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diff --git a/Documentation/00-INDEX b/Documentation/00-INDEX index 5b5aba404aac..1f3dbdfc9ae3 100644 --- a/Documentation/00-INDEX +++ b/Documentation/00-INDEX @@ -21,6 +21,9 @@ Changes - list of changes that break older software packages. CodingStyle - how the boss likes the C code in the kernel to look. +development-process/ + - An extended tutorial on how to work with the kernel development + process. DMA-API.txt - DMA API, pci_ API & extensions for non-consistent memory machines. DMA-ISA-LPC.txt diff --git a/Documentation/development-process/1.Intro b/Documentation/development-process/1.Intro new file mode 100644 index 000000000000..8cc2cba2b10d --- /dev/null +++ b/Documentation/development-process/1.Intro @@ -0,0 +1,274 @@ +1: A GUIDE TO THE KERNEL DEVELOPMENT PROCESS + +The purpose of this document is to help developers (and their managers) +work with the development community with a minimum of frustration. It is +an attempt to document how this community works in a way which is +accessible to those who are not intimately familiar with Linux kernel +development (or, indeed, free software development in general). While +there is some technical material here, this is very much a process-oriented +discussion which does not require a deep knowledge of kernel programming to +understand. + + +1.1: EXECUTIVE SUMMARY + +The rest of this section covers the scope of the kernel development process +and the kinds of frustrations that developers and their employers can +encounter there. There are a great many reasons why kernel code should be +merged into the official ("mainline") kernel, including automatic +availability to users, community support in many forms, and the ability to +influence the direction of kernel development. Code contributed to the +Linux kernel must be made available under a GPL-compatible license. + +Section 2 introduces the development process, the kernel release cycle, and +the mechanics of the merge window. The various phases in the patch +development, review, and merging cycle are covered. There is some +discussion of tools and mailing lists. Developers wanting to get started +with kernel development are encouraged to track down and fix bugs as an +initial exercise. + +Section 3 covers early-stage project planning, with an emphasis on +involving the development community as soon as possible. + +Section 4 is about the coding process; several pitfalls which have been +encountered by other developers are discussed. Some requirements for +patches are covered, and there is an introduction to some of the tools +which can help to ensure that kernel patches are correct. + +Section 5 talks about the process of posting patches for review. To be +taken seriously by the development community, patches must be properly +formatted and described, and they must be sent to the right place. +Following the advice in this section should help to ensure the best +possible reception for your work. + +Section 6 covers what happens after posting patches; the job is far from +done at that point. Working with reviewers is a crucial part of the +development process; this section offers a number of tips on how to avoid +problems at this important stage. Developers are cautioned against +assuming that the job is done when a patch is merged into the mainline. + +Section 7 introduces a couple of "advanced" topics: managing patches with +git and reviewing patches posted by others. + +Section 8 concludes the document with pointers to sources for more +information on kernel development. + + +1.2: WHAT THIS DOCUMENT IS ABOUT + +The Linux kernel, at over 6 million lines of code and well over 1000 active +contributors, is one of the largest and most active free software projects +in existence. Since its humble beginning in 1991, this kernel has evolved +into a best-of-breed operating system component which runs on pocket-sized +digital music players, desktop PCs, the largest supercomputers in +existence, and all types of systems in between. It is a robust, efficient, +and scalable solution for almost any situation. + +With the growth of Linux has come an increase in the number of developers +(and companies) wishing to participate in its development. Hardware +vendors want to ensure that Linux supports their products well, making +those products attractive to Linux users. Embedded systems vendors, who +use Linux as a component in an integrated product, want Linux to be as +capable and well-suited to the task at hand as possible. Distributors and +other software vendors who base their products on Linux have a clear +interest in the capabilities, performance, and reliability of the Linux +kernel. And end users, too, will often wish to change Linux to make it +better suit their needs. + +One of the most compelling features of Linux is that it is accessible to +these developers; anybody with the requisite skills can improve Linux and +influence the direction of its development. Proprietary products cannot +offer this kind of openness, which is a characteristic of the free software +process. But, if anything, the kernel is even more open than most other +free software projects. A typical three-month kernel development cycle can +involve over 1000 developers working for more than 100 different companies +(or for no company at all). + +Working with the kernel development community is not especially hard. But, +that notwithstanding, many potential contributors have experienced +difficulties when trying to do kernel work. The kernel community has +evolved its own distinct ways of operating which allow it to function +smoothly (and produce a high-quality product) in an environment where +thousands of lines of code are being changed every day. So it is not +surprising that Linux kernel development process differs greatly from +proprietary development methods. + +The kernel's development process may come across as strange and +intimidating to new developers, but there are good reasons and solid +experience behind it. A developer who does not understand the kernel +community's ways (or, worse, who tries to flout or circumvent them) will +have a frustrating experience in store. The development community, while +being helpful to those who are trying to learn, has little time for those +who will not listen or who do not care about the development process. + +It is hoped that those who read this document will be able to avoid that +frustrating experience. There is a lot of material here, but the effort +involved in reading it will be repaid in short order. The development +community is always in need of developers who will help to make the kernel +better; the following text should help you - or those who work for you - +join our community. + + +1.3: CREDITS + +This document was written by Jonathan Corbet, corbet@lwn.net. It has been +improved by comments from Johannes Berg, James Berry, Alex Chiang, Roland +Dreier, Randy Dunlap, Jake Edge, Jiri Kosina, Matt Mackall, Arthur Marsh, +Amanda McPherson, Andrew Morton, Andrew Price, Tsugikazu Shibata, and +Jochen Voß. + +This work was supported by the Linux Foundation; thanks especially to +Amanda McPherson, who saw the value of this effort and made it all happen. + + +1.4: THE IMPORTANCE OF GETTING CODE INTO THE MAINLINE + +Some companies and developers occasionally wonder why they should bother +learning how to work with the kernel community and get their code into the +mainline kernel (the "mainline" being the kernel maintained by Linus +Torvalds and used as a base by Linux distributors). In the short term, +contributing code can look like an avoidable expense; it seems easier to +just keep the code separate and support users directly. The truth of the +matter is that keeping code separate ("out of tree") is a false economy. + +As a way of illustrating the costs of out-of-tree code, here are a few +relevant aspects of the kernel development process; most of these will be +discussed in greater detail later in this document. Consider: + +- Code which has been merged into the mainline kernel is available to all + Linux users. It will automatically be present on all distributions which + enable it. There is no need for driver disks, downloads, or the hassles + of supporting multiple versions of multiple distributions; it all just + works, for the developer and for the user. Incorporation into the + mainline solves a large number of distribution and support problems. + +- While kernel developers strive to maintain a stable interface to user + space, the internal kernel API is in constant flux. The lack of a stable + internal interface is a deliberate design decision; it allows fundamental + improvements to be made at any time and results in higher-quality code. + But one result of that policy is that any out-of-tree code requires + constant upkeep if it is to work with new kernels. Maintaining + out-of-tree code requires significant amounts of work just to keep that + code working. + + Code which is in the mainline, instead, does not require this work as the + result of a simple rule requiring any developer who makes an API change + to also fix any code that breaks as the result of that change. So code + which has been merged into the mainline has significantly lower + maintenance costs. + +- Beyond that, code which is in the kernel will often be improved by other + developers. Surprising results can come from empowering your user + community and customers to improve your product. + +- Kernel code is subjected to review, both before and after merging into + the mainline. No matter how strong the original developer's skills are, + this review process invariably finds ways in which the code can be + improved. Often review finds severe bugs and security problems. This is + especially true for code which has been developed in a closed + environment; such code benefits strongly from review by outside + developers. Out-of-tree code is lower-quality code. + +- Participation in the development process is your way to influence the + direction of kernel development. Users who complain from the sidelines + are heard, but active developers have a stronger voice - and the ability + to implement changes which make the kernel work better for their needs. + +- When code is maintained separately, the possibility that a third party + will contribute a different implementation of a similar feature always + exists. Should that happen, getting your code merged will become much + harder - to the point of impossibility. Then you will be faced with the + unpleasant alternatives of either (1) maintaining a nonstandard feature + out of tree indefinitely, or (2) abandoning your code and migrating your + users over to the in-tree version. + +- Contribution of code is the fundamental action which makes the whole + process work. By contributing your code you can add new functionality to + the kernel and provide capabilities and examples which are of use to + other kernel developers. If you have developed code for Linux (or are + thinking about doing so), you clearly have an interest in the continued + success of this platform; contributing code is one of the best ways to + help ensure that success. + +All of the reasoning above applies to any out-of-tree kernel code, +including code which is distributed in proprietary, binary-only form. +There are, however, additional factors which should be taken into account +before considering any sort of binary-only kernel code distribution. These +include: + +- The legal issues around the distribution of proprietary kernel modules + are cloudy at best; quite a few kernel copyright holders believe that + most binary-only modules are derived products of the kernel and that, as + a result, their distribution is a violation of the GNU General Public + license (about which more will be said below). Your author is not a + lawyer, and nothing in this document can possibly be considered to be + legal advice. The true legal status of closed-source modules can only be + determined by the courts. But the uncertainty which haunts those modules + is there regardless. + +- Binary modules greatly increase the difficulty of debugging kernel + problems, to the point that most kernel developers will not even try. So + the distribution of binary-only modules will make it harder for your + users to get support from the community. + +- Support is also harder for distributors of binary-only modules, who must + provide a version of the module for every distribution and every kernel + version they wish to support. Dozens of builds of a single module can + be required to provide reasonably comprehensive coverage, and your users + will have to upgrade your module separately every time they upgrade their + kernel. + +- Everything that was said above about code review applies doubly to + closed-source code. Since this code is not available at all, it cannot + have been reviewed by the community and will, beyond doubt, have serious + problems. + +Makers of embedded systems, in particular, may be tempted to disregard much +of what has been said in this section in the belief that they are shipping +a self-contained product which uses a frozen kernel version and requires no +more development after its release. This argument misses the value of +widespread code review and the value of allowing your users to add +capabilities to your product. But these products, too, have a limited +commercial life, after which a new version must be released. At that +point, vendors whose code is in the mainline and well maintained will be +much better positioned to get the new