HOWTO do Linux kernel development

 

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.  It tries to not
contain anything related to the technical aspects of kernel programming,
but will help point you in the right direction for that.

If anything in this document becomes out of date, please send in patches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.


Introduction
------------

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 architecture-dependent
parts written in assembly. A good understanding of C is required for
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 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 it
adheres to the ISO C89 standard, it uses a number of extensions that are
not featured in the standard.  The kernel is a freestanding C
environment, with no reliance on the standard C library, so some
portions of the C standard are not supported.  Arbitrary long long
divisions and floating point are not allowed.  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 diverse group of people, with
high standards for coding, style and procedure.  These standards 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 standards ahead of time, as they are well
documented; do 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.

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


Documentation
------------

The Linux kernel source tree has a large range of documents that are
invaluable for 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 which explain how to use the feature.
When a kernel change causes the interface that the kernel exposes to
userspace to change, it is recommended that you send the information or
a patch to the manual pages explaining the change to the manual pages
maintainer at [e-mail], and CC the list
[e-mail].

Here is a list of files that are in the kernel source tree that are
required reading:
  README
    This file gives a short background on the Linux kernel and describes
    what is necessary to do to configure and build the kernel.  People
    who are new to the kernel should start here.

  Documentation/Changes
    This file gives a list of the minimum levels of various software
    packages that are necessary to build and run the kernel
    successfully.

  Documentation/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 for 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"
  ~akpm/stuff/tpp.txt
 "Linux kernel patch submission format"
  

  Documentation/stable_api_nonsense.txt
    This file describes the rationale behind the conscious decision to
    not 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 (or
 preventing rapid change)
    This document is crucial for understanding 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.

  Documentation/applying-patches.txt
    A good introduction describing exactly what a patch is and how to
    apply it to the different development branches of the kernel.

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:
 
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 an 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 describes
some basic logistical information, like how to compile a kernel and
apply 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:
 
It is a great place to start.  It describes a list of relatively simple
problems 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 be pointed 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 put 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:
 

Before making any actual modifications to the Linux kernel code, it is
imperative to understand how the code in question works.  For this
purpose, nothing is better than reading through it directly (most tricky
bits are commented well), perhaps even with the help of specialized
tools.  One such tool that is particularly recommended is the Linux
Cross-Reference project, which is able to present source code in a
self-referential, indexed webpage format. An excellent up-to-date
repository of the kernel code may be found at:
 ~qiyong/lxr/


The development process
-----------------------

Linux kernel development process currently consists of a few different
main kernel "branches" and lots of different subsystem-specific kernel
branches.  These different branches are:
  - main 2.6.x kernel tree
  - 2.6.x.y -stable kernel tree
  - 2.6.x -git kernel patches
  - 2.6.x -mm kernel patches
  - subsystem specific kernel trees and patches

2.6.x kernel tree
-----------------
2.6.x kernels are maintained by Linus Torvalds, and can be found on
kernel.org in the pub/linux/kernel/v2.6/ directory.  Its development
process is as follows:
  - As soon as a new kernel is released a two weeks window is open,
    during this period of time maintainers can submit big diffs to
    Linus, usually the patches that have already been included in the
    -mm kernel for a few weeks.  The preferred way to submit big changes
    is using git (the kernel's source management tool, more information
    can be found at ) but plain patches are also just
    fine.
  - After two weeks a -rc1 kernel is released it is now possible to push
    only patches that do not include new features that could affect the
    stability of the whole kernel.  Please note that a whole new driver
    (or filesystem) might be accepted after -rc1 because there is no
    risk of causing regressions with such a change as long as the change
    is self-contained and does not affect areas outside of the code that
    is being added.  git can be used to send patches to Linus after -rc1
    is released, but the patches need to also be sent to a public
    mailing list for review.
  - A new -rc is released whenever Linus deems the current git tree to
    be in a reasonably sane state adequate for testing.  The goal is to
    release a new -rc kernel every week.
  - Process continues until the kernel is considered "ready", the
    process should last around 6 weeks.
  - Known regressions in each release are periodically posted to the 
    linux-kernel mailing list.  The goal is to reduce the length of 
    that list to zero before declaring the kernel to be "ready," but, in
    the real world, a small number of regressions often remain at 
    release time.

It is worth mentioning what Andrew Morton wrote on the linux-kernel
mailing list about kernel releases:
 "Nobody knows when a k