Based on kernel version 4.1. Page generated on 2015-06-28 12:12 EST.
1 HOWTO do Linux kernel development 2 --------------------------------- 3 4 This is the be-all, end-all document on this topic. It contains 5 instructions on how to become a Linux kernel developer and how to learn 6 to work with the Linux kernel development community. It tries to not 7 contain anything related to the technical aspects of kernel programming, 8 but will help point you in the right direction for that. 9 10 If anything in this document becomes out of date, please send in patches 11 to the maintainer of this file, who is listed at the bottom of the 12 document. 13 14 15 Introduction 16 ------------ 17 18 So, you want to learn how to become a Linux kernel developer? Or you 19 have been told by your manager, "Go write a Linux driver for this 20 device." This document's goal is to teach you everything you need to 21 know to achieve this by describing the process you need to go through, 22 and hints on how to work with the community. It will also try to 23 explain some of the reasons why the community works like it does. 24 25 The kernel is written mostly in C, with some architecture-dependent 26 parts written in assembly. A good understanding of C is required for 27 kernel development. Assembly (any architecture) is not required unless 28 you plan to do low-level development for that architecture. Though they 29 are not a good substitute for a solid C education and/or years of 30 experience, the following books are good for, if anything, reference: 31 - "The C Programming Language" by Kernighan and Ritchie [Prentice Hall] 32 - "Practical C Programming" by Steve Oualline [O'Reilly] 33 - "C: A Reference Manual" by Harbison and Steele [Prentice Hall] 34 35 The kernel is written using GNU C and the GNU toolchain. While it 36 adheres to the ISO C89 standard, it uses a number of extensions that are 37 not featured in the standard. The kernel is a freestanding C 38 environment, with no reliance on the standard C library, so some 39 portions of the C standard are not supported. Arbitrary long long 40 divisions and floating point are not allowed. It can sometimes be 41 difficult to understand the assumptions the kernel has on the toolchain 42 and the extensions that it uses, and unfortunately there is no 43 definitive reference for them. Please check the gcc info pages (`info 44 gcc`) for some information on them. 45 46 Please remember that you are trying to learn how to work with the 47 existing development community. It is a diverse group of people, with 48 high standards for coding, style and procedure. These standards have 49 been created over time based on what they have found to work best for 50 such a large and geographically dispersed team. Try to learn as much as 51 possible about these standards ahead of time, as they are well 52 documented; do not expect people to adapt to you or your company's way 53 of doing things. 54 55 56 Legal Issues 57 ------------ 58 59 The Linux kernel source code is released under the GPL. Please see the 60 file, COPYING, in the main directory of the source tree, for details on 61 the license. If you have further questions about the license, please 62 contact a lawyer, and do not ask on the Linux kernel mailing list. The 63 people on the mailing lists are not lawyers, and you should not rely on 64 their statements on legal matters. 65 66 For common questions and answers about the GPL, please see: 67 http://www.gnu.org/licenses/gpl-faq.html 68 69 70 Documentation 71 ------------ 72 73 The Linux kernel source tree has a large range of documents that are 74 invaluable for learning how to interact with the kernel community. When 75 new features are added to the kernel, it is recommended that new 76 documentation files are also added which explain how to use the feature. 77 When a kernel change causes the interface that the kernel exposes to 78 userspace to change, it is recommended that you send the information or 79 a patch to the manual pages explaining the change to the manual pages 80 maintainer at email@example.com, and CC the list 81 firstname.lastname@example.org. 82 83 Here is a list of files that are in the kernel source tree that are 84 required reading: 85 README 86 This file gives a short background on the Linux kernel and describes 87 what is necessary to do to configure and build the kernel. People 88 who are new to the kernel should start here. 89 90 Documentation/Changes 91 This file gives a list of the minimum levels of various software 92 packages that are necessary to build and run the kernel 93 successfully. 94 95 Documentation/CodingStyle 96 This describes the Linux kernel coding style, and some of the 97 rationale behind it. All new code is expected to follow the 98 guidelines in this document. Most maintainers will only accept 99 patches if these rules are followed, and many people will only 100 review code if it is in the proper style. 