275 lines
15 KiB
Plaintext
Executable File
275 lines
15 KiB
Plaintext
Executable File
1: A GUIDE TO THE KERNEL DEVELOPMENT PROCESS
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The purpose of this document is to help developers (and their managers)
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work with the development community with a minimum of frustration. It is
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an attempt to document how this community works in a way which is
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accessible to those who are not intimately familiar with Linux kernel
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development (or, indeed, free software development in general). While
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there is some technical material here, this is very much a process-oriented
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discussion which does not require a deep knowledge of kernel programming to
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understand.
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1.1: EXECUTIVE SUMMARY
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The rest of this section covers the scope of the kernel development process
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and the kinds of frustrations that developers and their employers can
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encounter there. There are a great many reasons why kernel code should be
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merged into the official ("mainline") kernel, including automatic
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availability to users, community support in many forms, and the ability to
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influence the direction of kernel development. Code contributed to the
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Linux kernel must be made available under a GPL-compatible license.
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Section 2 introduces the development process, the kernel release cycle, and
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the mechanics of the merge window. The various phases in the patch
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development, review, and merging cycle are covered. There is some
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discussion of tools and mailing lists. Developers wanting to get started
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with kernel development are encouraged to track down and fix bugs as an
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initial exercise.
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Section 3 covers early-stage project planning, with an emphasis on
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involving the development community as soon as possible.
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Section 4 is about the coding process; several pitfalls which have been
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encountered by other developers are discussed. Some requirements for
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patches are covered, and there is an introduction to some of the tools
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which can help to ensure that kernel patches are correct.
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Section 5 talks about the process of posting patches for review. To be
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taken seriously by the development community, patches must be properly
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formatted and described, and they must be sent to the right place.
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Following the advice in this section should help to ensure the best
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possible reception for your work.
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Section 6 covers what happens after posting patches; the job is far from
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done at that point. Working with reviewers is a crucial part of the
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development process; this section offers a number of tips on how to avoid
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problems at this important stage. Developers are cautioned against
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assuming that the job is done when a patch is merged into the mainline.
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Section 7 introduces a couple of "advanced" topics: managing patches with
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git and reviewing patches posted by others.
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Section 8 concludes the document with pointers to sources for more
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information on kernel development.
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1.2: WHAT THIS DOCUMENT IS ABOUT
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The Linux kernel, at over 8 million lines of code and well over 1000
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contributors to each release, is one of the largest and most active free
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software projects in existence. Since its humble beginning in 1991, this
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kernel has evolved into a best-of-breed operating system component which
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runs on pocket-sized digital music players, desktop PCs, the largest
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supercomputers in existence, and all types of systems in between. It is a
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robust, efficient, and scalable solution for almost any situation.
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With the growth of Linux has come an increase in the number of developers
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(and companies) wishing to participate in its development. Hardware
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vendors want to ensure that Linux supports their products well, making
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those products attractive to Linux users. Embedded systems vendors, who
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use Linux as a component in an integrated product, want Linux to be as
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capable and well-suited to the task at hand as possible. Distributors and
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other software vendors who base their products on Linux have a clear
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interest in the capabilities, performance, and reliability of the Linux
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kernel. And end users, too, will often wish to change Linux to make it
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better suit their needs.
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One of the most compelling features of Linux is that it is accessible to
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these developers; anybody with the requisite skills can improve Linux and
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influence the direction of its development. Proprietary products cannot
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offer this kind of openness, which is a characteristic of the free software
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process. But, if anything, the kernel is even more open than most other
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free software projects. A typical three-month kernel development cycle can
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involve over 1000 developers working for more than 100 different companies
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(or for no company at all).
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Working with the kernel development community is not especially hard. But,
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that notwithstanding, many potential contributors have experienced
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difficulties when trying to do kernel work. The kernel community has
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evolved its own distinct ways of operating which allow it to function
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smoothly (and produce a high-quality product) in an environment where
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thousands of lines of code are being changed every day. So it is not
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surprising that Linux kernel development process differs greatly from
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proprietary development methods.