product ready for market quickly. + + +1.5: LICENSING + +Code is contributed to the Linux kernel under a number of licenses, but all +code must be compatible with version 2 of the GNU General Public License +(GPLv2), which is the license covering the kernel distribution as a whole. +In practice, that means that all code contributions are covered either by +GPLv2 (with, optionally, language allowing distribution under later +versions of the GPL) or the three-clause BSD license. Any contributions +which are not covered by a compatible license will not be accepted into the +kernel. + +Copyright assignments are not required (or requested) for code contributed +to the kernel. All code merged into the mainline kernel retains its +original ownership; as a result, the kernel now has thousands of owners. + +One implication of this ownership structure is that any attempt to change +the licensing of the kernel is doomed to almost certain failure. There are +few practical scenarios where the agreement of all copyright holders could +be obtained (or their code removed from the kernel). So, in particular, +there is no prospect of a migration to version 3 of the GPL in the +foreseeable future. + +It is imperative that all code contributed to the kernel be legitimately +free software. For that reason, code from anonymous (or pseudonymous) +contributors will not be accepted. All contributors are required to "sign +off" on their code, stating that the code can be distributed with the +kernel under the GPL. Code which has not been licensed as free software by +its owner, or which risks creating copyright-related problems for the +kernel (such as code which derives from reverse-engineering efforts lacking +proper safeguards) cannot be contributed. + +Questions about copyright-related issues are common on Linux development +mailing lists. Such questions will normally receive no shortage of +answers, but one should bear in mind that the people answering those +questions are not lawyers and cannot provide legal advice. If you have +legal questions relating to Linux source code, there is no substitute for +talking with a lawyer who understands this field. Relying on answers +obtained on technical mailing lists is a risky affair. diff --git a/Documentation/development-process/2.Process b/Documentation/development-process/2.Process new file mode 100644 index 000000000000..d750321acd5a --- /dev/null +++ b/Documentation/development-process/2.Process @@ -0,0 +1,459 @@ +2: HOW THE DEVELOPMENT PROCESS WORKS + +Linux kernel development in the early 1990's was a pretty loose affair, +with relatively small numbers of users and developers involved. With a +user base in the millions and with some 2,000 developers involved over the +course of one year, the kernel has since had to evolve a number of +processes to keep development happening smoothly. A solid understanding of +how the process works is required in order to be an effective part of it. + + +2.1: THE BIG PICTURE + +The kernel developers use a loosely time-based release process, with a new +major kernel release happening every two or three months. The recent +release history looks like this: + + 2.6.26 July 13, 2008 + 2.6.25 April 16, 2008 + 2.6.24 January 24, 2008 + 2.6.23 October 9, 2007 + 2.6.22 July 8, 2007 + 2.6.21 April 25, 2007 + 2.6.20 February 4, 2007 + +Every 2.6.x release is a major kernel release with new features, internal +API changes, and more. A typical 2.6 release can contain over 10,000 +changesets with changes to several hundred thousand lines of code. 2.6 is +thus the leading edge of Linux kernel development; the kernel uses a +rolling development model which is continually integrating major changes. + +A relatively straightforward discipline is followed with regard to the +merging of patches for each release. At the beginning of each development +cycle, the "merge window" is said to be open. At that time, code which is +deemed to be sufficiently stable (and which is accepted by the development +community) is merged into the mainline kernel. The bulk of changes for a +new development cycle (and all of the major changes) will be merged during +this time, at a rate approaching 1,000 changes ("patches," or "changesets") +per day. + +(As an aside, it is worth noting that the changes integrated during the +merge window do not come out of thin air; they have been collected, tested, +and staged ahead of time. How that process works will be described in +detail later on). + +The merge window lasts for two weeks. At the end of this time, Linus +Torvalds will declare that the window is closed and release the first of +the "rc" kernels. For the kernel which is destined to be 2.6.26, for +example, the release which happens at the end of the merge window will be +called 2.6.26-rc1. The -rc1 release is the signal that the time to merge +new features has passed, and that the time to stabilize the next kernel has +begun. + +Over the next six to ten weeks, only patches which fix problems should be +submitted to the mainline. On occasion a more significant change will be +allowed, but such occasions are rare; developers who try to merge new +features outside of the merge window tend to get an unfriendly reception. +As a general rule, if you miss the merge window for a given feature, the +best thing to do is to wait for the next development cycle. (An occasional +exception is made for drivers for previously-unsupported hardware; if they +touch no in-tree code, they cannot cause regressions and should be safe to +add at any time). + +As fixes make their way into the mainline, the patch rate will slow over +time. Linus releases new -rc kernels about once a week; a normal series +will get up to somewhere between -rc6 and -rc9 before the kernel is +considered to be sufficiently stable and the final 2.6.x release is made. +At that point the whole process starts over again. + +As an example, here is how the 2.6.25 development cycle went (all dates in +2008): + + January 24 2.6.24 stable release + February 10 2.6.25-rc1, merge window closes + February 15 2.6.25-rc2 + February 24 2.6.25-rc3 + March 4 2.6.25-rc4 + March 9 2.6.25-rc5 + March 16 2.6.25-rc6 + March 25 2.6.25-rc7 + April 1 2.6.25-rc8 + April 11 2.6.25-rc9 + April 16 2.6.25 stable release + +How do the developers decide when to close the development cycle and create +the stable release? The most significant metric used is the list of +regressions from previous releases. No bugs are welcome, but those which +break systems which worked in the past are considered to be especially +serious. For this reason, patches which cause regressions are looked upon +unfavorably and are quite likely to be reverted during the stabilization +period. + +The developers' goal is to fix all known regressions before the stable +release is made. In the real world, this kind of perfection is hard to +achieve; there are just too many variables in a project of this size. +There comes a point where delaying the final release just makes the problem +worse; the pile of changes waiting for the next merge window will grow +larger, creating even more regressions the next time around. So most 2.6.x +kernels go out with a handful of known regressions though, hopefully, none +of them are serious. + +Once a stable release is made, its ongoing maintenance is passed off to the +"stable team," currently comprised of Greg Kroah-Hartman and Chris Wright. +The stable team will release occasional updates to the stable release using +the 2.6.x.y numbering scheme. To be considered for an update release, a +patch must (1) fix a significant bug, and (2) already be merged into the +mainline for the next development kernel. Continuing our 2.6.25 example, +the history (as of this writing) is: + + May 1 2.6.25.1 + May 6 2.6.25.2 + May 9 2.6.25.3 + May 15 2.6.25.4 + June 7 2.6.25.5 + June 9 2.6.25.6 + June 16 2.6.25.7 + June 21 2.6.25.8 + June 24 2.6.25.9 + +Stable updates for a given kernel are made for approximately six months; +after that, the maintenance of stable releases is solely the responsibility +of the distributors which have shipped that particular kernel. + + +2.2: THE LIFECYCLE OF A PATCH + +Patches do not go directly from the developer's keyboard into the mainline +kernel. There is, instead, a somewhat involved (if somewhat informal) +process designed to ensure that each patch is reviewed for quality and that +each patch implements a change which is desirable to have in the mainline. +This process can happen quickly for minor fixes, or, in the case of large +and controversial changes, go on for years. Much developer frustration +comes from a lack of understanding of this process or from attempts to +circumvent it. + +In the hopes of reducing that frustration, this document will describe how +a patch gets into the kernel. What follows below is an introduction which +describes the process in a somewhat idealized way. A much more detailed +treatment will come in later sections. + +The stages that a patch goes through are, generally: + + - Design. This is where the real requirements for the patch - and the way + those requirements will be met - are laid out. Design work is often + done without involving the community, but it is better to do this work + in the open if at all possible; it can save a lot of time redesigning + things later. + + - Early review. Patches are posted to the relevant mailing list, and + developers on that list reply with any comments they may have. This + process should turn up any major problems with a patch if all goes + well. + + - Wider review. When the patch is getting close to ready for mainline + inclusion, it will be accepted by a relevant subsystem maintainer - + though this acceptance is not a guarantee that the patch will make it + all the way to the mainline. The patch will show up in the maintainer's + subsystem tree and into the staging trees (described below). When the + process works, this step leads to more extensive review of the patch and + the discovery of any problems resulting from the integration of this + patch with work being done by others. + + - Merging into the mainline. Eventually, a successful patch will be + merged into the mainline repository managed by Linus Torvalds. More + comments and/or problems may surface at this time; it is important that + the developer be responsive to these and fix any issues which arise. + + - Stable release. The number of users potentially affected by the patch + is now large, so, once again, new problems may arise. + + - Long-term maintenance. While it is certainly possible for a developer + to forget about code after merging it, that sort of behavior tends to + leave a poor impression in the development community. Merging code + eliminates some of the maintenance burden, in that others will fix + problems caused by API changes. But the original developer should + continue to take responsibility for the code if it is to remain useful + in the longer term. + +One of the largest mistakes made by kernel developers (or their employers) +is to try to cut the process down to a single "merging into the mainline" +step. This approach invariably leads to frustration for everybody +involved. + + +2.3: HOW PATCHES GET INTO THE KERNEL + +There is exactly one person who can merge patches into the mainline kernel +repository: Linus Torvalds. But, of the over 12,000 patches which went +into the 2.6.25 kernel, only 250 (around 2%) were directly chosen by Linus +himself. The kernel project has long since grown to a size where no single +developer could possibly inspect and select every patch unassisted. The +way the kernel developers have addressed this growth is through the use of +a lieutenant system built around a chain of trust. + +The kernel code base is logically broken down into a set of subsystems: +networking, specific architecture support, memory management, video +devices, etc. Most subsystems have a designated maintainer, a developer +who has overall responsibility for the code within that subsystem. These +subsystem maintainers are the gatekeepers (in a loose way) for the portion +of the kernel they manage; they are the ones who will (usually) accept a +patch for inclusion into the mainline kernel. + +Subsystem maintainers each manage their own version of the kernel source +tree, usually (but certainly not always) using the git source management +tool. Tools like git (and related tools like quilt or mercurial) allow +maintainers to track a list of patches, including authorship information +and other metadata. At any given time, the maintainer can identify which +patches in his or her repository are not found in the mainline. + +When the merge window opens, top-level maintainers will ask Linus to "pull" +the patches they have selected for merging from their repositories. If +Linus agrees, the stream of patches will flow up into his repository, +becoming part of the mainline kernel. The amount of attention that Linus +pays to specific patches received in a pull operation varies. It is clear +that, sometimes, he looks quite closely. But, as a general rule, Linus +trusts the subsystem maintainers to not send bad patches upstream. + +Subsystem maintainers, in turn, can pull patches from other maintainers. +For example, the networking tree is built from patches which accumulated +first in trees dedicated to network device drivers, wireless networking, +etc. This chain of repositories can be arbitrarily long, though it rarely +exceeds two or three links. Since each maintainer in the chain trusts +those managing lower-level trees, this process is known as the "chain of +trust." + +Clearly, in