Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932202AbVKNWUg (ORCPT ); Mon, 14 Nov 2005 17:20:36 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S932203AbVKNWUg (ORCPT ); Mon, 14 Nov 2005 17:20:36 -0500 Received: from mail.kroah.org ([69.55.234.183]:32140 "EHLO perch.kroah.org") by vger.kernel.org with ESMTP id S932202AbVKNWUg (ORCPT ); Mon, 14 Nov 2005 17:20:36 -0500 Date: Mon, 14 Nov 2005 14:07:09 -0800 From: Greg KH To: linux-kernel@vger.kernel.org Cc: greg@kroah.com Subject: [RFC] HOWTO do Linux kernel development Message-ID: <20051114220709.GA5234@kroah.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.5.11 Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 17971 Lines: 409 Over time, I get a lot of the same kind of emails from developers. Messages asking how to do this or that, or how this process works. I also see a lot of new developers make the same mistakes (wrong patch format, no signed-off-by:, not sent to the proper developer, wrong coding style, etc.) Along with these requests, I have heard a lot of complaints over time, about how there is no single place to go to to figure out how to do Linux kernel development, and where to point other people to. So, I've been working on a document for the past week or so to help alleviate a lot of these problems. If nothing else, it should be a place where anyone can point someone to when they ask the common questions, or do something in the not-correct way. I'd like to add this to the Linux kernel source tree, so it will be kept up to date over time, as things change (like the development process.) Ideally I'd like to put it in the main directory as HOWTO, but I don't know how others feel about this. Anyway, I'd like to get comments on what has been produced so far. I know the section about the development process is still not complete (it has a mark), and if anyone wants to fill that in, I'd really appreciate it. I would like to thank Pat Mochel, Hanna Linder, Randy Dunlap, Kay Sievers, Vojtech Pavlik, and Jan Kara for their review and comments on early drafts of this document. thanks, greg k-h ------------------------------ HOWTO do Linux kernel development --------------------------------- This is the be-all, end-all document on this topic. It contains instructions on how to become a Linux kernel developer and how to learn to work with the Linux kernel development community. If anything in this document becomes out of date, please send in patches to the maintainer of this file, who is listed at the bottom of the document. Intro ----- So, you want to learn how to become a Linux kernel developer? Or you have been told by your manager, "Go write a Linux driver for this device." This document's goal is to teach you everything you need to know to achieve this by describing the process you need to go through, and hints on how to work with the community. It will also try to explain some of the reasons why the community works like it does. The kernel is written mostly in C, with some architectural-dependent parts written in assembly. A good understanding of C is required to kernel development. Assembly (any architecture) is not required unless you plan to do low-level development for that architecture. Though they are not a good substitute for a solid C education and/or years of experience, the following books are good, if anything for reference: "The C Programming Language" by Kernighan and Ritchie [Prentice Hall] "Practical C Programming" by Steve Oualline [O'Reilly] "Programming the 80386" by Crawford and Gelsinger [Sybek] "UNIX Systems for Modern Architectures" by Curt Schimmel [Addison Wesley] The kernel is written using GNU C and the GNU toolchain. While it adheres to the ISO C99 (??) standard, it uses a number of extensions that are not featured in the standard. It can sometimes be difficult to understand the assumptions the kernel has on the toolchain and the extensions that it uses, and unfortunately there is no definitive reference for them. Please check the gcc info pages (`info gcc`) for some information on them. Please remember that you are trying to learn how to work with the existing development community. It is a very diverse group of people, with very high standards for coding, style and procedure. These procedures have been created over time based on what they have found to work best for such a large and geographically dispersed team. Try to learn as much as possible about these procedures ahead of time, as they are well documented, and not expect people to adapt to you, or your company's way of doing things. Legal Issues ------------ The Linux kernel source code is released under the GPL. Please see the file, COPYING, in the main directory of the source tree, for details on the license. If you have further questions about the license, please contact a lawyer, and do not ask on the Linux kernel mailing list. The people on the mailing lists are not lawyers, and you should not rely on their statements on legal matters. Documentation ------------ The Linux kernel source tree has a large range of documents that are invaluable in learning how to interact with the kernel community. When new features are added to the kernel, it is recommended that new documentation files are also added, that explain how to use the feature. Here is a list of files that are in the kernel source tree that are required reading: Documetation/CodingStyle This describes the Linux kernel coding style, and some of the rationale behind it. All new code is expected to follow the guidelines in this document. Most maintainers will only accept patches if these rules are followed, and many people will only review code if it is in the proper style. Documentation/SubmittingPatches Documentation/SubmittingDrivers These files describe in explicit detail how to successfully create and send a patch, including (but not limited to): - Email contents - Email format - Who to send it to Following these rules will not guarantee success (as all patches are subject