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The kernel's development process may come across as strange and
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intimidating to new developers, but there are good reasons and solid
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experience behind it. A developer who does not understand the kernel
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community's ways (or, worse, who tries to flout or circumvent them) will
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have a frustrating experience in store. The development community, while
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being helpful to those who are trying to learn, has little time for those
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who will not listen or who do not care about the development process.
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It is hoped that those who read this document will be able to avoid that
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frustrating experience. There is a lot of material here, but the effort
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involved in reading it will be repaid in short order. The development
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community is always in need of developers who will help to make the kernel
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better; the following text should help you - or those who work for you -
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join our community.
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1.3: CREDITS
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This document was written by Jonathan Corbet, corbet@lwn.net. It has been
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improved by comments from Johannes Berg, James Berry, Alex Chiang, Roland
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Dreier, Randy Dunlap, Jake Edge, Jiri Kosina, Matt Mackall, Arthur Marsh,
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Amanda McPherson, Andrew Morton, Andrew Price, Tsugikazu Shibata, and
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Jochen Voß.
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This work was supported by the Linux Foundation; thanks especially to
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Amanda McPherson, who saw the value of this effort and made it all happen.
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1.4: THE IMPORTANCE OF GETTING CODE INTO THE MAINLINE
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Some companies and developers occasionally wonder why they should bother
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learning how to work with the kernel community and get their code into the
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mainline kernel (the "mainline" being the kernel maintained by Linus
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Torvalds and used as a base by Linux distributors). In the short term,
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contributing code can look like an avoidable expense; it seems easier to
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just keep the code separate and support users directly. The truth of the
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matter is that keeping code separate ("out of tree") is a false economy.
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As a way of illustrating the costs of out-of-tree code, here are a few
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relevant aspects of the kernel development process; most of these will be
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discussed in greater detail later in this document. Consider:
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- Code which has been merged into the mainline kernel is available to all
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Linux users. It will automatically be present on all distributions which
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enable it. There is no need for driver disks, downloads, or the hassles
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of supporting multiple versions of multiple distributions; it all just
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works, for the developer and for the user. Incorporation into the
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mainline solves a large number of distribution and support problems.
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- While kernel developers strive to maintain a stable interface to user
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space, the internal kernel API is in constant flux. The lack of a stable
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internal interface is a deliberate design decision; it allows fundamental
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improvements to be made at any time and results in higher-quality code.
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But one result of that policy is that any out-of-tree code requires
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constant upkeep if it is to work with new kernels. Maintaining
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out-of-tree code requires significant amounts of work just to keep that
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code working.
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Code which is in the mainline, instead, does not require this work as the
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result of a simple rule requiring any developer who makes an API change
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to also fix any code that breaks as the result of that change. So code
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which has been merged into the mainline has significantly lower
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maintenance costs.
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- Beyond that, code which is in the kernel will often be improved by other
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developers. Surprising results can come from empowering your user
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community and customers to improve your product.
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- Kernel code is subjected to review, both before and after merging into
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the mainline. No matter how strong the original developer's skills are,
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this review process invariably finds ways in which the code can be
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improved. Often review finds severe bugs and security problems. This is
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especially true for code which has been developed in a closed
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environment; such code benefits strongly from review by outside
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developers. Out-of-tree code is lower-quality code.
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- Participation in the development process is your way to influence the
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direction of kernel development. Users who complain from the sidelines
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are heard, but active developers have a stronger voice - and the ability
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to implement changes which make the kernel work better for their needs.
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- When code is maintained separately, the possibility that a third party
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will contribute a different implementation of a similar feature always
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exists. Should that happen, getting your code merged will become much
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harder - to the point of impossibility. Then you will be faced with the
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unpleasant alternatives of either (1) maintaining a nonstandard feature
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out of tree indefinitely, or (2) abandoning your code and migrating your
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users over to the in-tree version.