101 102 Documentation/SubmittingPatches 103 Documentation/SubmittingDrivers 104 These files describe in explicit detail how to successfully create 105 and send a patch, including (but not limited to): 106 - Email contents 107 - Email format 108 - Who to send it to 109 Following these rules will not guarantee success (as all patches are 110 subject to scrutiny for content and style), but not following them 111 will almost always prevent it. 112 113 Other excellent descriptions of how to create patches properly are: 114 "The Perfect Patch" 115 http://www.ozlabs.org/~akpm/stuff/tpp.txt 116 "Linux kernel patch submission format" 117 http://linux.yyz.us/patch-format.html 118 119 Documentation/stable_api_nonsense.txt 120 This file describes the rationale behind the conscious decision to 121 not have a stable API within the kernel, including things like: 122 - Subsystem shim-layers (for compatibility?) 123 - Driver portability between Operating Systems. 124 - Mitigating rapid change within the kernel source tree (or 125 preventing rapid change) 126 This document is crucial for understanding the Linux development 127 philosophy and is very important for people moving to Linux from 128 development on other Operating Systems. 129 130 Documentation/SecurityBugs 131 If you feel you have found a security problem in the Linux kernel, 132 please follow the steps in this document to help notify the kernel 133 developers, and help solve the issue. 134 135 Documentation/ManagementStyle 136 This document describes how Linux kernel maintainers operate and the 137 shared ethos behind their methodologies. This is important reading 138 for anyone new to kernel development (or anyone simply curious about 139 it), as it resolves a lot of common misconceptions and confusion 140 about the unique behavior of kernel maintainers. 141 142 Documentation/stable_kernel_rules.txt 143 This file describes the rules on how the stable kernel releases 144 happen, and what to do if you want to get a change into one of these 145 releases. 146 147 Documentation/kernel-docs.txt 148 A list of external documentation that pertains to kernel 149 development. Please consult this list if you do not find what you 150 are looking for within the in-kernel documentation. 151 152 Documentation/applying-patches.txt 153 A good introduction describing exactly what a patch is and how to 154 apply it to the different development branches of the kernel. 155 156 The kernel also has a large number of documents that can be 157 automatically generated from the source code itself. This includes a 158 full description of the in-kernel API, and rules on how to handle 159 locking properly. The documents will be created in the 160 Documentation/DocBook/ directory and can be generated as PDF, 161 Postscript, HTML, and man pages by running: 162 make pdfdocs 163 make psdocs 164 make htmldocs 165 make mandocs 166 respectively from the main kernel source directory. 167 168 169 Becoming A Kernel Developer 170 --------------------------- 171 172 If you do not know anything about Linux kernel development, you should 173 look at the Linux KernelNewbies project: 174 http://kernelnewbies.org 175 It consists of a helpful mailing list where you can ask almost any type 176 of basic kernel development question (make sure to search the archives 177 first, before asking something that has already been answered in the 178 past.) It also has an IRC channel that you can use to ask questions in 179 real-time, and a lot of helpful documentation that is useful for 180 learning about Linux kernel development. 181 182 The website has basic information about code organization, subsystems, 183 and current projects (both in-tree and out-of-tree). It also describes 184 some basic logistical information, like how to compile a kernel and 185 apply a patch. 186 187 If you do not know where you want to start, but you want to look for 188 some task to start doing to join into the kernel development community, 189 go to the Linux Kernel Janitor's project: 190 http://kernelnewbies.org/KernelJanitors 191 It is a great place to start. It describes a list of relatively simple 192 problems that need to be cleaned up and fixed within the Linux kernel 193 source tree. Working with the developers in charge of this project, you 194 will learn the basics of getting your patch into the Linux kernel tree, 195 and possibly be pointed in the direction of what to go work on next, if 196 you do not already have an idea. 197 198 If you already have a chunk of code that you want to put into the kernel 199 tree, but need some help getting it in the proper form, the 200 kernel-mentors project was created to help you out with this. It is a 201 mailing list, and can be found at: 202 http://selenic.com/mailman/listinfo/kernel-mentors 203 204 Before making any actual modifications to the Linux kernel code, it is 