a system like this, getting patches into the kernel depends on +finding the right maintainer. Sending patches directly to Linus is not +normally the right way to go. + + +2.4: STAGING TREES + +The chain of subsystem trees guides the flow of patches into the kernel, +but it also raises an interesting question: what if somebody wants to look +at all of the patches which are being prepared for the next merge window? +Developers will be interested in what other changes are pending to see +whether there are any conflicts to worry about; a patch which changes a +core kernel function prototype, for example, will conflict with any other +patches which use the older form of that function. Reviewers and testers +want access to the changes in their integrated form before all of those +changes land in the mainline kernel. One could pull changes from all of +the interesting subsystem trees, but that would be a big and error-prone +job. + +The answer comes in the form of staging trees, where subsystem trees are +collected for testing and review. The older of these trees, maintained by +Andrew Morton, is called "-mm" (for memory management, which is how it got +started). The -mm tree integrates patches from a long list of subsystem +trees; it also has some patches aimed at helping with debugging. + +Beyond that, -mm contains a significant collection of patches which have +been selected by Andrew directly. These patches may have been posted on a +mailing list, or they may apply to a part of the kernel for which there is +no designated subsystem tree. As a result, -mm operates as a sort of +subsystem tree of last resort; if there is no other obvious path for a +patch into the mainline, it is likely to end up in -mm. Miscellaneous +patches which accumulate in -mm will eventually either be forwarded on to +an appropriate subsystem tree or be sent directly to Linus. In a typical +development cycle, approximately 10% of the patches going into the mainline +get there via -mm. + +The current -mm patch can always be found from the front page of + + http://kernel.org/ + +Those who want to see the current state of -mm can get the "-mm of the +moment" tree, found at: + + http://userweb.kernel.org/~akpm/mmotm/ + +Use of the MMOTM tree is likely to be a frustrating experience, though; +there is a definite chance that it will not even compile. + +The other staging tree, started more recently, is linux-next, maintained by +Stephen Rothwell. The linux-next tree is, by design, a snapshot of what +the mainline is expected to look like after the next merge window closes. +Linux-next trees are announced on the linux-kernel and linux-next mailing +lists when they are assembled; they can be downloaded from: + + http://www.kernel.org/pub/linux/kernel/people/sfr/linux-next/ + +Some information about linux-next has been gathered at: + + http://linux.f-seidel.de/linux-next/pmwiki/ + +How the linux-next tree will fit into the development process is still +changing. As of this writing, the first full development cycle involving +linux-next (2.6.26) is coming to an end; thus far, it has proved to be a +valuable resource for finding and fixing integration problems before the +beginning of the merge window. See http://lwn.net/Articles/287155/ for +more information on how linux-next has worked to set up the 2.6.27 merge +window. + +Some developers have begun to suggest that linux-next should be used as the +target for future development as well. The linux-next tree does tend to be +far ahead of the mainline and is more representative of the tree into which +any new work will be merged. The downside to this idea is that the +volatility of linux-next tends to make it a difficult development target. +See http://lwn.net/Articles/289013/ for more information on this topic, and +stay tuned; much is still in flux where linux-next is involved. + + +2.5: TOOLS + +As can be seen from the above text, the kernel development process depends +heavily on the ability to herd collections of patches in various +directions. The whole thing would not work anywhere near as well as it +does without suitably powerful tools. Tutorials on how to use these tools +are well beyond the scope of this document, but there is space for a few +pointers. + +By far the dominant source code management system used by the kernel +community is git. Git is one of a number of distributed version control +systems being developed in the free software community. It is well tuned +for kernel development, in that it performs quite well when dealing with +large repositories and large numbers of patches. It also has a reputation +for being difficult to learn and use, though it has gotten better over +time. Some sort of familiarity with git is almost a requirement for kernel +developers; even if they do not use it for their own work, they'll need git +to keep up with what other developers (and the mainline) are doing. + +Git is now packaged by almost all Linux distributions. There is a home +page at + + http://git.or.cz/ + +That page has pointers to documentation and tutorials. One should be +aware, in particular, of the Kernel Hacker's Guide to git, which has +information specific to kernel development: + + http://linux.yyz.us/git-howto.html + +Among the kernel developers who do not use git, the most popular choice is +almost certainly Mercurial: + + http://www.selenic.com/mercurial/ + +Mercurial shares many features with git, but it provides an interface which +many find easier to use. + +The other tool worth knowing about is Quilt: + + http://savannah.nongnu.org/projects/quilt/ + +Quilt is a patch management system, rather than a source code management +system. It does not track history over time; it is, instead, oriented +toward tracking a specific set of changes against an evolving code base. +Some major subsystem maintainers use quilt to manage patches intended to go +upstream. For the management of certain kinds of trees (-mm, for example), +quilt is the best tool for the job. + + +2.6: MAILING LISTS + +A great deal of Linux kernel development work is done by way of mailing +lists. It is hard to be a fully-functioning member of the community +without joining at least one list somewhere. But Linux mailing lists also +represent a potential hazard to developers, who risk getting buried under a +load of electronic mail, running afoul of the conventions used on the Linux +lists, or both. + +Most kernel mailing lists are run on vger.kernel.org; the master list can +be found at: + + http://vger.kernel.org/vger-lists.html + +There are lists hosted elsewhere, though; a number of them are at +lists.redhat.com. + +The core mailing list for kernel development is, of course, linux-kernel. +This list is an intimidating place to be; volume can reach 500 messages per +day, the amount of noise is high, the conversation can be severely +technical, and participants are not always concerned with showing a high +degree of politeness. But there is no other place where the kernel +development community comes together as a whole; developers who avoid this +list will miss important information. + +There are a few hints which can help with linux-kernel survival: + +- Have the list delivered to a separate folder, rather than your main + mailbox. One must be able to ignore the stream for sustained periods of + time. + +- Do not try to follow every conversation - nobody else does. It is + important to filter on both the topic of interest (though note that + long-running conversations can drift away from the original subject + without changing the email subject line) and the people who are + participating. + +- Do not feed the trolls. If somebody is trying to stir up an angry + response, ignore them. + +- When responding to linux-kernel email (or that on other lists) preserve + the Cc: header for all involved. In the absence of a strong reason (such + as an explicit request), you should never remove recipients. Always make + sure that the person you are responding to is in the Cc: list. This + convention also makes it unnecessary to explicitly ask to be copied on + replies to your postings. + +- Search the list archives (and the net as a whole) before asking + questions. Some developers can get impatient with people who clearly + have not done their homework. + +- Avoid top-posting (the practice of putting your answer above the quoted + text you are responding to). It makes your response harder to read and + makes a poor impression. + +- Ask on the correct mailing list. Linux-kernel may be the general meeting + point, but it is not the best place to find developers from all + subsystems. + +The last point - finding the correct mailing list - is a common place for +beginning developers to go wrong. Somebody who asks a networking-related +question on linux-kernel will almost certainly receive a polite suggestion +to ask on the netdev list instead, as that is the list frequented by most +networking developers. Other lists exist for the SCSI, video4linux, IDE, +filesystem, etc. subsystems. The best place to look for mailing lists is +in the MAINTAINERS file packaged with the kernel source. + + +2.7: GETTING STARTED WITH KERNEL DEVELOPMENT + +Questions about how to get started with the kernel development process are +common - from both individuals and companies. Equally common are missteps +which make the beginning of the relationship harder than it has to be. + +Companies often look to hire well-known developers to get a development +group started. This can, in fact, be an effective technique. But it also +tends to be expensive and does not do much to grow the pool of experienced +kernel developers. It is possible to bring in-house developers up to speed +on Linux kernel development, given the investment of a bit of time. Taking +this time can endow an employer with a group of developers who understand +the kernel and the company both, and who can help to train others as well. +Over the medium term, this is often the more profitable approach. + +Individual developers are often, understandably, at a loss for a place to +start. Beginning with a large project can be intimidating; one often wants +to test the waters with something smaller first. This is the point where +some developers jump into the creation of patches fixing spelling errors or +minor coding style issues. Unfortunately, such patches create a level of +noise which is distracting for the development community as a whole, so, +increasingly, they are looked down upon. New developers wishing to +introduce themselves to the community will not get the sort of reception +they wish for by these means. + +Andrew Morton gives this advice for aspiring kernel developers + + The #1 project for all kernel beginners should surely be "make sure + that the kernel runs perfectly at all times on all machines which + you can lay your hands on". Usually the way to do this is to work + with others on getting things fixed up (this can require + persistence!) but that's fine - it's a part of kernel development. + +(http://lwn.net/Articles/283982/). + +In the absence of obvious problems to fix, developers are advised to look +at the current lists of regressions and open bugs in general. There is +never any shortage of issues in need of fixing; by addressing these issues, +developers will gain experience with the process while, at the same time, +building respect with the rest of the development community. diff --git a/Documentation/development-process/3.Early-stage b/Documentation/development-process/3.Early-stage new file mode 100644 index 000000000000..307a159a70ca --- /dev/null +++ b/Documentation/development-process/3.Early-stage @@ -0,0 +1,195 @@ +3: EARLY-STAGE PLANNING + +When contemplating a Linux kernel development project, it can be tempting +to jump right in and start coding. As with any significant project, +though, much of the groundwork for success is best laid before the first +line of code is written. Some time spent in early planning and +communication can save far more time later on. + + +3.1: SPECIFYING THE PROBLEM + +Like any engineering project, a successful kernel enhancement starts with a +clear description of the problem to be solved. In some cases, this step is +easy: when a driver is needed for a specific piece of hardware, for +example. In others, though, it is tempting to confuse the real problem +with the proposed solution, and that can lead to difficulties. + +Consider an example: some years ago, developers working with Linux audio +sought a way to run applications without dropouts or other artifacts caused +by excessive latency in the system. The solution they arrived at was a +kernel module intended to hook into the Linux Security Module (LSM) +framework; this module could be configured to give specific applications +access to the realtime scheduler. This module was implemented and sent to +the linux-kernel mailing list, where it immediately ran into problems. + +To the audio developers, this security module was sufficient to solve their +immediate problem. To the wider kernel community, though, it was seen as a +misuse of the LSM framework (which is not intended to confer privileges +onto processes which they would not otherwise have) and a risk to system +stability. Their preferred solutions involved realtime scheduling access +via the rlimit mechanism for the short term, and ongoing latency reduction +work in the long term. + +The audio community, however, could not see past the particular solution +they had implemented; they were unwilling to accept alternatives. The +resulting disagreement left those developers feeling disillusioned with