significant
286 regressions discovered in a given 4.x kernel.
288 This is the recommended branch for users who want the most recent stable
289 kernel and are not interested in helping test development/experimental
292 If no 4.x.y kernel is available, then the highest numbered 4.x
293 kernel is the current stable kernel.
295 4.x.y are maintained by the "stable" team <stable@vger.kernel.org>, and
296 are released as needs dictate. The normal release period is approximately
297 two weeks, but it can be longer if there are no pressing problems. A
298 security-related problem, instead, can cause a release to happen almost
301 The file :ref:`Documentation/process/stable-kernel-rules.rst <stable_kernel_rules>`
302 in the kernel tree documents what kinds of changes are acceptable for
303 the -stable tree, and how the release process works.
308 These are daily snapshots of Linus' kernel tree which are managed in a
309 git repository (hence the name.) These patches are usually released
310 daily and represent the current state of Linus' tree. They are more
311 experimental than -rc kernels since they are generated automatically
312 without even a cursory glance to see if they are sane.
314 Subsystem Specific kernel trees and patches
315 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
317 The maintainers of the various kernel subsystems --- and also many
318 kernel subsystem developers --- expose their current state of
319 development in source repositories. That way, others can see what is
320 happening in the different areas of the kernel. In areas where
321 development is rapid, a developer may be asked to base his submissions
322 onto such a subsystem kernel tree so that conflicts between the
323 submission and other already ongoing work are avoided.
325 Most of these repositories are git trees, but there are also other SCMs
326 in use, or patch queues being published as quilt series. Addresses of
327 these subsystem repositories are listed in the MAINTAINERS file. Many
328 of them can be browsed at https://git.kernel.org/.
330 Before a proposed patch is committed to such a subsystem tree, it is
331 subject to review which primarily happens on mailing lists (see the
332 respective section below). For several kernel subsystems, this review
333 process is tracked with the tool patchwork. Patchwork offers a web
334 interface which shows patch postings, any comments on a patch or
335 revisions to it, and maintainers can mark patches as under review,
336 accepted, or rejected. Most of these patchwork sites are listed at
337 https://patchwork.kernel.org/.
339 4.x -next kernel tree for integration tests
340 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
342 Before updates from subsystem trees are merged into the mainline 4.x
343 tree, they need to be integration-tested. For this purpose, a special
344 testing repository exists into which virtually all subsystem trees are
345 pulled on an almost daily basis:
347 https://git.kernel.org/?p=linux/kernel/git/next/linux-next.git
349 This way, the -next kernel gives a summary outlook onto what will be
350 expected to go into the mainline kernel at the next merge period.
351 Adventurous testers are very welcome to runtime-test the -next kernel.
357 https://bugzilla.kernel.org is where the Linux kernel developers track kernel
358 bugs. Users are encouraged to report all bugs that they find in this
359 tool. For details on how to use the kernel bugzilla, please see:
361 https://bugzilla.kernel.org/page.cgi?id=faq.html
363 The file :ref:`admin-guide/reporting-bugs.rst <reportingbugs>`
364 in the main kernel source directory has a good
365 template for how to report a possible kernel bug, and details what kind
366 of information is needed by the kernel developers to help track down the
373 One of the best ways to put into practice your hacking skills is by fixing
374 bugs reported by other people. Not only you will help to make the kernel
375 more stable, you'll learn to fix real world problems and you will improve
376 your skills, and other developers will be aware of your presence. Fixing
377 bugs is one of the best ways to get merits among other developers, because
378 not many people like wasting time fixing other people's bugs.
380 To work in the already reported bug reports, go to https://bugzilla.kernel.org.
386 As some of the above documents describe, the majority of the core kernel
387 developers participate on the Linux Kernel Mailing list. Details on how
388 to subscribe and unsubscribe from the list can be found at:
390 http://vger.kernel.org/vger-lists.html#linux-kernel
392 There are archives of the mailing list on the web in many different
393 places. Use a search engine to find these archives. For example:
395 http://dir.gmane.org/gmane.linux.kernel
397 It is highly recommended that you search the archives about the topic
398 you want to bring up, before you post it to the list. A lot of things
399 already discussed in detail are only recorded at the mailing list
402 Most of the individual kernel subsystems also have their own separate
403 mailing list where they do their development efforts. See the
404 MAINTAINERS file for a list of what these lists are for the different
407 Many of the lists are hosted on kernel.org. Information on them can be
410 http://vger.kernel.org/vger-lists.html
412 Please remember to follow good behavioral habits when using the lists.
413 Though a bit cheesy, the following URL has some simple guidelines for
414 interacting with the list (or any list):
416 http://www.albion.com/netiquette/
418 If multiple people respond to your mail, the CC: list of recipients may
419 get pretty large. Don't remove anybody from the CC: list without a good
420 reason, or don't reply only to the list address. Get used to receiving the
421 mail twice, one from the sender and the one from the list, and don't try
422 to tune that by adding fancy mail-headers, people will not like it.
424 Remember to keep the context and the attribution of your replies intact,
425 keep the "John Kernelhacker wrote ...:" lines at the top of your reply, and
426 add your statements between the individual quoted sections instead of
427 writing at the top of the mail.
429 If you add patches to your mail, make sure they are plain readable text
430 as stated in :ref:`Documentation/process/submitting-patches.rst <submittingpatches>`.
431 Kernel developers don't want to deal with
432 attachments or compressed patches; they may want to comment on
433 individual lines of your patch, which works only that way. Make sure you
434 use a mail program that does not mangle spaces and tab characters. A
435 good first test is to send the mail to yourself and try to apply your
436 own patch by yourself. If that doesn't work, get your mail program fixed
437 or change it until it works.
439 Above all, please remember to show respect to other subscribers.
442 Working with the community
443 --------------------------
445 The goal of the kernel community is to provide the best possible kernel
446 there is. When you submit a patch for acceptance, it will be reviewed
447 on its technical merits and those alone. So, what should you be
452 - requests for change
453 - requests for justification
456 Remember, this is part of getting your patch into the kernel. You have
457 to be able to take criticism and comments about your patches, evaluate
458 them at a technical level and either rework your patches or provide
459 clear and concise reasoning as to why those changes should not be made.
460 If there are no responses to your posting, wait a few days and try
461 again, sometimes things get lost in the huge volume.
463 What should you not do?
465 - expect your patch to be accepted without question
468 - resubmit the patch without making any of the requested changes
470 In a community that is looking for the best technical solution possible,
471 there will always be differing opinions on how beneficial a patch is.
472 You have to be cooperative, and willing to adapt your idea to fit within
473 the kernel. Or at least be willing to prove your idea is worth it.
474 Remember, being wrong is acceptable as long as you are willing to work
475 toward a solution that is right.
477 It is normal that the answers to your first patch might simply be a list
478 of a dozen things you should correct. This does **not** imply that your
479 patch will not be accepted, and it is **not** meant against you
480 personally. Simply correct all issues raised against your patch and
484 Differences between the kernel community and corporate structures
485 -----------------------------------------------------------------
487 The kernel community works differently than most traditional corporate
488 development environments. Here are a list of things that you can try to
489 do to avoid problems:
491 Good things to say regarding your proposed changes:
493 - "This solves multiple problems."
494 - "This deletes 2000 lines of code."
495 - "Here is a patch that explains what I am trying to describe."
496 - "I tested it on 5 different architectures..."
497 - "Here is a series of small patches that..."
498 - "This increases performance on typical machines..."
500 Bad things you should avoid saying:
502 - "We did it this way in AIX/ptx/Solaris, so therefore it must be
504 - "I've being doing this for 20 years, so..."
505 - "This is required for my company to make money"
506 - "This is for our Enterprise product line."
507 - "Here is my 1000 page design document that describes my idea"
508 - "I've been working on this for 6 months..."
509 - "Here's a 5000 line patch that..."
510 - "I rewrote all of the current mess, and here it is..."
511 - "I have a deadline, and this patch needs to be applied now."
513 Another way the kernel community is different than most traditional
514 software engineering work environments is the faceless nature of
515 interaction. One benefit of using email and irc as the primary forms of
516 communication is the lack of discrimination based on gender or race.
517 The Linux kernel work environment is accepting of women and minorities
518 because all you are is an email address. The international aspect also
519 helps to level the playing field because you can't guess gender based on
520 a person's name. A man may be named Andrea and a woman may be named Pat.
