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

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