the +entire kernel development process; one of them went back to an audio list +and posted this: + + There are a number of very good Linux kernel developers, but they + tend to get outshouted by a large crowd of arrogant fools. Trying + to communicate user requirements to these people is a waste of + time. They are much too "intelligent" to listen to lesser mortals. + +(http://lwn.net/Articles/131776/). + +The reality of the situation was different; the kernel developers were far +more concerned about system stability, long-term maintenance, and finding +the right solution to the problem than they were with a specific module. +The moral of the story is to focus on the problem - not a specific solution +- and to discuss it with the development community before investing in the +creation of a body of code. + +So, when contemplating a kernel development project, one should obtain +answers to a short set of questions: + + - What, exactly, is the problem which needs to be solved? + + - Who are the users affected by this problem? Which use cases should the + solution address? + + - How does the kernel fall short in addressing that problem now? + +Only then does it make sense to start considering possible solutions. + + +3.2: EARLY DISCUSSION + +When planning a kernel development project, it makes great sense to hold +discussions with the community before launching into implementation. Early +communication can save time and trouble in a number of ways: + + - It may well be that the problem is addressed by the kernel in ways which + you have not understood. The Linux kernel is large and has a number of + features and capabilities which are not immediately obvious. Not all + kernel capabilities are documented as well as one might like, and it is + easy to miss things. Your author has seen the posting of a complete + driver which duplicated an existing driver that the new author had been + unaware of. Code which reinvents existing wheels is not only wasteful; + it will also not be accepted into the mainline kernel. + + - There may be elements of the proposed solution which will not be + acceptable for mainline merging. It is better to find out about + problems like this before writing the code. + + - It's entirely possible that other developers have thought about the + problem; they may have ideas for a better solution, and may be willing + to help in the creation of that solution. + +Years of experience with the kernel development community have taught a +clear lesson: kernel code which is designed and developed behind closed +doors invariably has problems which are only revealed when the code is +released into the community. Sometimes these problems are severe, +requiring months or years of effort before the code can be brought up to +the kernel community's standards. Some examples include: + + - The Devicescape network stack was designed and implemented for + single-processor systems. It could not be merged into the mainline + until it was made suitable for multiprocessor systems. Retrofitting + locking and such into code is a difficult task; as a result, the merging + of this code (now called mac80211) was delayed for over a year. + + - The Reiser4 filesystem included a number of capabilities which, in the + core kernel developers' opinion, should have been implemented in the + virtual filesystem layer instead. It also included features which could + not easily be implemented without exposing the system to user-caused + deadlocks. The late revelation of these problems - and refusal to + address some of them - has caused Reiser4 to stay out of the mainline + kernel. + + - The AppArmor security module made use of internal virtual filesystem + data structures in ways which were considered to be unsafe and + unreliable. This code has since been significantly reworked, but + remains outside of the mainline. + +In each of these cases, a great deal of pain and extra work could have been +avoided with some early discussion with the kernel developers. + + +3.3: WHO DO YOU TALK TO? + +When developers decide to take their plans public, the next question will +be: where do we start? The answer is to find the right mailing list(s) and +the right maintainer. For mailing lists, the best approach is to look in +the MAINTAINERS file for a relevant place to post. If there is a suitable +subsystem list, posting there is often preferable to posting on +linux-kernel; you are more likely to reach developers with expertise in the +relevant subsystem and the environment may be more supportive. + +Finding maintainers can be a bit harder. Again, the MAINTAINERS file is +the place to start. That file tends to not always be up to date, though, +and not all subsystems are represented there. The person listed in the +MAINTAINERS file may, in fact, not be the person who is actually acting in +that role currently. So, when there is doubt about who to contact, a +useful trick is to use git (and "git log" in particular) to see who is +currently active within the subsystem of interest. Look at who is writing +patches, and who, if anybody, is attaching Signed-off-by lines to those +patches. Those are the people who will be best placed to help with a new +development project. + +If all else fails, talking to Andrew Morton can be an effective way to +track down a maintainer for a specific piece of code. + + +3.4: WHEN TO POST? + +If possible, posting your plans during the early stages can only be +helpful. Describe the problem being solved and any plans that have been +made on how the implementation will be done. Any information you can +provide can help the development community provide useful input on the +project. + +One discouraging thing which can happen at this stage is not a hostile +reaction, but, instead, little or no reaction at all. The sad truth of the +matter is (1) kernel developers tend to be busy, (2) there is no shortage +of people with grand plans and little code (or even prospect of code) to +back them up, and (3) nobody is obligated to review or comment on ideas +posted by others. If a request-for-comments posting yields little in the +way of comments, do not assume that it means there is no interest in the +project. Unfortunately, you also cannot assume that there are no problems +with your idea. The best thing to do in this situation is to proceed, +keeping the community informed as you go. + + +3.5: GETTING OFFICIAL BUY-IN + +If your work is being done in a corporate environment - as most Linux +kernel work is - you must, obviously, have permission from suitably +empowered managers before you can post your company's plans or code to a +public mailing list. The posting of code which has not been cleared for +release under a GPL-compatible license can be especially problematic; the +sooner that a company's management and legal staff can agree on the posting +of a kernel development project, the better off everybody involved will be. + +Some readers may be thinking at this point that their kernel work is +intended to support a product which does not yet have an officially +acknowledged existence. Revealing their employer's plans on a public +mailing list may not be a viable option. In cases like this, it is worth +considering whether the secrecy is really necessary; there is often no real +need to keep development plans behind closed doors. + +That said, there are also cases where a company legitimately cannot +disclose its plans early in the development process. Companies with +experienced kernel developers may choose to proceed in an open-loop manner +on the assumption that they will be able to avoid serious integration +problems later. For companies without that sort of in-house expertise, the +best option is often to hire an outside developer to review the plans under +a non-disclosure agreement. The Linux Foundation operates an NDA program +designed to help with this sort of situation; more information can be found +at: + + http://www.linuxfoundation.org/en/NDA_program + +This kind of review is often enough to avoid serious problems later on +without requiring public disclosure of the project. diff --git a/Documentation/development-process/4.Coding b/Documentation/development-process/4.Coding new file mode 100644 index 000000000000..014aca8f14e2 --- /dev/null +++ b/Documentation/development-process/4.Coding @@ -0,0 +1,384 @@ +4: GETTING THE CODE RIGHT + +While there is much to be said for a solid and community-oriented design +process, the proof of any kernel development project is in the resulting +code. It is the code which will be examined by other developers and merged +(or not) into the mainline tree. So it is the quality of this code which +will determine the ultimate success of the project. + +This section will examine the coding process. We'll start with a look at a +number of ways in which kernel developers can go wrong. Then the focus +will shift toward doing things right and the tools which can help in that +quest. + + +4.1: PITFALLS + +* Coding style + +The kernel has long had a standard coding style, described in +Documentation/CodingStyle. For much of that time, the policies described +in that file were taken as being, at most, advisory. As a result, there is +a substantial amount of code in the kernel which does not meet the coding +style guidelines. The presence of that code leads to two independent +hazards for kernel developers. + +The first of these is to believe that the kernel coding standards do not +matter and are not enforced. The truth of the matter is that adding new +code to the kernel is very difficult if that code is not coded according to +the standard; many developers will request that the code be reformatted +before they will even review it. A code base as large as the kernel +requires some uniformity of code to make it possible for developers to +quickly understand any part of it. So there is no longer room for +strangely-formatted code. + +Occasionally, the kernel's coding style will run into conflict with an +employer's mandated style. In such cases, the kernel's style will have to +win before the code can be merged. Putting code into the kernel means +giving up a degree of control in a number of ways - including control over +how the code is formatted. + +The other trap is to assume that code which is already in the kernel is +urgently in need of coding style fixes. Developers may start to generate +reformatting patches as a way of gaining familiarity with the process, or +as a way of getting their name into the kernel changelogs - or both. But +pure coding style fixes are seen as noise by the development community; +they tend to get a chilly reception. So this type of patch is best +avoided. It is natural to fix the style of a piece of code while working +on it for other reasons, but coding style changes should not be made for +their own sake. + +The coding style document also should not be read as an absolute law which +can never be transgressed. If there is a good reason to go against the +style (a line which becomes far less readable if split to fit within the +80-column limit, for example), just do it. + + +* Abstraction layers + +Computer Science professors teach students to make extensive use of +abstraction layers in the name of flexibility and information hiding. +Certainly the kernel makes extensive use of abstraction; no project +involving several million lines of code could do otherwise and survive. +But experience has shown that excessive or premature abstraction can be +just as harmful as premature optimization. Abstraction should be used to +the level required and no further. + +At a simple level, consider a function which has an argument which is +always passed as zero by all callers. One could retain that argument just +in case somebody eventually needs to use the extra flexibility that it +provides. By that time, though, chances are good that the code which +implements this extra argument has been broken in some subtle way which was +never noticed - because it has never been used. Or, when the need for +extra flexibility arises, it does not do so in a way which matches the +programmer's early expectation. Kernel developers will routinely submit +patches to remove unused arguments; they should, in general, not be added +in the first place. + +Abstraction layers which hide access to hardware - often to allow the bulk +of a driver to be used with multiple operating systems - are especially +frowned upon. Such layers obscure the code and may impose a performance +penalty; they do not belong in the Linux kernel. + +On the other hand, if you find yourself copying significant amounts of code +from another kernel subsystem, it is time to ask whether it would, in fact, +make sense to pull out some of that code into a separate library or to +implement that functionality at a higher level. There is no value in +replicating the same code throughout the kernel. + + +* #ifdef and preprocessor use in general + +The C preprocessor seems to present a powerful temptation to some C +programmers, who see it as a way to efficiently encode a great deal of +flexibility into a source file. But the preprocessor is not C, and heavy +use of it results in code which is much harder for others to read and +harder for the compiler to check for correctness. Heavy preprocessor use +is almost always a sign of code which needs some cleanup work. + +Conditional compilation with #ifdef is, indeed, a powerful feature, and it +is used within the kernel. But there is little desire to see code which is +sprinkled liberally with #ifdef blocks. As a general rule, #ifdef use +should be confined to header