Introduction¶

So, you want to learn how to become a Linux kernel developer? Or youhave been told by your manager, “Go write a Linux driver for thisdevice.” This document’s goal is to teach you everything you need toknow 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 toexplain some of the reasons why the community works like it does.

The kernel is written mostly in C, with some architecture-dependentparts written in assembly. A good understanding of C is required forkernel development. Assembly (any architecture) is not required unlessyou plan to do low-level development for that architecture. Though theyare not a good substitute for a solid C education and/or years ofexperience, the following books are good for, if anything, reference:

• “The C Programming Language” by Kernighan and Ritchie [Prentice Hall]
• “Practical C Programming” by Steve Oualline [O’Reilly]
• “C: A Reference Manual” by Harbison and Steele [Prentice Hall]

The kernel is written using GNU C and the GNU toolchain. While itadheres to the ISO C89 standard, it uses a number of extensions that arenot featured in the standard. The kernel is a freestanding Cenvironment, with no reliance on the standard C library, so someportions of the C standard are not supported. Arbitrary long longdivisions and floating point are not allowed. It can sometimes bedifficult to understand the assumptions the kernel has on the toolchainand the extensions that it uses, and unfortunately there is nodefinitive reference for them. Please check the gcc info pages (infogcc) for some information on them.

Please remember that you are trying to learn how to work with theexisting development community. It is a diverse group of people, withhigh standards for coding, style and procedure. These standards havebeen created over time based on what they have found to work best forsuch a large and geographically dispersed team. Try to learn as much aspossible about these standards ahead of time, as they are welldocumented; do not expect people to adapt to you or your company’s wayof doing things.

Legal Issues¶

The Linux kernel source code is released under the GPL. Please see the fileCOPYING in the main directory of the source tree. The Linux kernel licensingrules and how to useSPDX identifiers in source code aredescribed inDocumentation/process/license-rules.rst.If you have further questions about the license, please contact a lawyer, and donot ask on the Linux kernel mailing list. The people on the mailing lists arenot lawyers, and you should not rely on their statements on legal matters.

For common questions and answers about the GPL, please see:

https://www.gnu.org/licenses/gpl-faq.html

Documentation¶

The Linux kernel source tree has a large range of documents that areinvaluable for learning how to interact with the kernel community. Whennew features are added to the kernel, it is recommended that newdocumentation files are also added which explain how to use the feature.When a kernel change causes the interface that the kernel exposes touserspace to change, it is recommended that you send the information ora patch to the manual pages explaining the change to the manual pagesmaintainer atmtk.manpages@gmail.com, and CC the listlinux-api@vger.kernel.org.

Here is a list of files that are in the kernel source tree that arerequired reading:

Documentation/admin-guide/README.rst

This file gives a short background on the Linux kernel and describeswhat is necessary to do to configure and build the kernel. Peoplewho are new to the kernel should start here.

Documentation/process/changes.rst

This file gives a list of the minimum levels of various softwarepackages that are necessary to build and run the kernelsuccessfully.

Documentation/process/coding-style.rst

This describes the Linux kernel coding style, and some of therationale behind it. All new code is expected to follow theguidelines in this document. Most maintainers will only acceptpatches if these rules are followed, and many people will onlyreview code if it is in the proper style.

Documentation/process/submitting-patches.rst andDocumentation/process/submitting-drivers.rst

These files describe in explicit detail how to successfully createand 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 aresubject to scrutiny for content and style), but not following themwill almost always prevent it.

Other excellent descriptions of how to create patches properly are:

“The Perfect Patch”

https://www.ozlabs.org/~akpm/stuff/tpp.txt

“Linux kernel patch submission format”

https://web.archive.org/web/20180829112450/http://linux.yyz.us/patch-format.html

Documentation/process/stable-api-nonsense.rst

This file describes the rationale behind the conscious decision tonot have a stable API within the kernel, including things like:

• Subsystem shim-layers (for compatibility?)
• Driver portability between Operating Systems.
• Mitigating rapid change within the kernel source tree (orpreventing rapid change)

This document is crucial for understanding the Linux developmentphilosophy and is very important for people moving to Linux fromdevelopment on other Operating Systems.

Documentation/admin-guide/security-bugs.rst

If you feel you have found a security problem in the Linux kernel,please follow the steps in this document to help notify the kerneldevelopers, and help solve the issue.

Documentation/process/management-style.rst

This document describes how Linux kernel maintainers operate and theshared ethos behind their methodologies. This is important readingfor anyone new to kernel development (or anyone simply curious aboutit), as it resolves a lot of common misconceptions and confusionabout the unique behavior of kernel maintainers.

Documentation/process/stable-kernel-rules.rst

This file describes the rules on how the stable kernel releaseshappen, and what to do if you want to get a change into one of thesereleases.

Documentation/process/kernel-docs.rst

A list of external documentation that pertains to kerneldevelopment. Please consult this list if you do not find what youare looking for within the in-kernel documentation.