205 imperative to understand how the code in question works. For this 206 purpose, nothing is better than reading through it directly (most tricky 207 bits are commented well), perhaps even with the help of specialized 208 tools. One such tool that is particularly recommended is the Linux 209 Cross-Reference project, which is able to present source code in a 210 self-referential, indexed webpage format. An excellent up-to-date 211 repository of the kernel code may be found at: 212 http://lxr.linux.no/+trees 213 214 215 The development process 216 ----------------------- 217 218 Linux kernel development process currently consists of a few different 219 main kernel "branches" and lots of different subsystem-specific kernel 220 branches. These different branches are: 221 - main 3.x kernel tree 222 - 3.x.y -stable kernel tree 223 - 3.x -git kernel patches 224 - subsystem specific kernel trees and patches 225 - the 3.x -next kernel tree for integration tests 226 227 3.x kernel tree 228 ----------------- 229 3.x kernels are maintained by Linus Torvalds, and can be found on 230 kernel.org in the pub/linux/kernel/v3.x/ directory. Its development 231 process is as follows: 232 - As soon as a new kernel is released a two weeks window is open, 233 during this period of time maintainers can submit big diffs to 234 Linus, usually the patches that have already been included in the 235 -next kernel for a few weeks. The preferred way to submit big changes 236 is using git (the kernel's source management tool, more information 237 can be found at http://git-scm.com/) but plain patches are also just 238 fine. 239 - After two weeks a -rc1 kernel is released it is now possible to push 240 only patches that do not include new features that could affect the 241 stability of the whole kernel. Please note that a whole new driver 242 (or filesystem) might be accepted after -rc1 because there is no 243 risk of causing regressions with such a change as long as the change 244 is self-contained and does not affect areas outside of the code that 245 is being added. git can be used to send patches to Linus after -rc1 246 is released, but the patches need to also be sent to a public 247 mailing list for review. 248 - A new -rc is released whenever Linus deems the current git tree to 249 be in a reasonably sane state adequate for testing. The goal is to 250 release a new -rc kernel every week. 251 - Process continues until the kernel is considered "ready", the 252 process should last around 6 weeks. 253 - Known regressions in each release are periodically posted to the 254 linux-kernel mailing list. The goal is to reduce the length of 255 that list to zero before declaring the kernel to be "ready," but, in 256 the real world, a small number of regressions often remain at 257 release time. 258 259 It is worth mentioning what Andrew Morton wrote on the linux-kernel 260 mailing list about kernel releases: 261 "Nobody knows when a kernel will be released, because it's 262 released according to perceived bug status, not according to a 263 preconceived timeline." 264 265 3.x.y -stable kernel tree 266 --------------------------- 267 Kernels with 3-part versions are -stable kernels. They contain 268 relatively small and critical fixes for security problems or significant 269 regressions discovered in a given 3.x kernel. 270 271 This is the recommended branch for users who want the most recent stable 272 kernel and are not interested in helping test development/experimental 273 versions. 274 275 If no 3.x.y kernel is available, then the highest numbered 3.x 276 kernel is the current stable kernel. 277 278 3.x.y are maintained by the "stable" team <email@example.com>, and 279 are released as needs dictate. The normal release period is approximately 280 two weeks, but it can be longer if there are no pressing problems. A 281 security-related problem, instead, can cause a release to happen almost 282 instantly. 283 284 The file Documentation/stable_kernel_rules.txt in the kernel tree 285 documents what kinds of changes are acceptable for the -stable tree, and 286 how the release process works. 287 288 3.x -git patches 289 ------------------ 290 These are daily snapshots of Linus' kernel tree which are managed in a 291 git repository (hence the name.) These patches are usually released 292 daily and represent the current state of Linus' tree. They are more 293 experimental than -rc kernels since they are generated automatically 294 without even a cursory glance to see if they are sane. 295 296 Subsystem Specific kernel trees and patches 297 ------------------------------------------- 298 The maintainers of the various kernel subsystems --- and also many 299 kernel subsystem developers --- expose their current state of 300 development in source repositories. That way, others can see what is 301 happening in the different areas of the kernel. In areas where 302 development is rapid, a developer may be asked to base his submissions 303 onto such a subsystem kernel tree so that conflicts between the 304 submission and other already ongoing work are avoided. 