files whenever possible. +Conditionally-compiled code can be confined to functions which, if the code +is not to be present, simply become empty. The compiler will then quietly +optimize out the call to the empty function. The result is far cleaner +code which is easier to follow. + +C preprocessor macros present a number of hazards, including possible +multiple evaluation of expressions with side effects and no type safety. +If you are tempted to define a macro, consider creating an inline function +instead. The code which results will be the same, but inline functions are +easier to read, do not evaluate their arguments multiple times, and allow +the compiler to perform type checking on the arguments and return value. + + +* Inline functions + +Inline functions present a hazard of their own, though. Programmers can +become enamored of the perceived efficiency inherent in avoiding a function +call and fill a source file with inline functions. Those functions, +however, can actually reduce performance. Since their code is replicated +at each call site, they end up bloating the size of the compiled kernel. +That, in turn, creates pressure on the processor's memory caches, which can +slow execution dramatically. Inline functions, as a rule, should be quite +small and relatively rare. The cost of a function call, after all, is not +that high; the creation of large numbers of inline functions is a classic +example of premature optimization. + +In general, kernel programmers ignore cache effects at their peril. The +classic time/space tradeoff taught in beginning data structures classes +often does not apply to contemporary hardware. Space *is* time, in that a +larger program will run slower than one which is more compact. + + +* Locking + +In May, 2006, the "Devicescape" networking stack was, with great +fanfare, released under the GPL and made available for inclusion in the +mainline kernel. This donation was welcome news; support for wireless +networking in Linux was considered substandard at best, and the Devicescape +stack offered the promise of fixing that situation. Yet, this code did not +actually make it into the mainline until June, 2007 (2.6.22). What +happened? + +This code showed a number of signs of having been developed behind +corporate doors. But one large problem in particular was that it was not +designed to work on multiprocessor systems. Before this networking stack +(now called mac80211) could be merged, a locking scheme needed to be +retrofitted onto it. + +Once upon a time, Linux kernel code could be developed without thinking +about the concurrency issues presented by multiprocessor systems. Now, +however, this document is being written on a dual-core laptop. Even on +single-processor systems, work being done to improve responsiveness will +raise the level of concurrency within the kernel. The days when kernel +code could be written without thinking about locking are long past. + +Any resource (data structures, hardware registers, etc.) which could be +accessed concurrently by more than one thread must be protected by a lock. +New code should be written with this requirement in mind; retrofitting +locking after the fact is a rather more difficult task. Kernel developers +should take the time to understand the available locking primitives well +enough to pick the right tool for the job. Code which shows a lack of +attention to concurrency will have a difficult path into the mainline. + + +* Regressions + +One final hazard worth mentioning is this: it can be tempting to make a +change (which may bring big improvements) which causes something to break +for existing users. This kind of change is called a "regression," and +regressions have become most unwelcome in the mainline kernel. With few +exceptions, changes which cause regressions will be backed out if the +regression cannot be fixed in a timely manner. Far better to avoid the +regression in the first place. + +It is often argued that a regression can be justified if it causes things +to work for more people than it creates problems for. Why not make a +change if it brings new functionality to ten systems for each one it +breaks? The best answer to this question was expressed by Linus in July, +2007: + + So we don't fix bugs by introducing new problems. That way lies + madness, and nobody ever knows if you actually make any real + progress at all. Is it two steps forwards, one step back, or one + step forward and two steps back? + +(http://lwn.net/Articles/243460/). + +An especially unwelcome type of regression is any sort of change to the +user-space ABI. Once an interface has been exported to user space, it must +be supported indefinitely. This fact makes the creation of user-space +interfaces particularly challenging: since they cannot be changed in +incompatible ways, they must be done right the first time. For this +reason, a great deal of thought, clear documentation, and wide review for +user-space interfaces is always required. + + + +4.2: CODE CHECKING TOOLS + +For now, at least, the writing of error-free code remains an ideal that few +of us can reach. What we can hope to do, though, is to catch and fix as +many of those errors as possible before our code goes into the mainline +kernel. To that end, the kernel developers have put together an impressive +array of tools which can catch a wide variety of obscure problems in an +automated way. Any problem caught by the computer is a problem which will +not afflict a user later on, so it stands to reason that the automated +tools should be used whenever possible. + +The first step is simply to heed the warnings produced by the compiler. +Contemporary versions of gcc can detect (and warn about) a large number of +potential errors. Quite often, these warnings point to real problems. +Code submitted for review should, as a rule, not produce any compiler +warnings. When silencing warnings, take care to understand the real cause +and try to avoid "fixes" which make the warning go away without addressing +its cause. + +Note that not all compiler warnings are enabled by default. Build the +kernel with "make EXTRA_CFLAGS=-W" to get the full set. + +The kernel provides several configuration options which turn on debugging +features; most of these are found in the "kernel hacking" submenu. Several +of these options should be turned on for any kernel used for development or +testing purposes. In particular, you should turn on: + + - ENABLE_WARN_DEPRECATED, ENABLE_MUST_CHECK, and FRAME_WARN to get an + extra set of warnings for problems like the use of deprecated interfaces + or ignoring an important return value from a function. The output + generated by these warnings can be verbose, but one need not worry about + warnings from other parts of the kernel. + + - DEBUG_OBJECTS will add code to track the lifetime of various objects + created by the kernel and warn when things are done out of order. If + you are adding a subsystem which creates (and exports) complex objects + of its own, consider adding support for the object debugging + infrastructure. + + - DEBUG_SLAB can find a variety of memory allocation and use errors; it + should be used on most development kernels. + + - DEBUG_SPINLOCK, DEBUG_SPINLOCK_SLEEP, and DEBUG_MUTEXES will find a + number of common locking errors. + +There are quite a few other debugging options, some of which will be +discussed below. Some of them have a significant performance impact and +should not be used all of the time. But some time spent learning the +available options will likely be paid back many times over in short order. + +One of the heavier debugging tools is the locking checker, or "lockdep." +This tool will track the acquisition and release of every lock (spinlock or +mutex) in the system, the order in which locks are acquired relative to +each other, the current interrupt environment, and more. It can then +ensure that locks are always acquired in the same order, that the same +interrupt assumptions apply in all situations, and so on. In other words, +lockdep can find a number of scenarios in which the system could, on rare +occasion, deadlock. This kind of problem can be painful (for both +developers and users) in a deployed system; lockdep allows them to be found +in an automated manner ahead of time. Code with any sort of non-trivial +locking should be run with lockdep enabled before being submitted for +inclusion. + +As a diligent kernel programmer, you will, beyond doubt, check the return +status of any operation (such as a memory allocation) which can fail. The +fact of the matter, though, is that the resulting failure recovery paths +are, probably, completely untested. Untested code tends to be broken code; +you could be much more confident of your code if all those error-handling +paths had been exercised a few times. + +The kernel provides a fault injection framework which can do exactly that, +especially where memory allocations are involved. With fault injection +enabled, a configurable percentage of memory allocations will be made to +fail; these failures can be restricted to a specific range of code. +Running with fault injection enabled allows the programmer to see how the +code responds when things go badly. See +Documentation/fault-injection/fault-injection.text for more information on +how to use this facility. + +Other kinds of errors can be found with the "sparse" static analysis tool. +With sparse, the programmer can be warned about confusion between +user-space and kernel-space addresses, mixture of big-endian and +small-endian quantities, the passing of integer values where a set of bit +flags is expected, and so on. Sparse must be installed separately (it can +be found at http://www.kernel.org/pub/software/devel/sparse/ if your +distributor does not package it); it can then be run on the code by adding +"C=1" to your make command. + +Other kinds of portability errors are best found by compiling your code for +other architectures. If you do not happen to have an S/390 system or a +Blackfin development board handy, you can still perform the compilation +step. A large set of cross compilers for x86 systems can be found at + + http://www.kernel.org/pub/tools/crosstool/ + +Some time spent installing and using these compilers will help avoid +embarrassment later. + + +4.3: DOCUMENTATION + +Documentation has often been more the exception than the rule with kernel +development. Even so, adequate documentation will help to ease the merging +of new code into the kernel, make life easier for other developers, and +will be helpful for your users. In many cases, the addition of +documentation has become essentially mandatory. + +The first piece of documentation for any patch is its associated +changelog. Log entries should describe the problem being solved, the form +of the solution, the people who worked on the patch, any relevant +effects on performance, and anything else that might be needed to +understand the patch. + +Any code which adds a new user-space interface - including new sysfs or +/proc files - should include documentation of that interface which enables +user-space developers to know what they are working with. See +Documentation/ABI/README for a description of how this documentation should +be formatted and what information needs to be provided. + +The file Documentation/kernel-parameters.txt describes all of the kernel's +boot-time parameters. Any patch which adds new parameters should add the +appropriate entries to this file. + +Any new configuration options must be accompanied by help text which +clearly explains the options and when the user might want to select them. + +Internal API information for many subsystems is documented by way of +specially-formatted comments; these comments can be extracted and formatted +in a number of ways by the "kernel-doc" script. If you are working within +a subsystem which has kerneldoc comments, you should maintain them and add +them, as appropriate, for externally-available functions. Even in areas +which have not been so documented, there is no harm in adding kerneldoc +comments for the future; indeed, this can be a useful activity for +beginning kernel developers. The format of these comments, along with some +information on how to create kerneldoc templates can be found in the file +Documentation/kernel-doc-nano-HOWTO.txt. + +Anybody who reads through a significant amount of existing kernel code will +note that, often, comments are most notable by their absence. Once again, +the expectations for new code are higher than they were in the past; +merging uncommented code will be harder. That said, there is little desire +for verbosely-commented code. The code should, itself, be readable, with +comments explaining the more subtle aspects. + +Certain things should always be commented. Uses of memory barriers should +be accompanied by a line explaining why the barrier is necessary. The +locking rules for data structures generally need to be explained somewhere. +Major data structures need comprehensive documentation in general. +Non-obvious dependencies between separate bits of code should be pointed +out. Anything which might tempt a code janitor to make an incorrect +"cleanup" needs a comment saying why it is done the way it is. And so on. + + +4.4: INTERNAL API CHANGES + +The binary interface provided by the kernel to user space cannot be broken +except under the most severe circumstances. The kernel's internal +programming interfaces, instead, are highly fluid and can be changed when +the need arises. If you find yourself having to work around a kernel API, +or simply