213 lines
11 KiB
Plaintext
213 lines
11 KiB
Plaintext
3: EARLY-STAGE PLANNING
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When contemplating a Linux kernel development project, it can be tempting
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to jump right in and start coding. As with any significant project,
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though, much of the groundwork for success is best laid before the first
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line of code is written. Some time spent in early planning and
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communication can save far more time later on.
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3.1: SPECIFYING THE PROBLEM
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Like any engineering project, a successful kernel enhancement starts with a
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clear description of the problem to be solved. In some cases, this step is
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easy: when a driver is needed for a specific piece of hardware, for
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example. In others, though, it is tempting to confuse the real problem
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with the proposed solution, and that can lead to difficulties.
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Consider an example: some years ago, developers working with Linux audio
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sought a way to run applications without dropouts or other artifacts caused
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by excessive latency in the system. The solution they arrived at was a
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kernel module intended to hook into the Linux Security Module (LSM)
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framework; this module could be configured to give specific applications
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access to the realtime scheduler. This module was implemented and sent to
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the linux-kernel mailing list, where it immediately ran into problems.
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To the audio developers, this security module was sufficient to solve their
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immediate problem. To the wider kernel community, though, it was seen as a
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misuse of the LSM framework (which is not intended to confer privileges
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onto processes which they would not otherwise have) and a risk to system
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stability. Their preferred solutions involved realtime scheduling access
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via the rlimit mechanism for the short term, and ongoing latency reduction
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work in the long term.
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The audio community, however, could not see past the particular solution
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they had implemented; they were unwilling to accept alternatives. The
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resulting disagreement left those developers feeling disillusioned with the
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entire kernel development process; one of them went back to an audio list
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and posted this:
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There are a number of very good Linux kernel developers, but they
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tend to get outshouted by a large crowd of arrogant fools. Trying
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to communicate user requirements to these people is a waste of
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time. They are much too "intelligent" to listen to lesser mortals.
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(http://lwn.net/Articles/131776/).
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The reality of the situation was different; the kernel developers were far
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more concerned about system stability, long-term maintenance, and finding
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the right solution to the problem than they were with a specific module.
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The moral of the story is to focus on the problem - not a specific solution
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- and to discuss it with the development community before investing in the
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creation of a body of code.
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So, when contemplating a kernel development project, one should obtain
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answers to a short set of questions:
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- What, exactly, is the problem which needs to be solved?
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- Who are the users affected by this problem? Which use cases should the
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solution address?
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- How does the kernel fall short in addressing that problem now?
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Only then does it make sense to start considering possible solutions.
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3.2: EARLY DISCUSSION
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When planning a kernel development project, it makes great sense to hold
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discussions with the community before launching into implementation. Early
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communication can save time and trouble in a number of ways:
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- It may well be that the problem is addressed by the kernel in ways which
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you have not understood. The Linux kernel is large and has a number of
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features and capabilities which are not immediately obvious. Not all
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kernel capabilities are documented as well as one might like, and it is
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easy to miss things. Your author has seen the posting of a complete
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driver which duplicated an existing driver that the new author had been
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unaware of. Code which reinvents existing wheels is not only wasteful;
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it will also not be accepted into the mainline kernel.
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- There may be elements of the proposed solution which will not be
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acceptable for mainline merging. It is better to find out about
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problems like this before writing the code.
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- It's entirely possible that other developers have thought about the
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problem; they may have ideas for a better solution, and may be willing
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to help in the creation of that solution.
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Years of experience with the kernel development community have taught a
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clear lesson: kernel code which is designed and developed behind closed
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doors invariably has problems which are only revealed when the code is
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released into the community. Sometimes these problems are severe,
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requiring months or years of effort before the code can be brought up to
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the kernel community's standards. Some examples include:
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- The Devicescape network stack was designed and implemented for
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single-processor systems. It could not be merged into the mainline
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until it was made suitable for multiprocessor systems. Retrofitting
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locking and such into code is a difficult task; as a result, the merging
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of this code (now called mac80211) was delayed for over a year.
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- The Reiser4 filesystem included a number of capabilities which, in the
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core kernel developers' opinion, should have been implemented in the
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virtual filesystem layer instead. It also included features which could
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not easily be implemented without exposing the system to user-caused
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deadlocks. The late revelation of these problems - and refusal to
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address some of them - has caused Reiser4 to stay out of the mainline
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kernel.