305 306 Most of these repositories are git trees, but there are also other SCMs 307 in use, or patch queues being published as quilt series. Addresses of 308 these subsystem repositories are listed in the MAINTAINERS file. Many 309 of them can be browsed at http://git.kernel.org/. 310 311 Before a proposed patch is committed to such a subsystem tree, it is 312 subject to review which primarily happens on mailing lists (see the 313 respective section below). For several kernel subsystems, this review 314 process is tracked with the tool patchwork. Patchwork offers a web 315 interface which shows patch postings, any comments on a patch or 316 revisions to it, and maintainers can mark patches as under review, 317 accepted, or rejected. Most of these patchwork sites are listed at 318 http://patchwork.kernel.org/. 319 320 3.x -next kernel tree for integration tests 321 --------------------------------------------- 322 Before updates from subsystem trees are merged into the mainline 3.x 323 tree, they need to be integration-tested. For this purpose, a special 324 testing repository exists into which virtually all subsystem trees are 325 pulled on an almost daily basis: 326 http://git.kernel.org/?p=linux/kernel/git/next/linux-next.git 327 328 This way, the -next kernel gives a summary outlook onto what will be 329 expected to go into the mainline kernel at the next merge period. 330 Adventurous testers are very welcome to runtime-test the -next kernel. 331 332 333 Bug Reporting 334 ------------- 335 336 bugzilla.kernel.org is where the Linux kernel developers track kernel 337 bugs. Users are encouraged to report all bugs that they find in this 338 tool. For details on how to use the kernel bugzilla, please see: 339 http://bugzilla.kernel.org/page.cgi?id=faq.html 340 341 The file REPORTING-BUGS in the main kernel source directory has a good 342 template for how to report a possible kernel bug, and details what kind 343 of information is needed by the kernel developers to help track down the 344 problem. 345 346 347 Managing bug reports 348 -------------------- 349 350 One of the best ways to put into practice your hacking skills is by fixing 351 bugs reported by other people. Not only you will help to make the kernel 352 more stable, you'll learn to fix real world problems and you will improve 353 your skills, and other developers will be aware of your presence. Fixing 354 bugs is one of the best ways to get merits among other developers, because 355 not many people like wasting time fixing other people's bugs. 356 357 To work in the already reported bug reports, go to http://bugzilla.kernel.org. 358 If you want to be advised of the future bug reports, you can subscribe to the 359 bugme-new mailing list (only new bug reports are mailed here) or to the 360 bugme-janitor mailing list (every change in the bugzilla is mailed here) 361 362 http://lists.linux-foundation.org/mailman/listinfo/bugme-new 363 http://lists.linux-foundation.org/mailman/listinfo/bugme-janitors 364 365 366 367 Mailing lists 368 ------------- 369 370 As some of the above documents describe, the majority of the core kernel 371 developers participate on the Linux Kernel Mailing list. Details on how 372 to subscribe and unsubscribe from the list can be found at: 373 http://vger.kernel.org/vger-lists.html#linux-kernel 374 There are archives of the mailing list on the web in many different 375 places. Use a search engine to find these archives. For example: 376 http://dir.gmane.org/gmane.linux.kernel 377 It is highly recommended that you search the archives about the topic 378 you want to bring up, before you post it to the list. A lot of things 379 already discussed in detail are only recorded at the mailing list 380 archives. 381 382 Most of the individual kernel subsystems also have their own separate 383 mailing list where they do their development efforts. See the 384 MAINTAINERS file for a list of what these lists are for the different 385 groups. 386 387 Many of the lists are hosted on kernel.org. Information on them can be 388 found at: 389 http://vger.kernel.org/vger-lists.html 390 391 Please remember to follow good behavioral habits when using the lists. 