ia64/linux-2.6.18-xen.hg

view Documentation/HOWTO @ 854:950b9eb27661

usbback: fix urb interval value for interrupt urbs.

Signed-off-by: Noboru Iwamatsu <n_iwamatsu@jp.fujitsu.com>
author Keir Fraser <keir.fraser@citrix.com>
date Mon Apr 06 13:51:20 2009 +0100 (2009-04-06)
parents 831230e53067
children
line source
1 HOWTO do Linux kernel development
2 ---------------------------------
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.
10 If anything in this document becomes out of date, please send in patches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.
15 Introduction
16 ------------
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.
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]
34 The kernel is written using GNU C and the GNU toolchain. While it
35 adheres to the ISO C89 standard, it uses a number of extensions that are
36 not featured in the standard. The kernel is a freestanding C
37 environment, with no reliance on the standard C library, so some
38 portions of the C standard are not supported. Arbitrary long long
39 divisions and floating point are not allowed. It can sometimes be
40 difficult to understand the assumptions the kernel has on the toolchain
41 and the extensions that it uses, and unfortunately there is no
42 definitive reference for them. Please check the gcc info pages (`info
43 gcc`) for some information on them.
45 Please remember that you are trying to learn how to work with the
46 existing development community. It is a diverse group of people, with
47 high standards for coding, style and procedure. These standards have
48 been created over time based on what they have found to work best for
49 such a large and geographically dispersed team. Try to learn as much as
50 possible about these standards ahead of time, as they are well
51 documented; do not expect people to adapt to you or your company's way
52 of doing things.
55 Legal Issues
56 ------------
58 The Linux kernel source code is released under the GPL. Please see the
59 file, COPYING, in the main directory of the source tree, for details on
60 the license. If you have further questions about the license, please
61 contact a lawyer, and do not ask on the Linux kernel mailing list. The
62 people on the mailing lists are not lawyers, and you should not rely on
63 their statements on legal matters.
65 For common questions and answers about the GPL, please see:
66 http://www.gnu.org/licenses/gpl-faq.html
69 Documentation
70 ------------
72 The Linux kernel source tree has a large range of documents that are
73 invaluable for learning how to interact with the kernel community. When
74 new features are added to the kernel, it is recommended that new
75 documentation files are also added which explain how to use the feature.
76 When a kernel change causes the interface that the kernel exposes to
77 userspace to change, it is recommended that you send the information or
78 a patch to the manual pages explaining the change to the manual pages
79 maintainer at mtk-manpages@gmx.net.
81 Here is a list of files that are in the kernel source tree that are
82 required reading:
83 README
84 This file gives a short background on the Linux kernel and describes
85 what is necessary to do to configure and build the kernel. People
86 who are new to the kernel should start here.
88 Documentation/Changes
89 This file gives a list of the minimum levels of various software
90 packages that are necessary to build and run the kernel
91 successfully.
93 Documentation/CodingStyle
94 This describes the Linux kernel coding style, and some of the
95 rationale behind it. All new code is expected to follow the
96 guidelines in this document. Most maintainers will only accept
97 patches if these rules are followed, and many people will only
98 review code if it is in the proper style.
100 Documentation/SubmittingPatches
101 Documentation/SubmittingDrivers
102 These files describe in explicit detail how to successfully create
103 and send a patch, including (but not limited to):
104 - Email contents
105 - Email format
106 - Who to send it to
107 Following these rules will not guarantee success (as all patches are
108 subject to scrutiny for content and style), but not following them
109 will almost always prevent it.
111 Other excellent descriptions of how to create patches properly are:
112 "The Perfect Patch"
113 http://www.zip.com.au/~akpm/linux/patches/stuff/tpp.txt
114 "Linux kernel patch submission format"
115 http://linux.yyz.us/patch-format.html
117 Documentation/stable_api_nonsense.txt
118 This file describes the rationale behind the conscious decision to
119 not have a stable API within the kernel, including things like:
120 - Subsystem shim-layers (for compatibility?)
121 - Driver portability between Operating Systems.