not using a specific functionality because it does not meet your +needs, that may be a sign that the API needs to change. As a kernel +developer, you are empowered to make such changes. + +There are, of course, some catches. API changes can be made, but they need +to be well justified. So any patch making an internal API change should be +accompanied by a description of what the change is and why it is +necessary. This kind of change should also be broken out into a separate +patch, rather than buried within a larger patch. + +The other catch is that a developer who changes an internal API is +generally charged with the task of fixing any code within the kernel tree +which is broken by the change. For a widely-used function, this duty can +lead to literally hundreds or thousands of changes - many of which are +likely to conflict with work being done by other developers. Needless to +say, this can be a large job, so it is best to be sure that the +justification is solid. + +When making an incompatible API change, one should, whenever possible, +ensure that code which has not been updated is caught by the compiler. +This will help you to be sure that you have found all in-tree uses of that +interface. It will also alert developers of out-of-tree code that there is +a change that they need to respond to. Supporting out-of-tree code is not +something that kernel developers need to be worried about, but we also do +not have to make life harder for out-of-tree developers than it it needs to +be. diff --git a/Documentation/development-process/5.Posting b/Documentation/development-process/5.Posting new file mode 100644 index 000000000000..dd48132a74dd --- /dev/null +++ b/Documentation/development-process/5.Posting @@ -0,0 +1,278 @@ +5: POSTING PATCHES + +Sooner or later, the time comes when your work is ready to be presented to +the community for review and, eventually, inclusion into the mainline +kernel. Unsurprisingly, the kernel development community has evolved a set +of conventions and procedures which are used in the posting of patches; +following them will make life much easier for everybody involved. This +document will attempt to cover these expectations in reasonable detail; +more information can also be found in the files SubmittingPatches, +SubmittingDrivers, and SubmitChecklist in the kernel documentation +directory. + + +5.1: WHEN TO POST + +There is a constant temptation to avoid posting patches before they are +completely "ready." For simple patches, that is not a problem. If the +work being done is complex, though, there is a lot to be gained by getting +feedback from the community before the work is complete. So you should +consider posting in-progress work, or even making a git tree available so +that interested developers can catch up with your work at any time. + +When posting code which is not yet considered ready for inclusion, it is a +good idea to say so in the posting itself. Also mention any major work +which remains to be done and any known problems. Fewer people will look at +patches which are known to be half-baked, but those who do will come in +with the idea that they can help you drive the work in the right direction. + + +5.2: BEFORE CREATING PATCHES + +There are a number of things which should be done before you consider +sending patches to the development community. These include: + + - Test the code to the extent that you can. Make use of the kernel's + debugging tools, ensure that the kernel will build with all reasonable + combinations of configuration options, use cross-compilers to build for + different architectures, etc. + + - Make sure your code is compliant with the kernel coding style + guidelines. + + - Does your change have performance implications? If so, you should run + benchmarks showing what the impact (or benefit) of your change is; a + summary of the results should be included with the patch. + + - Be sure that you have the right to post the code. If this work was done + for an employer, the employer likely has a right to the work and must be + agreeable with its release under the GPL. + +As a general rule, putting in some extra thought before posting code almost +always pays back the effort in short order. + + +5.3: PATCH PREPARATION + +The preparation of patches for posting can be a surprising amount of work, +but, once again, attempting to save time here is not generally advisable +even in the short term. + +Patches must be prepared against a specific version of the kernel. As a +general rule, a patch should be based on the current mainline as found in +Linus's git tree. It may become necessary to make versions against -mm, +linux-next, or a subsystem tree, though, to facilitate wider testing and +review. Depending on the area of your patch and what is going on +elsewhere, basing a patch against these other trees can require a +significant amount of work resolving conflicts and dealing with API +changes. + +Only the most simple changes should be formatted as a single patch; +everything else should be made as a logical series of changes. Splitting +up patches is a bit of an art; some developers spend a long time figuring +out how to do it in the way that the community expects. There are a few +rules of thumb, however, which can help considerably: + + - The patch series you post will almost certainly not be the series of + changes found in your working revision control system. Instead, the + changes you have made need to be considered in their final form, then + split apart in ways which make sense. The developers are interested in + discrete, self-contained changes, not the path you took to get to those + changes. + + - Each logically independent change should be formatted as a separate + patch. These changes can be small ("add a field to this structure") or + large (adding a significant new driver, for example), but they should be + conceptually small and amenable to a one-line description. Each patch + should make a specific change which can be reviewed on its own and + verified to do what it says it does. + + - As a way of restating the guideline above: do not mix different types of + changes in the same patch. If a single patch fixes a critical security + bug, rearranges a few structures, and reformats the code, there is a + good chance that it will be passed over and the important fix will be + lost. + + - Each patch should yield a kernel which builds and runs properly; if your + patch series is interrupted in the middle, the result should still be a + working kernel. Partial application of a patch series is a common + scenario when the "git bisect" tool is used to find regressions; if the + result is a broken kernel, you will make life harder for developers and + users who are engaging in the noble work of tracking down problems. + + - Do not overdo it, though. One developer recently posted a set of edits + to a single file as 500 separate patches - an act which did not make him + the most popular person on the kernel mailing list. A single patch can + be reasonably large as long as it still contains a single *logical* + change. + + - It can be tempting to add a whole new infrastructure with a series of + patches, but to leave that infrastructure unused until the final patch + in the series enables the whole thing. This temptation should be + avoided if possible; if that series adds regressions, bisection will + finger the last patch as the one which caused the problem, even though + the real bug is elsewhere. Whenever possible, a patch which adds new + code should make that code active immediately. + +Working to create the perfect patch series can be a frustrating process +which takes quite a bit of time and thought after the "real work" has been +done. When done properly, though, it is time well spent. + + +5.4: PATCH FORMATTING + +So now you have a perfect series of patches for posting, but the work is +not done quite yet. Each patch needs to be formatted into a message which +quickly and clearly communicates its purpose to the rest of the world. To +that end, each patch will be composed of the following: + + - An optional "From" line naming the author of the patch. This line is + only necessary if you are passing on somebody else's patch via email, + but it never hurts to add it when in doubt. + + - A one-line description of what the patch does. This message should be + enough for a reader who sees it with no other context to figure out the + scope of the patch; it is the line that will show up in the "short form" + changelogs. This message is usually formatted with the relevant + subsystem name first, followed by the purpose of the patch. For + example: + + gpio: fix build on CONFIG_GPIO_SYSFS=n + + - A blank line followed by a detailed description of the contents of the + patch. This description can be as long as is required; it should say + what the patch does and why it should be applied to the kernel. + + - One or more tag lines, with, at a minimum, one Signed-off-by: line from + the author of the patch. Tags will be described in more detail below. + +The above three items should, normally, be the text used when committing +the change to a revision control system. They are followed by: + + - The patch itself, in the unified ("-u") patch format. Using the "-p" + option to diff will associate function names with changes, making the + resulting patch easier for others to read. + +You should avoid including changes to irrelevant files (those generated by +the build process, for example, or editor backup files) in the patch. The +file "dontdiff" in the Documentation directory can help in this regard; +pass it to diff with the "-X" option. + +The tags mentioned above are used to describe how various developers have +been associated with the development of this patch. They are described in +detail in the SubmittingPatches document; what follows here is a brief +summary. Each of these lines has the format: + + tag: Full Name <email address> optional-other-stuff + +The tags in common use are: + + - Signed-off-by: this is a developer's certification that he or she has + the right to submit the patch for inclusion into the kernel. It is an + agreement to the Developer's Certificate of Origin, the full text of + which can be found in Documentation/SubmittingPatches. Code without a + proper signoff cannot be merged into the mainline. + + - Acked-by: indicates an agreement by another developer (often a + maintainer of the relevant code) that the patch is appropriate for + inclusion into the kernel. + + - Tested-by: states that the named person has tested the patch and found + it to work. + + - Reviewed-by: the named developer has reviewed the patch for correctness; + see the reviewer's statement in Documentation/SubmittingPatches for more + detail. + + - Reported-by: names a user who reported a problem which is fixed by this + patch; this tag is used to give credit to the (often underappreciated) + people who test our code and let us know when things do not work + correctly. + + - Cc: the named person received a copy of the patch and had the + opportunity to comment on it. + +Be careful in the addition of tags to your patches: only Cc: is appropriate +for addition without the explicit permission of the person named. + + +5.5: SENDING THE PATCH + +Before you mail your patches, there are a couple of other things you should +take care of: + + - Are you sure that your mailer will not corrupt the patches? Patches + which have had gratuitous white-space changes or line wrapping performed + by the mail client will not apply at the other end, and often will not + be examined in any detail. If there is any doubt at all, mail the patch + to yourself and convince yourself that it shows up intact. + + Documentation/email-clients.txt has some helpful hints on making + specific mail clients work for sending patches. + + - Are you sure your patch is free of silly mistakes? You should always + run patches through scripts/checkpatch.pl and address the complaints it + comes up with. Please bear in mind that checkpatch.pl, while being the + embodiment of a fair amount of thought about what kernel patches should + look like, is not smarter than you. If fixing a checkpatch.pl complaint + would make the code worse, don't do it. + +Patches should always be sent as plain text. Please do not send them as +attachments; that makes it much harder for reviewers to quote sections of +the patch in their replies. Instead, just put the patch directly into your +message. + +When mailing patches, it is important to send copies to anybody who might +be interested in it. Unlike some other projects, the kernel encourages +people to err on the side of sending too many copies; don't assume that the +relevant people will see your posting on the mailing lists. In particular, +copies should go to: + + - The maintainer(s) of the affected subsystem(s). As described earlier, + the MAINTAINERS file is the first place to look for these people. + + - Other developers who have been working in the same area - especially + those who might be working there now. Using git to see who else has + modified the files you are working on can be helpful. + + - If you are responding to a bug report or a feature request, copy the + original poster as well. + + - Send a copy to the relevant mailing list, or, if nothing else applies, + the linux-kernel list. + + - If you are fixing a bug, think about whether the fix should go into the + next stable update. If so, stable@kernel.org should get a copy of the + patch. Also add a "Cc: stable@kernel.org" to the tags within the patch + itself; that will cause the