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 If anything in this document becomes out of date, please send in patches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