122 - Mitigating rapid change within the kernel source tree (or
123 preventing rapid change)
124 This document is crucial for understanding the Linux development
125 philosophy and is very important for people moving to Linux from
126 development on other Operating Systems.
128 Documentation/SecurityBugs
129 If you feel you have found a security problem in the Linux kernel,
130 please follow the steps in this document to help notify the kernel
131 developers, and help solve the issue.
133 Documentation/ManagementStyle
134 This document describes how Linux kernel maintainers operate and the
135 shared ethos behind their methodologies. This is important reading
136 for anyone new to kernel development (or anyone simply curious about
137 it), as it resolves a lot of common misconceptions and confusion
138 about the unique behavior of kernel maintainers.
140 Documentation/stable_kernel_rules.txt
141 This file describes the rules on how the stable kernel releases
142 happen, and what to do if you want to get a change into one of these
143 releases.
145 Documentation/kernel-docs.txt
146 A list of external documentation that pertains to kernel
147 development. Please consult this list if you do not find what you
148 are looking for within the in-kernel documentation.
150 Documentation/applying-patches.txt
151 A good introduction describing exactly what a patch is and how to
152 apply it to the different development branches of the kernel.
154 The kernel also has a large number of documents that can be
155 automatically generated from the source code itself. This includes a
156 full description of the in-kernel API, and rules on how to handle
157 locking properly. The documents will be created in the
158 Documentation/DocBook/ directory and can be generated as PDF,
159 Postscript, HTML, and man pages by running:
160 make pdfdocs
161 make psdocs
162 make htmldocs
163 make mandocs
164 respectively from the main kernel source directory.
167 Becoming A Kernel Developer
168 ---------------------------
170 If you do not know anything about Linux kernel development, you should
171 look at the Linux KernelNewbies project:
172 http://kernelnewbies.org
173 It consists of a helpful mailing list where you can ask almost any type
174 of basic kernel development question (make sure to search the archives
175 first, before asking something that has already been answered in the
176 past.) It also has an IRC channel that you can use to ask questions in
177 real-time, and a lot of helpful documentation that is useful for
178 learning about Linux kernel development.
180 The website has basic information about code organization, subsystems,
181 and current projects (both in-tree and out-of-tree). It also describes
182 some basic logistical information, like how to compile a kernel and
183 apply a patch.
185 If you do not know where you want to start, but you want to look for
186 some task to start doing to join into the kernel development community,
187 go to the Linux Kernel Janitor's project:
188 http://janitor.kernelnewbies.org/
189 It is a great place to start. It describes a list of relatively simple
190 problems that need to be cleaned up and fixed within the Linux kernel
191 source tree. Working with the developers in charge of this project, you
192 will learn the basics of getting your patch into the Linux kernel tree,
193 and possibly be pointed in the direction of what to go work on next, if
194 you do not already have an idea.
196 If you already have a chunk of code that you want to put into the kernel
197 tree, but need some help getting it in the proper form, the
198 kernel-mentors project was created to help you out with this. It is a
199 mailing list, and can be found at:
200 http://selenic.com/mailman/listinfo/kernel-mentors
202 Before making any actual modifications to the Linux kernel code, it is
203 imperative to understand how the code in question works. For this
204 purpose, nothing is better than reading through it directly (most tricky
205 bits are commented well), perhaps even with the help of specialized
206 tools. One such tool that is particularly recommended is the Linux
207 Cross-Reference project, which is able to present source code in a
208 self-referential, indexed webpage format. An excellent up-to-date
209 repository of the kernel code may be found at:
210 http://sosdg.org/~coywolf/lxr/
213 The development process
214 -----------------------
216 Linux kernel development process currently consists of a few different
217 main kernel "branches" and lots of different subsystem-specific kernel
218 branches. These different branches are:
219 - main 2.6.x kernel tree
220 - 2.6.x.y -stable kernel tree
221 - 2.6.x -git kernel patches
222 - 2.6.x -mm kernel patches
223 - subsystem specific kernel trees and patches
225 2.6.x kernel tree
226 -----------------
227 2.6.x kernels are maintained by Linus Torvalds, and can be found on
228 kernel.org in the pub/linux/kernel/v2.6/ directory. Its development
229 process is as follows:
230 - As soon as a new kernel is released a two weeks window is open,
231 during this period of time maintainers can submit big diffs to
232 Linus, usually the patches that have already been included in the
233 -mm kernel for a few weeks. The preferred way to submit big changes
234 is using git (the kernel's source management tool, more information
235 can be found at http://git.or.cz/) but plain patches are also just
236 fine.