stable team to get a notification when your + fix goes into the mainline. + +When selecting recipients for a patch, it is good to have an idea of who +you think will eventually accept the patch and get it merged. While it +is possible to send patches directly to Linus Torvalds and have him merge +them, things are not normally done that way. Linus is busy, and there are +subsystem maintainers who watch over specific parts of the kernel. Usually +you will be wanting that maintainer to merge your patches. If there is no +obvious maintainer, Andrew Morton is often the patch target of last resort. + +Patches need good subject lines. The canonical format for a patch line is +something like: + + [PATCH nn/mm] subsys: one-line description of the patch + +where "nn" is the ordinal number of the patch, "mm" is the total number of +patches in the series, and "subsys" is the name of the affected subsystem. +Clearly, nn/mm can be omitted for a single, standalone patch. + +If you have a significant series of patches, it is customary to send an +introductory description as part zero. This convention is not universally +followed though; if you use it, remember that information in the +introduction does not make it into the kernel changelogs. So please ensure +that the patches, themselves, have complete changelog information. + +In general, the second and following parts of a multi-part patch should be +sent as a reply to the first part so that they all thread together at the +receiving end. Tools like git and quilt have commands to mail out a set of +patches with the proper threading. If you have a long series, though, and +are using git, please provide the --no-chain-reply-to option to avoid +creating exceptionally deep nesting. diff --git a/Documentation/development-process/6.Followthrough b/Documentation/development-process/6.Followthrough new file mode 100644 index 000000000000..a8fba3d83a85 --- /dev/null +++ b/Documentation/development-process/6.Followthrough @@ -0,0 +1,202 @@ +6: FOLLOWTHROUGH + +At this point, you have followed the guidelines given so far and, with the +addition of your own engineering skills, have posted a perfect series of +patches. One of the biggest mistakes that even experienced kernel +developers can make is to conclude that their work is now done. In truth, +posting patches indicates a transition into the next stage of the process, +with, possibly, quite a bit of work yet to be done. + +It is a rare patch which is so good at its first posting that there is no +room for improvement. The kernel development process recognizes this fact, +and, as a result, is heavily oriented toward the improvement of posted +code. You, as the author of that code, will be expected to work with the +kernel community to ensure that your code is up to the kernel's quality +standards. A failure to participate in this process is quite likely to +prevent the inclusion of your patches into the mainline. + + +6.1: WORKING WITH REVIEWERS + +A patch of any significance will result in a number of comments from other +developers as they review the code. Working with reviewers can be, for +many developers, the most intimidating part of the kernel development +process. Life can be made much easier, though, if you keep a few things in +mind: + + - If you have explained your patch well, reviewers will understand its + value and why you went to the trouble of writing it. But that value + will not keep them from asking a fundamental question: what will it be + like to maintain a kernel with this code in it five or ten years later? + Many of the changes you may be asked to make - from coding style tweaks + to substantial rewrites - come from the understanding that Linux will + still be around and under development a decade from now. + + - Code review is hard work, and it is a relatively thankless occupation; + people remember who wrote kernel code, but there is little lasting fame + for those who reviewed it. So reviewers can get grumpy, especially when + they see the same mistakes being made over and over again. If you get a + review which seems angry, insulting, or outright offensive, resist the + impulse to respond in kind. Code review is about the code, not about + the people, and code reviewers are not attacking you personally. + + - Similarly, code reviewers are not trying to promote their employers' + agendas at the expense of your own. Kernel developers often expect to + be working on the kernel years from now, but they understand that their + employer could change. They truly are, almost without exception, + working toward the creation of the best kernel they can; they are not + trying to create discomfort for their employers' competitors. + +What all of this comes down to is that, when reviewers send you comments, +you need to pay attention to the technical observations that they are +making. Do not let their form of expression or your own pride keep that +from happening. When you get review comments on a patch, take the time to +understand what the reviewer is trying to say. If possible, fix the things +that the reviewer is asking you to fix. And respond back to the reviewer: +thank them, and describe how you will answer their questions. + +Note that you do not have to agree with every change suggested by +reviewers. If you believe that the reviewer has misunderstood your code, +explain what is really going on. If you have a technical objection to a +suggested change, describe it and justify your solution to the problem. If +your explanations make sense, the reviewer will accept them. Should your +explanation not prove persuasive, though, especially if others start to +agree with the reviewer, take some time to think things over again. It can +be easy to become blinded by your own solution to a problem to the point +that you don't realize that something is fundamentally wrong or, perhaps, +you're not even solving the right problem. + +One fatal mistake is to ignore review comments in the hope that they will +go away. They will not go away. If you repost code without having +responded to the comments you got the time before, you're likely to find +that your patches go nowhere. + +Speaking of reposting code: please bear in mind that reviewers are not +going to remember all the details of the code you posted the last time +around. So it is always a good idea to remind reviewers of previously +raised issues and how you dealt with them; the patch changelog is a good +place for this kind of information. Reviewers should not have to search +through list archives to familiarize themselves with what was said last +time; if you help them get a running start, they will be in a better mood +when they revisit your code. + +What if you've tried to do everything right and things still aren't going +anywhere? Most technical disagreements can be resolved through discussion, +but there are times when somebody simply has to make a decision. If you +honestly believe that this decision is going against you wrongly, you can +always try appealing to a higher power. As of this writing, that higher +power tends to be Andrew Morton. Andrew has a great deal of respect in the +kernel development community; he can often unjam a situation which seems to +be hopelessly blocked. Appealing to Andrew should not be done lightly, +though, and not before all other alternatives have been explored. And bear +in mind, of course, that he may not agree with you either. + + +6.2: WHAT HAPPENS NEXT + +If a patch is considered to be a good thing to add to the kernel, and once +most of the review issues have been resolved, the next step is usually +entry into a subsystem maintainer's tree. How that works varies from one +subsystem to the next; each maintainer has his or her own way of doing +things. In particular, there may be more than one tree - one, perhaps, +dedicated to patches planned for the next merge window, and another for +longer-term work. + +For patches applying to areas for which there is no obvious subsystem tree +(memory management patches, for example), the default tree often ends up +being -mm. Patches which affect multiple subsystems can also end up going +through the -mm tree. + +Inclusion into a subsystem tree can bring a higher level of visibility to a +patch. Now other developers working with that tree will get the patch by +default. Subsystem trees typically feed into -mm and linux-next as well, +making their contents visible to the development community as a whole. At +this point, there's a good chance that you will get more comments from a +new set of reviewers; these comments need to be answered as in the previous +round. + +What may also happen at this point, depending on the nature of your patch, +is that conflicts with work being done by others turn up. In the worst +case, heavy patch conflicts can result in some work being put on the back +burner so that the remaining patches can be worked into shape and merged. +Other times, conflict resolution will involve working with the other +developers and, possibly, moving some patches between trees to ensure that +everything applies cleanly. This work can be a pain, but count your +blessings: before the advent of the linux-next tree, these conflicts often +only turned up during the merge window and had to be addressed in a hurry. +Now they can be resolved at leisure, before the merge window opens. + +Some day, if all goes well, you'll log on and see that your patch has been +merged into the mainline kernel. Congratulations! Once the celebration is +complete (and you have added yourself to the MAINTAINERS file), though, it +is worth remembering an important little fact: the job still is not done. +Merging into the mainline brings its own challenges. + +To begin with, the visibility of your patch has increased yet again. There +may be a new round of comments from developers who had not been aware of +the patch before. It may be tempting to ignore them, since there is no +longer any question of your code being merged. Resist that temptation, +though; you still need to be responsive to developers who have questions or +suggestions. + +More importantly, though: inclusion into the mainline puts your code into +the hands of a much larger group of testers. Even if you have contributed +a driver for hardware which is not yet available, you will be surprised by +how many people will build your code into their kernels. And, of course, +where there are testers, there will be bug reports. + +The worst sort of bug reports are regressions. If your patch causes a +regression, you'll find an uncomfortable number of eyes upon you; +regressions need to be fixed as soon as possible. If you are unwilling or +unable to fix the regression (and nobody else does it for you), your patch +will almost certainly be removed during the stabilization period. Beyond +negating all of the work you have done to get your patch into the mainline, +having a patch pulled as the result of a failure to fix a regression could +well make it harder for you to get work merged in the future. + +After any regressions have been dealt with, there may be other, ordinary +bugs to deal with. The stabilization period is your best opportunity to +fix these bugs and ensure that your code's debut in a mainline kernel +release is as solid as possible. So, please, answer bug reports, and fix +the problems if at all possible. That's what the stabilization period is +for; you can start creating cool new patches once any problems with the old +ones have been taken care of. + +And don't forget that there are other milestones which may also create bug +reports: the next mainline stable release, when prominent distributors pick +up a version of the kernel containing your patch, etc. Continuing to +respond to these reports is a matter of basic pride in your work. If that +is insufficient motivation, though, it's also worth considering that the +development community remembers developers who lose interest in their code +after it's merged. The next time you post a patch, they will be evaluating +it with the assumption that you will not be around to maintain it +afterward. + + +6.3: OTHER THINGS THAT CAN HAPPEN + +One day, you may open your mail client and see that somebody has mailed you +a patch to your code. That is one of the advantages of having your code +out there in the open, after all. If you agree with the patch, you can +either forward it on to the subsystem maintainer (be sure to include a +proper From: line so that the attribution is correct, and add a signoff of +your own), or send an Acked-by: response back and let the original poster +send it upward. + +If you disagree with the patch, send a polite response explaining why. If +possible, tell the author what changes need to be made to make the patch +acceptable to you. There is a certain resistance to merging patches which +are opposed by the author and maintainer of the code, but it only goes so +far. If you are seen as needlessly blocking good work, those patches will +eventually flow around you and get into the mainline anyway. In the Linux +kernel, nobody has absolute veto power over any code. Except maybe Linus. + +On very rare occasion, you may see something completely different: another +developer posts a different solution to your problem. At that point, +chances are that one of the two patches will not be merged, and "mine was +here first" is not considered to be a compelling technical argument. If +somebody else's patch displaces yours and gets into the mainline, there is +really only one way to respond: be pleased that your problem got solved and +get on with your work. Having