521 Most women who have worked in the Linux kernel and have expressed an
522 opinion have had positive experiences.
524 The language barrier can cause problems for some people who are not
525 comfortable with English. A good grasp of the language can be needed in
526 order to get ideas across properly on mailing lists, so it is
527 recommended that you check your emails to make sure they make sense in
528 English before sending them.
531 Break up your changes
532 ---------------------
534 The Linux kernel community does not gladly accept large chunks of code
535 dropped on it all at once. The changes need to be properly introduced,
536 discussed, and broken up into tiny, individual portions. This is almost
537 the exact opposite of what companies are used to doing. Your proposal
538 should also be introduced very early in the development process, so that
539 you can receive feedback on what you are doing. It also lets the
540 community feel that you are working with them, and not simply using them
541 as a dumping ground for your feature. However, don't send 50 emails at
542 one time to a mailing list, your patch series should be smaller than
543 that almost all of the time.
545 The reasons for breaking things up are the following:
547 1) Small patches increase the likelihood that your patches will be
548 applied, since they don't take much time or effort to verify for
549 correctness. A 5 line patch can be applied by a maintainer with
550 barely a second glance. However, a 500 line patch may take hours to
551 review for correctness (the time it takes is exponentially
552 proportional to the size of the patch, or something).
554 Small patches also make it very easy to debug when something goes
555 wrong. It's much easier to back out patches one by one than it is
556 to dissect a very large patch after it's been applied (and broken
559 2) It's important not only to send small patches, but also to rewrite
560 and simplify (or simply re-order) patches before submitting them.
562 Here is an analogy from kernel developer Al Viro:
564 *"Think of a teacher grading homework from a math student. The
565 teacher does not want to see the student's trials and errors
566 before they came up with the solution. They want to see the
567 cleanest, most elegant answer. A good student knows this, and
568 would never submit her intermediate work before the final
571 *The same is true of kernel development. The maintainers and
572 reviewers do not want to see the thought process behind the
573 solution to the problem one is solving. They want to see a
574 simple and elegant solution."*
576 It may be challenging to keep the balance between presenting an elegant
577 solution and working together with the community and discussing your
578 unfinished work. Therefore it is good to get early in the process to
579 get feedback to improve your work, but also keep your changes in small
580 chunks that they may get already accepted, even when your whole task is
581 not ready for inclusion now.
583 Also realize that it is not acceptable to send patches for inclusion
584 that are unfinished and will be "fixed up later."
590 Along with breaking up your patches, it is very important for you to let
591 the Linux community know why they should add this change. New features
592 must be justified as being needed and useful.
598 When sending in your patches, pay special attention to what you say in
599 the text in your email. This information will become the ChangeLog
600 information for the patch, and will be preserved for everyone to see for
601 all time. It should describe the patch completely, containing:
603 - why the change is necessary
604 - the overall design approach in the patch
605 - implementation details
608 For more details on what this should all look like, please see the
609 ChangeLog section of the document:
612 http://www.ozlabs.org/~akpm/stuff/tpp.txt
615 All of these things are sometimes very hard to do. It can take years to
616 perfect these practices (if at all). It's a continuous process of
617 improvement that requires a lot of patience and determination. But
618 don't give up, it's possible. Many have done it before, and each had to
619 start exactly where you are now.
626 Thanks to Paolo Ciarrocchi who allowed the "Development Process"
627 (https://lwn.net/Articles/94386/) section
628 to be based on text he had written, and to Randy Dunlap and Gerrit
629 Huizenga for some of the list of things you should and should not say.
630 Also thanks to Pat Mochel, Hanna Linder, Randy Dunlap, Kay Sievers,
631 Vojtech Pavlik, Jan Kara, Josh Boyer, Kees Cook, Andrew Morton, Andi
632 Kleen, Vadim Lobanov, Jesper Juhl, Adrian Bunk, Keri Harris, Frans Pop,
633 David A. Wheeler, Junio Hamano, Michael Kerrisk, and Alex Shepard for
634 their review, comments, and contributions. Without their help, this
635 document would not have been possible.
639 Maintainer: Greg Kroah-Hartman <greg@kroah.com>