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- The AppArmor security module made use of internal virtual filesystem
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data structures in ways which were considered to be unsafe and
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unreliable. This concern (among others) kept AppArmor out of the
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mainline for years.
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In each of these cases, a great deal of pain and extra work could have been
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avoided with some early discussion with the kernel developers.
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3.3: WHO DO YOU TALK TO?
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When developers decide to take their plans public, the next question will
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be: where do we start? The answer is to find the right mailing list(s) and
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the right maintainer. For mailing lists, the best approach is to look in
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the MAINTAINERS file for a relevant place to post. If there is a suitable
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subsystem list, posting there is often preferable to posting on
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linux-kernel; you are more likely to reach developers with expertise in the
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relevant subsystem and the environment may be more supportive.
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Finding maintainers can be a bit harder. Again, the MAINTAINERS file is
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the place to start. That file tends to not always be up to date, though,
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and not all subsystems are represented there. The person listed in the
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MAINTAINERS file may, in fact, not be the person who is actually acting in
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that role currently. So, when there is doubt about who to contact, a
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useful trick is to use git (and "git log" in particular) to see who is
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currently active within the subsystem of interest. Look at who is writing
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patches, and who, if anybody, is attaching Signed-off-by lines to those
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patches. Those are the people who will be best placed to help with a new
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development project.
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The task of finding the right maintainer is sometimes challenging enough
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that the kernel developers have added a script to ease the process:
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.../scripts/get_maintainer.pl
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This script will return the current maintainer(s) for a given file or
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directory when given the "-f" option. If passed a patch on the
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command line, it will list the maintainers who should probably receive
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copies of the patch. There are a number of options regulating how hard
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get_maintainer.pl will search for maintainers; please be careful about
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using the more aggressive options as you may end up including developers
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who have no real interest in the code you are modifying.
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If all else fails, talking to Andrew Morton can be an effective way to
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track down a maintainer for a specific piece of code.
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3.4: WHEN TO POST?
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If possible, posting your plans during the early stages can only be
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helpful. Describe the problem being solved and any plans that have been
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made on how the implementation will be done. Any information you can
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provide can help the development community provide useful input on the
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project.
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One discouraging thing which can happen at this stage is not a hostile
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reaction, but, instead, little or no reaction at all. The sad truth of the
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matter is (1) kernel developers tend to be busy, (2) there is no shortage
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of people with grand plans and little code (or even prospect of code) to
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back them up, and (3) nobody is obligated to review or comment on ideas
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posted by others. Beyond that, high-level designs often hide problems
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which are only reviewed when somebody actually tries to implement those
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designs; for that reason, kernel developers would rather see the code.
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If a request-for-comments posting yields little in the way of comments, do
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not assume that it means there is no interest in the project.
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Unfortunately, you also cannot assume that there are no problems with your
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idea. The best thing to do in this situation is to proceed, keeping the
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community informed as you go.
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3.5: GETTING OFFICIAL BUY-IN
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If your work is being done in a corporate environment - as most Linux
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kernel work is - you must, obviously, have permission from suitably
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empowered managers before you can post your company's plans or code to a
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public mailing list. The posting of code which has not been cleared for
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release under a GPL-compatible license can be especially problematic; the
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sooner that a company's management and legal staff can agree on the posting
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of a kernel development project, the better off everybody involved will be.
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Some readers may be thinking at this point that their kernel work is
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intended to support a product which does not yet have an officially
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acknowledged existence. Revealing their employer's plans on a public
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mailing list may not be a viable option. In cases like this, it is worth
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considering whether the secrecy is really necessary; there is often no real
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need to keep development plans behind closed doors.
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That said, there are also cases where a company legitimately cannot
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disclose its plans early in the development process. Companies with
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experienced kernel developers may choose to proceed in an open-loop manner
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on the assumption that they will be able to avoid serious integration
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problems later. For companies without that sort of in-house expertise, the
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best option is often to hire an outside developer to review the plans under
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a non-disclosure agreement. The Linux Foundation operates an NDA program
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designed to help with this sort of situation; more information can be found
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at:
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http://www.linuxfoundation.org/en/NDA_program
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This kind of review is often enough to avoid serious problems later on
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without requiring public disclosure of the project.
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