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 If anything in this document becomes out of date, please send in patches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