392 Though a bit cheesy, the following URL has some simple guidelines for 393 interacting with the list (or any list): 394 http://www.albion.com/netiquette/ 395 396 If multiple people respond to your mail, the CC: list of recipients may 397 get pretty large. Don't remove anybody from the CC: list without a good 398 reason, or don't reply only to the list address. Get used to receiving the 399 mail twice, one from the sender and the one from the list, and don't try 400 to tune that by adding fancy mail-headers, people will not like it. 401 402 Remember to keep the context and the attribution of your replies intact, 403 keep the "John Kernelhacker wrote ...:" lines at the top of your reply, and 404 add your statements between the individual quoted sections instead of 405 writing at the top of the mail. 406 407 If you add patches to your mail, make sure they are plain readable text 408 as stated in Documentation/SubmittingPatches. Kernel developers don't 409 want to deal with attachments or compressed patches; they may want 410 to comment on individual lines of your patch, which works only that way. 411 Make sure you use a mail program that does not mangle spaces and tab 412 characters. A good first test is to send the mail to yourself and try 413 to apply your own patch by yourself. If that doesn't work, get your 414 mail program fixed or change it until it works. 415 416 Above all, please remember to show respect to other subscribers. 417 418 419 Working with the community 420 -------------------------- 421 422 The goal of the kernel community is to provide the best possible kernel 423 there is. When you submit a patch for acceptance, it will be reviewed 424 on its technical merits and those alone. So, what should you be 425 expecting? 426 - criticism 427 - comments 428 - requests for change 429 - requests for justification 430 - silence 431 432 Remember, this is part of getting your patch into the kernel. You have 433 to be able to take criticism and comments about your patches, evaluate 434 them at a technical level and either rework your patches or provide 435 clear and concise reasoning as to why those changes should not be made. 436 If there are no responses to your posting, wait a few days and try 437 again, sometimes things get lost in the huge volume. 438 439 What should you not do? 440 - expect your patch to be accepted without question 441 - become defensive 442 - ignore comments 443 - resubmit the patch without making any of the requested changes 444 445 In a community that is looking for the best technical solution possible, 446 there will always be differing opinions on how beneficial a patch is. 447 You have to be cooperative, and willing to adapt your idea to fit within 448 the kernel. Or at least be willing to prove your idea is worth it. 449 Remember, being wrong is acceptable as long as you are willing to work 450 toward a solution that is right. 451 452 It is normal that the answers to your first patch might simply be a list 453 of a dozen things you should correct. This does _not_ imply that your 454 patch will not be accepted, and it is _not_ meant against you 455 personally. Simply correct all issues raised against your patch and 456 resend it. 457 458 459 Differences between the kernel community and corporate structures 460 ----------------------------------------------------------------- 461 462 The kernel community works differently than most traditional corporate 463 development environments. Here are a list of things that you can try to 464 do to avoid problems: 465 Good things to say regarding your proposed changes: 466 - "This solves multiple problems." 467 - "This deletes 2000 lines of code." 468 - "Here is a patch that explains what I am trying to describe." 469 - "I tested it on 5 different architectures..." 470 - "Here is a series of small patches that..." 471 - "This increases performance on typical machines..." 472 473 Bad things you should avoid saying: 474 - "We did it this way in AIX/ptx/Solaris, so therefore it must be 475 good..." 476 - "I've being doing this for 20 years, so..." 477 - "This is required for my company to make money" 478 - "This is for our Enterprise product line." 479 - "Here is my 1000 page design document that describes my idea" 480 - "I've been working on this for 6 months..." 481 - "Here's a 5000 line patch that..." 482 - "I rewrote all of the current mess, and here it is..." 483 - "I have a deadline, and this patch needs to be applied now." 484 485 Another way the kernel community is different than most traditional 486 software engineering work environments is the faceless nature of 487 interaction. One benefit of using email and irc as the primary forms of 488 communication is the lack of discrimination based on gender or race. 489 The Linux kernel work environment is accepting of women and minorities 490 because all you are is an email address. The international aspect also 491 helps to level the playing field because you can't guess gender based on 492 a person's name. A man may be named Andrea and a woman may be named Pat. 493 Most women who have worked in the Linux kernel and have expressed an 494 opinion have had positive experiences. 