237 - After two weeks a -rc1 kernel is released it is now possible to push
238 only patches that do not include new features that could affect the
239 stability of the whole kernel. Please note that a whole new driver
240 (or filesystem) might be accepted after -rc1 because there is no
241 risk of causing regressions with such a change as long as the change
242 is self-contained and does not affect areas outside of the code that
243 is being added. git can be used to send patches to Linus after -rc1
244 is released, but the patches need to also be sent to a public
245 mailing list for review.
246 - A new -rc is released whenever Linus deems the current git tree to
247 be in a reasonably sane state adequate for testing. The goal is to
248 release a new -rc kernel every week.
249 - Process continues until the kernel is considered "ready", the
250 process should last around 6 weeks.
252 It is worth mentioning what Andrew Morton wrote on the linux-kernel
253 mailing list about kernel releases:
254 "Nobody knows when a kernel will be released, because it's
255 released according to perceived bug status, not according to a
256 preconceived timeline."
258 2.6.x.y -stable kernel tree
259 ---------------------------
260 Kernels with 4 digit versions are -stable kernels. They contain
261 relatively small and critical fixes for security problems or significant
262 regressions discovered in a given 2.6.x kernel.
264 This is the recommended branch for users who want the most recent stable
265 kernel and are not interested in helping test development/experimental
266 versions.
268 If no 2.6.x.y kernel is available, then the highest numbered 2.6.x
269 kernel is the current stable kernel.
271 2.6.x.y are maintained by the "stable" team <stable@kernel.org>, and are
272 released almost every other week.
274 The file Documentation/stable_kernel_rules.txt in the kernel tree
275 documents what kinds of changes are acceptable for the -stable tree, and
276 how the release process works.
278 2.6.x -git patches
279 ------------------
280 These are daily snapshots of Linus' kernel tree which are managed in a
281 git repository (hence the name.) These patches are usually released
282 daily and represent the current state of Linus' tree. They are more
283 experimental than -rc kernels since they are generated automatically
284 without even a cursory glance to see if they are sane.
286 2.6.x -mm kernel patches
287 ------------------------
288 These are experimental kernel patches released by Andrew Morton. Andrew
289 takes all of the different subsystem kernel trees and patches and mushes
290 them together, along with a lot of patches that have been plucked from
291 the linux-kernel mailing list. This tree serves as a proving ground for
292 new features and patches. Once a patch has proved its worth in -mm for
293 a while Andrew or the subsystem maintainer pushes it on to Linus for
294 inclusion in mainline.
296 It is heavily encouraged that all new patches get tested in the -mm tree
297 before they are sent to Linus for inclusion in the main kernel tree.
299 These kernels are not appropriate for use on systems that are supposed
300 to be stable and they are more risky to run than any of the other
301 branches.
303 If you wish to help out with the kernel development process, please test
304 and use these kernel releases and provide feedback to the linux-kernel
305 mailing list if you have any problems, and if everything works properly.
307 In addition to all the other experimental patches, these kernels usually
308 also contain any changes in the mainline -git kernels available at the
309 time of release.
311 The -mm kernels are not released on a fixed schedule, but usually a few
312 -mm kernels are released in between each -rc kernel (1 to 3 is common).
314 Subsystem Specific kernel trees and patches
315 -------------------------------------------
316 A number of the different kernel subsystem developers expose their
317 development trees so that others can see what is happening in the
318 different areas of the kernel. These trees are pulled into the -mm
319 kernel releases as described above.