Documentation/process/applying-patches.rst

A good introduction describing exactly what a patch is and how toapply it to the different development branches of the kernel.

The kernel also has a large number of documents that can beautomatically generated from the source code itself or fromReStructuredText markups (ReST), like this one. This includes afull description of the in-kernel API, and rules on how to handlelocking properly.

All such documents can be generated as PDF or HTML by running:

make pdfdocsmake htmldocs

respectively from the main kernel source directory.

The documents that uses ReST markup will be generated at Documentation/output.They can also be generated on LaTeX and ePub formats with:

make latexdocsmake epubdocs

Becoming A Kernel Developer¶

If you do not know anything about Linux kernel development, you shouldlook at the Linux KernelNewbies project:

https://kernelnewbies.org

It consists of a helpful mailing list where you can ask almost any typeof basic kernel development question (make sure to search the archivesfirst, before asking something that has already been answered in thepast.) It also has an IRC channel that you can use to ask questions inreal-time, and a lot of helpful documentation that is useful forlearning 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 describessome basic logistical information, like how to compile a kernel andapply a patch.

If you do not know where you want to start, but you want to look forsome task to start doing to join into the kernel development community,go to the Linux Kernel Janitor’s project:

https://kernelnewbies.org/KernelJanitors

It is a great place to start. It describes a list of relatively simpleproblems that need to be cleaned up and fixed within the Linux kernelsource tree. Working with the developers in charge of this project, youwill learn the basics of getting your patch into the Linux kernel tree,and possibly be pointed in the direction of what to go work on next, ifyou do not already have an idea.

Before making any actual modifications to the Linux kernel code, it isimperative to understand how the code in question works. For thispurpose, nothing is better than reading through it directly (most trickybits are commented well), perhaps even with the help of specializedtools. One such tool that is particularly recommended is the LinuxCross-Reference project, which is able to present source code in aself-referential, indexed webpage format. An excellent up-to-daterepository of the kernel code may be found at:

https://elixir.bootlin.com/

The development process¶

Linux kernel development process currently consists of a few differentmain kernel “branches” and lots of different subsystem-specific kernelbranches. These different branches are:

• Linus’s mainline tree
• Various stable trees with multiple major numbers
• Subsystem-specific trees
• linux-next integration testing tree

Bug Reporting¶

https://bugzilla.kernel.org is where the Linux kernel developers track kernelbugs. Users are encouraged to report all bugs that they find in thistool. For details on how to use the kernel bugzilla, please see:

https://bugzilla.kernel.org/page.cgi?id=faq.html

The fileadmin-guide/reporting-bugs.rstin the main kernel source directory has a goodtemplate for how to report a possible kernel bug, and details what kindof information is needed by the kernel developers to help track down theproblem.

Managing bug reports¶

One of the best ways to put into practice your hacking skills is by fixingbugs reported by other people. Not only you will help to make the kernelmore stable, but you’ll also learn to fix real world problems and you willimprove your skills, and other developers will be aware of your presence.Fixing bugs is one of the best ways to get merits among other developers,because not many people like wasting time fixing other people’s bugs.

To work in the already reported bug reports, go tohttps://bugzilla.kernel.org.

Mailing lists¶

As some of the above documents describe, the majority of the core kerneldevelopers participate on the Linux Kernel Mailing list. Details on howto 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 differentplaces. Use a search engine to find these archives. For example:

http://dir.gmane.org/gmane.linux.kernel

It is highly recommended that you search the archives about the topicyou want to bring up, before you post it to the list. A lot of thingsalready discussed in detail are only recorded at the mailing listarchives.

Most of the individual kernel subsystems also have their own separatemailing list where they do their development efforts. See theMAINTAINERS file for a list of what these lists are for the differentgroups.

Many of the lists are hosted on kernel.org. Information on them can befound at:

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 forinteracting with the list (or any list):

http://www.albion.com/netiquette/

If multiple people respond to your mail, the CC: list of recipients mayget pretty large. Don’t remove anybody from the CC: list without a goodreason, or don’t reply only to the list address. Get used to receiving themail twice, one from the sender and the one from the list, and don’t tryto 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 your reply, andadd your statements between the individual quoted sections instead ofwriting at the top of the mail.

If you add patches to your mail, make sure they are plain readable textas stated inDocumentation/process/submitting-patches.rst.Kernel developers don’t want to deal withattachments or compressed patches; they may want to comment onindividual lines of your patch, which works only that way. Make sure youuse a mail program that does not mangle spaces and tab characters. Agood first test is to send the mail to yourself and try to apply yourown patch by yourself. If that doesn’t work, get your mail program fixedor change it until it works.

Above all, please remember to show respect to other subscribers.

Working with the community¶

The goal of the kernel community is to provide the best possible kernelthere is. When you submit a patch for acceptance, it will be reviewedon its technical merits and those alone. So, what should you beexpecting?

• criticism
• comments
• requests for change
• requests for justification
• silence

Remember, this is part of getting your patch into the kernel. You haveto be able to take criticism and comments about your patches, evaluatethem at a technical level and either rework your patches or provideclear and concise reasoning as to why those changes should not be made.If there are no responses to your posting, wait a few days and tryagain, sometimes things get lost in the huge volume.