495 496 The language barrier can cause problems for some people who are not 497 comfortable with English. A good grasp of the language can be needed in 498 order to get ideas across properly on mailing lists, so it is 499 recommended that you check your emails to make sure they make sense in 500 English before sending them. 501 502 503 Break up your changes 504 --------------------- 505 506 The Linux kernel community does not gladly accept large chunks of code 507 dropped on it all at once. The changes need to be properly introduced, 508 discussed, and broken up into tiny, individual portions. This is almost 509 the exact opposite of what companies are used to doing. Your proposal 510 should also be introduced very early in the development process, so that 511 you can receive feedback on what you are doing. It also lets the 512 community feel that you are working with them, and not simply using them 513 as a dumping ground for your feature. However, don't send 50 emails at 514 one time to a mailing list, your patch series should be smaller than 515 that almost all of the time. 516 517 The reasons for breaking things up are the following: 518 519 1) Small patches increase the likelihood that your patches will be 520 applied, since they don't take much time or effort to verify for 521 correctness. A 5 line patch can be applied by a maintainer with 522 barely a second glance. However, a 500 line patch may take hours to 523 review for correctness (the time it takes is exponentially 524 proportional to the size of the patch, or something). 525 526 Small patches also make it very easy to debug when something goes 527 wrong. It's much easier to back out patches one by one than it is 528 to dissect a very large patch after it's been applied (and broken 529 something). 530 531 2) It's important not only to send small patches, but also to rewrite 532 and simplify (or simply re-order) patches before submitting them. 533 534 Here is an analogy from kernel developer Al Viro: 535 "Think of a teacher grading homework from a math student. The 536 teacher does not want to see the student's trials and errors 537 before they came up with the solution. They want to see the 538 cleanest, most elegant answer. A good student knows this, and 539 would never submit her intermediate work before the final 540 solution." 541 542 The same is true of kernel development. The maintainers and 543 reviewers do not want to see the thought process behind the 544 solution to the problem one is solving. They want to see a 545 simple and elegant solution." 546 547 It may be challenging to keep the balance between presenting an elegant 548 solution and working together with the community and discussing your 549 unfinished work. Therefore it is good to get early in the process to 550 get feedback to improve your work, but also keep your changes in small 551 chunks that they may get already accepted, even when your whole task is 552 not ready for inclusion now. 553 554 Also realize that it is not acceptable to send patches for inclusion 555 that are unfinished and will be "fixed up later." 556 557 558 Justify your change 559 ------------------- 560 561 Along with breaking up your patches, it is very important for you to let 562 the Linux community know why they should add this change. New features 563 must be justified as being needed and useful. 564 565 566 Document your change 567 -------------------- 568 569 When sending in your patches, pay special attention to what you say in 570 the text in your email. This information will become the ChangeLog 571 information for the patch, and will be preserved for everyone to see for 572 all time. It should describe the patch completely, containing: 573 - why the change is necessary 574 - the overall design approach in the patch 575 - implementation details 576 - testing results 577 578 For more details on what this should all look like, please see the 579 ChangeLog section of the document: 580 "The Perfect Patch" 581 http://www.ozlabs.org/~akpm/stuff/tpp.txt 582 583 584 585 586 All of these things are sometimes very hard to do. It can take years to 587 perfect these practices (if at all). It's a continuous process of 588 improvement that requires a lot of patience and determination. But 589 don't give up, it's possible. Many have done it before, and each had to 590 start exactly where you are now. 591 592 593 594 595 ---------- 596 Thanks to Paolo Ciarrocchi who allowed the "Development Process" 597 (http://lwn.net/Articles/94386/) section 598 to be based on text he had written, and to Randy Dunlap and Gerrit 599 Huizenga for some of the list of things you should and should not say. 600 Also thanks to Pat Mochel, Hanna Linder, Randy Dunlap, Kay Sievers, 601 Vojtech Pavlik, Jan Kara, Josh Boyer, Kees Cook, Andrew Morton, Andi 602 Kleen, Vadim Lobanov, Jesper Juhl, Adrian Bunk, Keri Harris, Frans Pop, 603 David A. Wheeler, Junio Hamano, Michael Kerrisk, and Alex Shepard for 604 their review, comments, and contributions. Without their help, this 605 document would not have been possible. 606 607 608 609 Maintainer: Greg Kroah-Hartman <firstname.lastname@example.org>