321 Here is a list of some of the different kernel trees available:
322 git trees:
323 - Kbuild development tree, Sam Ravnborg <sam@ravnborg.org>
324 kernel.org:/pub/scm/linux/kernel/git/sam/kbuild.git
326 - ACPI development tree, Len Brown <len.brown@intel.com>
327 kernel.org:/pub/scm/linux/kernel/git/lenb/linux-acpi-2.6.git
329 - Block development tree, Jens Axboe <axboe@suse.de>
330 kernel.org:/pub/scm/linux/kernel/git/axboe/linux-2.6-block.git
332 - DRM development tree, Dave Airlie <airlied@linux.ie>
333 kernel.org:/pub/scm/linux/kernel/git/airlied/drm-2.6.git
335 - ia64 development tree, Tony Luck <tony.luck@intel.com>
336 kernel.org:/pub/scm/linux/kernel/git/aegl/linux-2.6.git
338 - ieee1394 development tree, Jody McIntyre <scjody@modernduck.com>
339 kernel.org:/pub/scm/linux/kernel/git/scjody/ieee1394.git
341 - infiniband, Roland Dreier <rolandd@cisco.com>
342 kernel.org:/pub/scm/linux/kernel/git/roland/infiniband.git
344 - libata, Jeff Garzik <jgarzik@pobox.com>
345 kernel.org:/pub/scm/linux/kernel/git/jgarzik/libata-dev.git
347 - network drivers, Jeff Garzik <jgarzik@pobox.com>
348 kernel.org:/pub/scm/linux/kernel/git/jgarzik/netdev-2.6.git
350 - pcmcia, Dominik Brodowski <linux@dominikbrodowski.net>
351 kernel.org:/pub/scm/linux/kernel/git/brodo/pcmcia-2.6.git
353 - SCSI, James Bottomley <James.Bottomley@SteelEye.com>
354 kernel.org:/pub/scm/linux/kernel/git/jejb/scsi-misc-2.6.git
356 Other git kernel trees can be found listed at http://kernel.org/git
358 quilt trees:
359 - USB, PCI, Driver Core, and I2C, Greg Kroah-Hartman <gregkh@suse.de>
360 kernel.org/pub/linux/kernel/people/gregkh/gregkh-2.6/
363 Bug Reporting
364 -------------
366 bugzilla.kernel.org is where the Linux kernel developers track kernel
367 bugs. Users are encouraged to report all bugs that they find in this
368 tool. For details on how to use the kernel bugzilla, please see:
369 http://test.kernel.org/bugzilla/faq.html
371 The file REPORTING-BUGS in the main kernel source directory has a good
372 template for how to report a possible kernel bug, and details what kind
373 of information is needed by the kernel developers to help track down the
374 problem.
377 Mailing lists
378 -------------
380 As some of the above documents describe, the majority of the core kernel
381 developers participate on the Linux Kernel Mailing list. Details on how
382 to subscribe and unsubscribe from the list can be found at:
383 http://vger.kernel.org/vger-lists.html#linux-kernel
384 There are archives of the mailing list on the web in many different
385 places. Use a search engine to find these archives. For example:
386 http://dir.gmane.org/gmane.linux.kernel
387 It is highly recommended that you search the archives about the topic
388 you want to bring up, before you post it to the list. A lot of things
389 already discussed in detail are only recorded at the mailing list
390 archives.
392 Most of the individual kernel subsystems also have their own separate
393 mailing list where they do their development efforts. See the
394 MAINTAINERS file for a list of what these lists are for the different
395 groups.
397 Many of the lists are hosted on kernel.org. Information on them can be
398 found at:
399 http://vger.kernel.org/vger-lists.html
401 Please remember to follow good behavioral habits when using the lists.
402 Though a bit cheesy, the following URL has some simple guidelines for
403 interacting with the list (or any list):
404 http://www.albion.com/netiquette/
406 If multiple people respond to your mail, the CC: list of recipients may
407 get pretty large. Don't remove anybody from the CC: list without a good
408 reason, or don't reply only to the list address. Get used to receiving the
409 mail twice, one from the sender and the one from the list, and don't try
410 to tune that by adding fancy mail-headers, people will not like it.