What should you not do?

• expect your patch to be accepted without question
• become defensive
• ignore comments
• resubmit the patch without making any of the requested changes

In a community that is looking for the best technical solution possible,there will always be differing opinions on how beneficial a patch is.You have to be cooperative, and willing to adapt your idea to fit withinthe kernel. Or at least be willing to prove your idea is worth it.Remember, being wrong is acceptable as long as you are willing to worktoward a solution that is right.

It is normal that the answers to your first patch might simply be a listof a dozen things you should correct. This doesnot imply that yourpatch will not be accepted, and it isnot meant against youpersonally. Simply correct all issues raised against your patch andresend it.

Differences between the kernel community and corporate structures¶

The kernel community works differently than most traditional corporatedevelopment environments. Here are a list of things that you can try todo 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 begood…”
• “I’ve being doing this for 20 years, so…”
• “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 traditionalsoftware engineering work environments is the faceless nature ofinteraction. One benefit of using email and irc as the primary forms ofcommunication is the lack of discrimination based on gender or race.The Linux kernel work environment is accepting of women and minoritiesbecause all you are is an email address. The international aspect alsohelps to level the playing field because you can’t guess gender based ona 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 anopinion have had positive experiences.

The language barrier can cause problems for some people who are notcomfortable with English. A good grasp of the language can be needed inorder to get ideas across properly on mailing lists, so it isrecommended that you check your emails to make sure they make sense inEnglish before sending them.

Break up your changes¶

The Linux kernel community does not gladly accept large chunks of codedropped on it all at once. The changes need to be properly introduced,discussed, and broken up into tiny, individual portions. This is almostthe exact opposite of what companies are used to doing. Your proposalshould also be introduced very early in the development process, so thatyou can receive feedback on what you are doing. It also lets thecommunity feel that you are working with them, and not simply using themas a dumping ground for your feature. However, don’t send 50 emails atone time to a mailing list, your patch series should be smaller thanthat almost all of the time.

The reasons for breaking things up are the following:

1. Small patches increase the likelihood that your patches will beapplied, since they don’t take much time or effort to verify forcorrectness. A 5 line patch can be applied by a maintainer withbarely a second glance. However, a 500 line patch may take hours toreview for correctness (the time it takes is exponentiallyproportional to the size of the patch, or something).

   Small patches also make it very easy to debug when something goeswrong. It’s much easier to back out patches one by one than it isto dissect a very large patch after it’s been applied (and brokensomething).

2. It’s important not only to send small patches, but also to rewriteand 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. Theteacher does not want to see the student’s trials and errorsbefore they came up with the solution. They want to see thecleanest, most elegant answer. A good student knows this, andwould never submit her intermediate work before the finalsolution.

The same is true of kernel development. The maintainers andreviewers do not want to see the thought process behind thesolution to the problem one is solving. They want to see asimple and elegant solution.”

It may be challenging to keep the balance between presenting an elegantsolution and working together with the community and discussing yourunfinished work. Therefore it is good to get early in the process toget feedback to improve your work, but also keep your changes in smallchunks that they may get already accepted, even when your whole task isnot ready for inclusion now.

Also realize that it is not acceptable to send patches for inclusionthat are unfinished and will be “fixed up later.”

Justify your change¶

Along with breaking up your patches, it is very important for you to letthe Linux community know why they should add this change. New featuresmust be justified as being needed and useful.

Document your change¶

When sending in your patches, pay special attention to what you say inthe text in your email. This information will become the ChangeLoginformation for the patch, and will be preserved for everyone to see forall time. It should describe the patch completely, containing:

• why the change is necessary
• the overall design approach in the patch
• implementation details
• testing results

For more details on what this should all look like, please see theChangeLog section of the document:

“The Perfect Patch”

https://www.ozlabs.org/~akpm/stuff/tpp.txt

All of these things are sometimes very hard to do. It can take years toperfect these practices (if at all). It’s a continuous process ofimprovement that requires a lot of patience and determination. Butdon’t give up, it’s possible. Many have done it before, and each had tostart exactly where you are now.

Thanks to Paolo Ciarrocchi who allowed the “Development Process”(https://lwn.net/Articles/94386/) sectionto be based on text he had written, and to Randy Dunlap and GerritHuizenga for some of the list of things you should and should not say.Also thanks to Pat Mochel, Hanna Linder, Randy Dunlap, Kay Sievers,Vojtech Pavlik, Jan Kara, Josh Boyer, Kees Cook, Andrew Morton, AndiKleen, Vadim Lobanov, Jesper Juhl, Adrian Bunk, Keri Harris, Frans Pop,David A. Wheeler, Junio Hamano, Michael Kerrisk, and Alex Shepard fortheir review, comments, and contributions. Without their help, thisdocument would not have been possible.

Maintainer: Greg Kroah-Hartman <greg@kroah.com>