one's work shoved aside in this manner can +be hurtful and discouraging, but the community will remember your reaction +long after they have forgotten whose patch actually got merged. diff --git a/Documentation/development-process/7.AdvancedTopics b/Documentation/development-process/7.AdvancedTopics new file mode 100644 index 000000000000..a2cf74093aa1 --- /dev/null +++ b/Documentation/development-process/7.AdvancedTopics @@ -0,0 +1,173 @@ +7: ADVANCED TOPICS + +At this point, hopefully, you have a handle on how the development process +works. There is still more to learn, however! This section will cover a +number of topics which can be helpful for developers wanting to become a +regular part of the Linux kernel development process. + +7.1: MANAGING PATCHES WITH GIT + +The use of distributed version control for the kernel began in early 2002, +when Linus first started playing with the proprietary BitKeeper +application. While BitKeeper was controversial, the approach to software +version management it embodied most certainly was not. Distributed version +control enabled an immediate acceleration of the kernel development +project. In current times, there are several free alternatives to +BitKeeper. For better or for worse, the kernel project has settled on git +as its tool of choice. + +Managing patches with git can make life much easier for the developer, +especially as the volume of those patches grows. Git also has its rough +edges and poses certain hazards; it is a young and powerful tool which is +still being civilized by its developers. This document will not attempt to +teach the reader how to use git; that would be sufficient material for a +long document in its own right. Instead, the focus here will be on how git +fits into the kernel development process in particular. Developers who +wish to come up to speed with git will find more information at: + + http://git.or.cz/ + + http://www.kernel.org/pub/software/scm/git/docs/user-manual.html + +and on various tutorials found on the web. + +The first order of business is to read the above sites and get a solid +understanding of how git works before trying to use it to make patches +available to others. A git-using developer should be able to obtain a copy +of the mainline repository, explore the revision history, commit changes to +the tree, use branches, etc. An understanding of git's tools for the +rewriting of history (such as rebase) is also useful. Git comes with its +own terminology and concepts; a new user of git should know about refs, +remote branches, the index, fast-forward merges, pushes and pulls, detached +heads, etc. It can all be a little intimidating at the outset, but the +concepts are not that hard to grasp with a bit of study. + +Using git to generate patches for submission by email can be a good +exercise while coming up to speed. + +When you are ready to start putting up git trees for others to look at, you +will, of course, need a server that can be pulled from. Setting up such a +server with git-daemon is relatively straightforward if you have a system +which is accessible to the Internet. Otherwise, free, public hosting sites +(Github, for example) are starting to appear on the net. Established +developers can get an account on kernel.org, but those are not easy to come +by; see http://kernel.org/faq/ for more information. + +The normal git workflow involves the use of a lot of branches. Each line +of development can be separated into a separate "topic branch" and +maintained independently. Branches in git are cheap, there is no reason to +not make free use of them. And, in any case, you should not do your +development in any branch which you intend to ask others to pull from. +Publicly-available branches should be created with care; merge in patches +from development branches when they are in complete form and ready to go - +not before. + +Git provides some powerful tools which can allow you to rewrite your +development history. An inconvenient patch (one which breaks bisection, +say, or which has some other sort of obvious bug) can be fixed in place or +made to disappear from the history entirely. A patch series can be +rewritten as if it had been written on top of today's mainline, even though +you have been working on it for months. Changes can be transparently +shifted from one branch to another. And so on. Judicious use of git's +ability to revise history can help in the creation of clean patch sets with +fewer problems. + +Excessive use of this capability can lead to other problems, though, beyond +a simple obsession for the creation of the perfect project history. +Rewriting history will rewrite the changes contained in that history, +turning a tested (hopefully) kernel tree into an untested one. But, beyond +that, developers cannot easily collaborate if they do not have a shared +view of the project history; if you rewrite history which other developers +have pulled into their repositories, you will make life much more difficult +for those developers. So a simple rule of thumb applies here: history +which has been exported to others should generally be seen as immutable +thereafter. + +So, once you push a set of changes to your publicly-available server, those +changes should not be rewritten. Git will attempt to enforce this rule if +you try to push changes which do not result in a fast-forward merge +(i.e. changes which do not share the same history). It is possible to +override this check, and there may be times when it is necessary to rewrite +an exported tree. Moving changesets between trees to avoid conflicts in +linux-next is one example. But such actions should be rare. This is one +of the reasons why development should be done in private branches (which +can be rewritten if necessary) and only moved into public branches when +it's in a reasonably advanced state. + +As the mainline (or other tree upon which a set of changes is based) +advances, it is tempting to merge with that tree to stay on the leading +edge. For a private branch, rebasing can be an easy way to keep up with +another tree, but rebasing is not an option once a tree is exported to the +world. Once that happens, a full merge must be done. Merging occasionally +makes good sense, but overly frequent merges can clutter the history +needlessly. Suggested technique in this case is to merge infrequently, and +generally only at specific release points (such as a mainline -rc +release). If you are nervous about specific changes, you can always +perform test merges in a private branch. The git "rerere" tool can be +useful in such situations; it remembers how merge conflicts were resolved +so that you don't have to do the same work twice. + +One of the biggest recurring complaints about tools like git is this: the +mass movement of patches from one repository to another makes it easy to +slip in ill-advised changes which go into the mainline below the review +radar. Kernel developers tend to get unhappy when they see that kind of +thing happening; putting up a git tree with unreviewed or off-topic patches +can affect your ability to get trees pulled in the future. Quoting Linus: + + You can send me patches, but for me to pull a git patch from you, I + need to know that you know what you're doing, and I need to be able + to trust things *without* then having to go and check every + individual change by hand. + +(http://lwn.net/Articles/224135/). + +To avoid this kind of situation, ensure that all patches within a given +branch stick closely to the associated topic; a "driver fixes" branch +should not be making changes to the core memory management code. And, most +importantly, do not use a git tree to bypass the review process. Post an +occasional summary of the tree to the relevant list, and, when the time is +right, request that the tree be included in linux-next. + +If and when others start to send patches for inclusion into your tree, +don't forget to review them. Also ensure that you maintain the correct +authorship information; the git "am" tool does its best in this regard, but +you may have to add a "From:" line to the patch if it has been relayed to +you via a third party. + +When requesting a pull, be sure to give all the relevant information: where +your tree is, what branch to pull, and what changes will result from the +pull. The git request-pull command can be helpful in this regard; it will +format the request as other developers expect, and will also check to be +sure that you have remembered to push those changes to the public server. + + +7.2: REVIEWING PATCHES + +Some readers will certainly object to putting this section with "advanced +topics" on the grounds that even beginning kernel developers should be +reviewing patches. It is certainly true that there is no better way to +learn how to program in the kernel environment than by looking at code +posted by others. In addition, reviewers are forever in short supply; by +looking at code you can make a significant contribution to the process as a +whole. + +Reviewing code can be an intimidating prospect, especially for a new kernel +developer who may well feel nervous about questioning code - in public - +which has been posted by those with more experience. Even code written by +the most experienced developers can be improved, though. Perhaps the best +piece of advice for reviewers (all reviewers) is this: phrase review +comments as questions rather than criticisms. Asking "how does the lock +get released in this path?" will always work better than stating "the +locking here is wrong." + +Different developers will review code from different points of view. Some +are mostly concerned with coding style and whether code lines have trailing +white space. Others will focus primarily on whether the change implemented +by the patch as a whole is a good thing for the kernel or not. Yet others +will check for problematic locking, excessive stack usage, possible +security issues, duplication of code found elsewhere, adequate +documentation, adverse effects on performance, user-space ABI changes, etc. +All types of review, if they lead to better code going into the kernel, are +welcome and worthwhile. + + diff --git a/Documentation/development-process/8.Conclusion b/Documentation/development-process/8.Conclusion new file mode 100644 index 000000000000..1990ab4b4949 --- /dev/null +++ b/Documentation/development-process/8.Conclusion @@ -0,0 +1,74 @@ +8: FOR MORE INFORMATION + +There are numerous sources of information on Linux kernel development and +related topics. First among those will always be the Documentation +directory found in the kernel source distribution. The top-level HOWTO +file is an important starting point; SubmittingPatches and +SubmittingDrivers are also something which all kernel developers should +read. Many internal kernel APIs are documented using the kerneldoc +mechanism; "make htmldocs" or "make pdfdocs" can be used to generate those +documents in HTML or PDF format (though the version of TeX shipped by some +distributions runs into internal limits and fails to process the documents +properly). + +Various web sites discuss kernel development at all levels of detail. Your +author would like to humbly suggest http://lwn.net/ as a source; +information on many specific kernel topics can be found via the LWN kernel +index at: + + http://lwn.net/Kernel/Index/ + +Beyond that, a valuable resource for kernel developers is: + + http://kernelnewbies.org/ + +Information about the linux-next tree gathers at: + + http://linux.f-seidel.de/linux-next/pmwiki/ + +And, of course, one should not forget http://kernel.org/, the definitive +location for kernel release information. + +There are a number of books on kernel development: + + Linux Device Drivers, 3rd Edition (Jonathan Corbet, Alessandro + Rubini, and Greg Kroah-Hartman). Online at + http://lwn.net/Kernel/LDD3/. + + Linux Kernel Development (Robert Love). + + Understanding the Linux Kernel (Daniel Bovet and Marco Cesati). + +All of these books suffer from a common fault, though: they tend to be +somewhat obsolete by the time they hit the shelves, and they have been on +the shelves for a while now. Still, there is quite a bit of good +information to be found there. + +Documentation for git can be found at: + + http://www.kernel.org/pub/software/scm/git/docs/ + + http://www.kernel.org/pub/software/scm/git/docs/user-manual.html + + +9: CONCLUSION + +Congratulations to anybody who has made it through this long-winded +document. Hopefully it has provided a helpful understanding of how the +Linux kernel is developed and how you can participate in that process. + +In the end, it's the participation that matters. Any open source software +project is no more than the sum of what its contributors put into it. The +Linux kernel has progressed as quickly and as well as it has because it has +been helped by an impressively large group of developers, all of whom are +working to make it better. The kernel is a premier example of what can be +done when thousands of people work together toward a common goal. + +The kernel can always benefit from a larger developer base, though. There +is always more work to do. But, just as importantly, most other +participants in the Linux ecosystem can benefit through contributing to the +kernel. Getting code into the mainline is the key to higher code quality, +lower maintenance and distribution costs, a higher level of influence over +the direction of kernel development, and more. It is a situation where +everybody involved wins. Fire up your editor and come join us; you will be +more than welcome. |