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- Contribution of code is the fundamental action which makes the whole
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process work. By contributing your code you can add new functionality to
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the kernel and provide capabilities and examples which are of use to
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other kernel developers. If you have developed code for Linux (or are
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thinking about doing so), you clearly have an interest in the continued
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success of this platform; contributing code is one of the best ways to
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help ensure that success.
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All of the reasoning above applies to any out-of-tree kernel code,
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including code which is distributed in proprietary, binary-only form.
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There are, however, additional factors which should be taken into account
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before considering any sort of binary-only kernel code distribution. These
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include:
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- The legal issues around the distribution of proprietary kernel modules
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are cloudy at best; quite a few kernel copyright holders believe that
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most binary-only modules are derived products of the kernel and that, as
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a result, their distribution is a violation of the GNU General Public
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license (about which more will be said below). Your author is not a
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lawyer, and nothing in this document can possibly be considered to be
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legal advice. The true legal status of closed-source modules can only be
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determined by the courts. But the uncertainty which haunts those modules
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is there regardless.
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- Binary modules greatly increase the difficulty of debugging kernel
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problems, to the point that most kernel developers will not even try. So
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the distribution of binary-only modules will make it harder for your
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users to get support from the community.
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- Support is also harder for distributors of binary-only modules, who must
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provide a version of the module for every distribution and every kernel
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version they wish to support. Dozens of builds of a single module can
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be required to provide reasonably comprehensive coverage, and your users
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will have to upgrade your module separately every time they upgrade their
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kernel.
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- Everything that was said above about code review applies doubly to
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closed-source code. Since this code is not available at all, it cannot
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have been reviewed by the community and will, beyond doubt, have serious
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problems.
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Makers of embedded systems, in particular, may be tempted to disregard much
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of what has been said in this section in the belief that they are shipping
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a self-contained product which uses a frozen kernel version and requires no
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more development after its release. This argument misses the value of
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widespread code review and the value of allowing your users to add
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capabilities to your product. But these products, too, have a limited
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commercial life, after which a new version must be released. At that
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point, vendors whose code is in the mainline and well maintained will be
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much better positioned to get the new product ready for market quickly.
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1.5: LICENSING
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Code is contributed to the Linux kernel under a number of licenses, but all
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code must be compatible with version 2 of the GNU General Public License
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(GPLv2), which is the license covering the kernel distribution as a whole.
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In practice, that means that all code contributions are covered either by
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GPLv2 (with, optionally, language allowing distribution under later
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versions of the GPL) or the three-clause BSD license. Any contributions
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which are not covered by a compatible license will not be accepted into the
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kernel.
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Copyright assignments are not required (or requested) for code contributed
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to the kernel. All code merged into the mainline kernel retains its
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original ownership; as a result, the kernel now has thousands of owners.
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One implication of this ownership structure is that any attempt to change
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the licensing of the kernel is doomed to almost certain failure. There are
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few practical scenarios where the agreement of all copyright holders could
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be obtained (or their code removed from the kernel). So, in particular,
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there is no prospect of a migration to version 3 of the GPL in the
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foreseeable future.
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It is imperative that all code contributed to the kernel be legitimately
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free software. For that reason, code from anonymous (or pseudonymous)
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contributors will not be accepted. All contributors are required to "sign
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off" on their code, stating that the code can be distributed with the
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kernel under the GPL. Code which has not been licensed as free software by
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its owner, or which risks creating copyright-related problems for the
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kernel (such as code which derives from reverse-engineering efforts lacking
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proper safeguards) cannot be contributed.
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Questions about copyright-related issues are common on Linux development
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mailing lists. Such questions will normally receive no shortage of
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answers, but one should bear in mind that the people answering those
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questions are not lawyers and cannot provide legal advice. If you have
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legal questions relating to Linux source code, there is no substitute for
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talking with a lawyer who understands this field. Relying on answers
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obtained on technical mailing lists is a risky affair.
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