to scrutiny of content and style), but not following them will almost always prevent it. Other excellent descriptions of how to create patches properly are: "The Perfect Patch" http://www.zip.com.au/~akpm/linux/patches/stuff/tpp.txt "Linux kernel patch submission format" http://linux.yyz.us/patch-format.html Documentation/stable_api_nonsense.txt This file describes the rationale behind the conscious decision to not have a stable API within the kernel, including for things like: - Subsystem shim-layers (for compatibility?) - Driver portability between Operating Systems. - Mitigating rapid change within the kernel source tree (or preventing rapid change) This document is crucial for understand the Linux development philosophy and is very important for people moving to Linux from development on other Operating Systems. Documentation/SecurityBugs If you feel you have found a security problem in the Linux kernel, please follow the steps in this document to help notify the kernel developers, and help solve the issue. Documentation/ManagementStyle This document describes how Linux kernel maintainers operate and the shared ethos behind their methodologies. This is important reading for anyone new to kernel development (or anyone simply curious about it), as it resolves a lot of common misconceptions and confusion about the unique behavior of kernel maintainers. Documentation/stable_kernel_rules.txt This file describes the rules on how the stable kernel releases happen, and what to do if you want to get a change into one of these releases. Documentation/kernel-docs.txt A list of external documentation that pertains to kernel development. Please consult this list, if you do not find what you are looking for within the in-kernel documentation. The kernel also has a large number of documents that can be automatically generated from the source code itself. This includes a full description of the in-kernel api, and rules on how to handle locking properly. The documents will be created in the Documentation/DocBook/ directory and can be generated as PDF, Postscript, HTML, and man pages by running: make pdfdocs make psdocs make htmldocs make mandocs respectively from the main kernel source directory. Becoming A Kernel Developer --------------------------- If you do not know anything about Linux kernel development, you should look at the Linux KernelNewbies project: http://kernelnewbies.org It consists of a helpful mailing list, where you can ask almost any type of basic kernel development question (make sure to search the archives first, before asking something that has already been answered in the past.) It also has a IRC channel that you can use to ask questions in real-time, and a lot of helpful documentation that is useful for learning about Linux kernel development. The website has basic information about code organization, subsystems, and current projects (both in-tree and out-of-tree). It also basic logistical information, like compiling a kernel and applying a patch. If you do not know where you want to start, but you want to look for some task to start doing to join into the kernel development community, go to the Linux Kernel Janitor's project: http://janitor.kernelnewbies.org/ It is a great place to start. It describes a list of relatively simple tasks that need to be cleaned up and fixed within the Linux kernel source tree. Working with the developers in charge of this project, you will learn the basics of getting your patch into the Linux kernel tree, and possibly point you in the direction of what to go work on next, if you do not already have an idea. If you already have a chunk of code that you want to have go into the kernel tree, but need some help getting it in the proper form, the kernel-mentors project was created to help you out with this. It is a mailing list, and can be found at: http://selenic.com/mailman/listinfo/kernel-mentors The development process ----------------------- Mailing lists ------------- As some of the above documents describe, the majority of the core kernel developers participate on the Linux Kernel Mailing list. Details on how to subscribe and unsubscribe from the list, can be found at: http://vger.kernel.org/vger-lists.html#linux-kernel There are archives of the mailing list on the web in many different places. Use a search engine to find these archives. It is highly recommended that you search the archives about the topic you want to bring up, before you post it to the list. A lot of things are already discussed in detail and are only recorded at the mailing list archives. Most of the individual kernel subsystems also have their own separate mailing list where they do their development efforts. See the MAINTAINERS file for a list of what these lists are, for the different groups. Many of the lists are hosted on kernel.org. Information on them can be found here: http://vger.kernel.org/vger-lists.html Please remember to follow good behavioral habits when using the lists. Though a bit cheesy, the following URL has some simple guidelines for interacting with the list (or any list): http://www.albion.com/netiquette/ If multiple people respond to your mail, the CC: list of recipients may get pretty large. Don't remove anybody from the CC: list without a good reason, or don't reply only to the list address. Get used to receive the mail twice, one from the sender and the one from the list and don't try to tune that by adding fancy mail-headers, people will not like it. Remember to keep the context and the attribution of your replies intact, keep the "John Kernelhacker wrote ...:" lines at the top of you reply and add your statements between the individual quoted sections instead of writing at the top of the mail. If you add patches to your mail, make sure they are plain readable text as stated in Documentation/SubmittingPatches. Kernel developers don't want to deal with attachments or compressed patches, they may want to comment individual lines of your patch, which