412 Remember to keep the context and the attribution of your replies intact,
413 keep the "John Kernelhacker wrote ...:" lines at the top of your reply, and
414 add your statements between the individual quoted sections instead of
415 writing at the top of the mail.
417 If you add patches to your mail, make sure they are plain readable text
418 as stated in Documentation/SubmittingPatches. Kernel developers don't
419 want to deal with attachments or compressed patches; they may want
420 to comment on individual lines of your patch, which works only that way.
421 Make sure you use a mail program that does not mangle spaces and tab
422 characters. A good first test is to send the mail to yourself and try
423 to apply your own patch by yourself. If that doesn't work, get your
424 mail program fixed or change it until it works.
426 Above all, please remember to show respect to other subscribers.
429 Working with the community
430 --------------------------
432 The goal of the kernel community is to provide the best possible kernel
433 there is. When you submit a patch for acceptance, it will be reviewed
434 on its technical merits and those alone. So, what should you be
435 expecting?
436 - criticism
437 - comments
438 - requests for change
439 - requests for justification
440 - silence
442 Remember, this is part of getting your patch into the kernel. You have
443 to be able to take criticism and comments about your patches, evaluate
444 them at a technical level and either rework your patches or provide
445 clear and concise reasoning as to why those changes should not be made.
446 If there are no responses to your posting, wait a few days and try
447 again, sometimes things get lost in the huge volume.
449 What should you not do?
450 - expect your patch to be accepted without question
451 - become defensive
452 - ignore comments
453 - resubmit the patch without making any of the requested changes
455 In a community that is looking for the best technical solution possible,
456 there will always be differing opinions on how beneficial a patch is.
457 You have to be cooperative, and willing to adapt your idea to fit within
458 the kernel. Or at least be willing to prove your idea is worth it.
459 Remember, being wrong is acceptable as long as you are willing to work
460 toward a solution that is right.
462 It is normal that the answers to your first patch might simply be a list
463 of a dozen things you should correct. This does _not_ imply that your
464 patch will not be accepted, and it is _not_ meant against you
465 personally. Simply correct all issues raised against your patch and
466 resend it.
469 Differences between the kernel community and corporate structures
470 -----------------------------------------------------------------
472 The kernel community works differently than most traditional corporate
473 development environments. Here are a list of things that you can try to
474 do to try to avoid problems:
475 Good things to say regarding your proposed changes:
476 - "This solves multiple problems."
477 - "This deletes 2000 lines of code."
478 - "Here is a patch that explains what I am trying to describe."
479 - "I tested it on 5 different architectures..."
480 - "Here is a series of small patches that..."
481 - "This increases performance on typical machines..."
483 Bad things you should avoid saying:
484 - "We did it this way in AIX/ptx/Solaris, so therefore it must be
485 good..."
486 - "I've being doing this for 20 years, so..."
487 - "This is required for my company to make money"
488 - "This is for our Enterprise product line."
489 - "Here is my 1000 page design document that describes my idea"
490 - "I've been working on this for 6 months..."
491 - "Here's a 5000 line patch that..."
492 - "I rewrote all of the current mess, and here it is..."
493 - "I have a deadline, and this patch needs to be applied now."
495 Another way the kernel community is different than most traditional
496 software engineering work environments is the faceless nature of
497 interaction. One benefit of using email and irc as the primary forms of
498 communication is the lack of discrimination based on gender or race.
499 The Linux kernel work environment is accepting of women and minorities
500 because all you are is an email address. The international aspect also
501 helps to level the playing field because you can't guess gender based on
502 a person's name. A man may be named Andrea and a woman may be named Pat.
503 Most women who have worked in the Linux kernel and have expressed an
504 opinion have had positive experiences.