works only that way. Make sure you use a mail program that does not mangle spaces and tab characters. A good first test is to send the mail to yourself and try to apply your own patch by yourself. If that doesn't work, get your mail program fixed or change it until it works. Above all, please remember to show respect to other subscribers. Working with the community -------------------------- The kernel community works differently than most traditional corporate development environments. Here are a list of things that you can try to do to try to avoid problems: Good things to say regarding your proposed changes: - "This solves multiple problems." - "This deletes 2000 lines of code." - "Here is a patch that explains what I am trying to describe." - "I tested it on 5 different architectures..." - "Here is a series of small patches that..." - "This increases performance on typical machines..." Bad things you should avoid saying: - "We did it this way in AIX/ptx/Solaris, so therefore it must be good..." - "I've being doing this for 20 years, so..." - "It makes this proprietary benchmark go faster" - "This is required for my company to make money" - "This is for our Enterprise product line." - "Here is my 1000 page design document that describes my idea" - "I've been working on this for 6 months..." - "Here's a 5000 line patch that..." - "I rewrote all of the current mess, and here it is..." - "I have a deadline, and this patch needs to be applied now." Another way the kernel community is different than most traditional software engineering work environments is the faceless nature of interaction. One benefit of using email and irc as the primary forms of communication is the lack of discrimination based on gender or race. The Linux kernel work environment is accepting of women and minorities because all you are is an email address. The international aspect also helps to level the playing field because you can't guess gender based on a person's name. A man may be named Andrea and a woman may be named Pat. Most women who have worked in the Linux kernel and have expressed an opinion have had positive experiences. Here is a group that is a good starting point for women interested in contributing to Linux: http://www.linuxchix.org/ The language barrier can be present for some people who are not comfortable with English. A good grasp of the language can be needed in order to get ideas across properly on mailing lists, so it is recommended that you check your emails to make sure they make sense in English before sending them. Break your changes up --------------------- The Linux kernel community does not gladly accept large chunks of code dropped on it all at once. The changes need to be properly introduced, discussed, and broken up into tiny, individual portions. This is almost exactly opposite of what companies are used to doing. Your proposal should also be introduced very early in the development process, so that you can receive feedback on what you are doing. It also lets the community feel that you are working with them, and not simply using them as a dumping ground for your feature. However, don't send 50 emails at one time to a mailing list, your patch series should be smaller than that almost all of the time. The reasons for breaking things up are the following: 1) Small patches increase the likelihood that your patches will be applied, since they don't take much time or effort to verify for correctness. A 5 line patch can be applied by a maintainer with barely a second glance. But, a 500 line patch may take hours to review for correctness (the time it takes is exponentially proportional to the size of the patch, or something). Small patches also make it very easy to debug when something goes wrong. It's much easier to back out patches one by one, than it is to dissect a very large patch after it's been applied (and broken something). 2) It's important not only to send small patches, but also to rewrite and simplify (or simply re-order) patches before submitting them. Here is an analogy from kernel developer Al Viro: "Think of a teacher grading homework from a math student. The teacher does not want to see the student's trials and errors before they came up with the solution. They want to see the cleanest, most elegant answer. A good student knows this, and would never submit her intermediate work before the final solution." The same is true of kernel development. The maintainers and reviewers do not want to see the thought process behind the solution to the problem one is solving. They want to see a simple and elegant solution." That may be challenging to keep the balance between presenting an elegant solution and working together with the community and discuss your unfinished work. Therefore it is good to get early in the process to get feedback to improve your work, but also keep your changes in small chunks that they may get already accepted, even when your whole task is not ready for inclusion now. Also realize that it is not acceptable to send patches for inclusion that are unfinished and will be "fixed up later." Justify your change ------------------- Along with breaking up your patches, it is very important for you to let the Linux community know why they should add this change. New features must be justified as being needed and useful. All of these things are sometimes very hard to do. It can take years to perfect these practices (if at all). It's a continuous process of improvement that requires a lot of patience and determination. But, don't give up. It's possible. Many have done it before, and each had to start exactly where you are now. ---------- Thanks to Randy Dunlap and Gerrit Huizenga for the list of things you should and should not say. Also thanks to Pat Mochel, Hanna Linder, Randy Dunlap, Kay Sievers, Vojtech Pavlik, and Jan Kara for their review and comments on early drafts of this document. Maintainer: Greg Kroah-Hartman - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/