506 The language barrier can cause problems for some people who are not
507 comfortable with English. A good grasp of the language can be needed in
508 order to get ideas across properly on mailing lists, so it is
509 recommended that you check your emails to make sure they make sense in
510 English before sending them.
513 Break up your changes
514 ---------------------
516 The Linux kernel community does not gladly accept large chunks of code
517 dropped on it all at once. The changes need to be properly introduced,
518 discussed, and broken up into tiny, individual portions. This is almost
519 the exact opposite of what companies are used to doing. Your proposal
520 should also be introduced very early in the development process, so that
521 you can receive feedback on what you are doing. It also lets the
522 community feel that you are working with them, and not simply using them
523 as a dumping ground for your feature. However, don't send 50 emails at
524 one time to a mailing list, your patch series should be smaller than
525 that almost all of the time.
527 The reasons for breaking things up are the following:
529 1) Small patches increase the likelihood that your patches will be
530 applied, since they don't take much time or effort to verify for
531 correctness. A 5 line patch can be applied by a maintainer with
532 barely a second glance. However, a 500 line patch may take hours to
533 review for correctness (the time it takes is exponentially
534 proportional to the size of the patch, or something).
536 Small patches also make it very easy to debug when something goes
537 wrong. It's much easier to back out patches one by one than it is
538 to dissect a very large patch after it's been applied (and broken
539 something).
541 2) It's important not only to send small patches, but also to rewrite
542 and simplify (or simply re-order) patches before submitting them.
544 Here is an analogy from kernel developer Al Viro:
545 "Think of a teacher grading homework from a math student. The
546 teacher does not want to see the student's trials and errors
547 before they came up with the solution. They want to see the
548 cleanest, most elegant answer. A good student knows this, and
549 would never submit her intermediate work before the final
550 solution."
552 The same is true of kernel development. The maintainers and
553 reviewers do not want to see the thought process behind the
554 solution to the problem one is solving. They want to see a
555 simple and elegant solution."
557 It may be challenging to keep the balance between presenting an elegant
558 solution and working together with the community and discussing your
559 unfinished work. Therefore it is good to get early in the process to
560 get feedback to improve your work, but also keep your changes in small
561 chunks that they may get already accepted, even when your whole task is
562 not ready for inclusion now.
564 Also realize that it is not acceptable to send patches for inclusion
565 that are unfinished and will be "fixed up later."
568 Justify your change
569 -------------------
571 Along with breaking up your patches, it is very important for you to let
572 the Linux community know why they should add this change. New features
573 must be justified as being needed and useful.
576 Document your change
577 --------------------
579 When sending in your patches, pay special attention to what you say in
580 the text in your email. This information will become the ChangeLog
581 information for the patch, and will be preserved for everyone to see for
582 all time. It should describe the patch completely, containing:
583 - why the change is necessary
584 - the overall design approach in the patch
585 - implementation details
586 - testing results
588 For more details on what this should all look like, please see the
589 ChangeLog section of the document:
590 "The Perfect Patch"
591 http://www.zip.com.au/~akpm/linux/patches/stuff/tpp.txt
596 All of these things are sometimes very hard to do. It can take years to
597 perfect these practices (if at all). It's a continuous process of
598 improvement that requires a lot of patience and determination. But
599 don't give up, it's possible. Many have done it before, and each had to
600 start exactly where you are now.
605 ----------
606 Thanks to Paolo Ciarrocchi who allowed the "Development Process"
607 (http://linux.tar.bz/articles/2.6-development_process) section
608 to be based on text he had written, and to Randy Dunlap and Gerrit
609 Huizenga for some of the list of things you should and should not say.
610 Also thanks to Pat Mochel, Hanna Linder, Randy Dunlap, Kay Sievers,
611 Vojtech Pavlik, Jan Kara, Josh Boyer, Kees Cook, Andrew Morton, Andi
612 Kleen, Vadim Lobanov, Jesper Juhl, Adrian Bunk, Keri Harris, Frans Pop,
613 David A. Wheeler, Junio Hamano, Michael Kerrisk, and Alex Shepard for
614 their review, comments, and contributions. Without their help, this
615 document would not have been possible.
619 Maintainer: Greg Kroah-Hartman <greg@kroah.com>