1.. _development_early_stage:
2
3Early-stage planning
4====================
5
6When contemplating a Linux kernel development project, it can be tempting
7to jump right in and start coding.  As with any significant project,
8though, much of the groundwork for success is best laid before the first
9line of code is written.  Some time spent in early planning and
10communication can save far more time later on.
11
12
13Specifying the problem
14----------------------
15
16Like any engineering project, a successful kernel enhancement starts with a
17clear description of the problem to be solved.  In some cases, this step is
18easy: when a driver is needed for a specific piece of hardware, for
19example.  In others, though, it is tempting to confuse the real problem
20with the proposed solution, and that can lead to difficulties.
21
22Consider an example: some years ago, developers working with Linux audio
23sought a way to run applications without dropouts or other artifacts caused
24by excessive latency in the system.  The solution they arrived at was a
25kernel module intended to hook into the Linux Security Module (LSM)
26framework; this module could be configured to give specific applications
27access to the realtime scheduler.  This module was implemented and sent to
28the linux-kernel mailing list, where it immediately ran into problems.
29
30To the audio developers, this security module was sufficient to solve their
31immediate problem.  To the wider kernel community, though, it was seen as a
32misuse of the LSM framework (which is not intended to confer privileges
33onto processes which they would not otherwise have) and a risk to system
34stability.  Their preferred solutions involved realtime scheduling access
35via the rlimit mechanism for the short term, and ongoing latency reduction
36work in the long term.
37
38The audio community, however, could not see past the particular solution
39they had implemented; they were unwilling to accept alternatives.  The
40resulting disagreement left those developers feeling disillusioned with the
41entire kernel development process; one of them went back to an audio list
42and posted this:
43
44	There are a number of very good Linux kernel developers, but they
45	tend to get outshouted by a large crowd of arrogant fools. Trying
46	to communicate user requirements to these people is a waste of
47	time. They are much too "intelligent" to listen to lesser mortals.
48
49(http://lwn.net/Articles/131776/).
50
51The reality of the situation was different; the kernel developers were far
52more concerned about system stability, long-term maintenance, and finding
53the right solution to the problem than they were with a specific module.
54The moral of the story is to focus on the problem - not a specific solution
55- and to discuss it with the development community before investing in the
56creation of a body of code.
57
58So, when contemplating a kernel development project, one should obtain
59answers to a short set of questions:
60
61 - What, exactly, is the problem which needs to be solved?
62
63 - Who are the users affected by this problem?  Which use cases should the
64   solution address?
65
66 - How does the kernel fall short in addressing that problem now?
67
68Only then does it make sense to start considering possible solutions.
69
70
71Early discussion
72----------------
73
74When planning a kernel development project, it makes great sense to hold
75discussions with the community before launching into implementation.  Early
76communication can save time and trouble in a number of ways:
77
78 - It may well be that the problem is addressed by the kernel in ways which
79   you have not understood.  The Linux kernel is large and has a number of
80   features and capabilities which are not immediately obvious.  Not all
81   kernel capabilities are documented as well as one might like, and it is
82   easy to miss things.  Your author has seen the posting of a complete
83   driver which duplicated an existing driver that the new author had been
84   unaware of.  Code which reinvents existing wheels is not only wasteful;
85   it will also not be accepted into the mainline kernel.
86
87 - There may be elements of the proposed solution which will not be
88   acceptable for mainline merging.  It is better to find out about
89   problems like this before writing the code.
90
91 - It's entirely possible that other developers have thought about the
92   problem; they may have ideas for a better solution, and may be willing
93   to help in the creation of that solution.
94
95Years of experience with the kernel development community have taught a
96clear lesson: kernel code which is designed and developed behind closed
97doors invariably has problems which are only revealed when the code is
98released into the community.  Sometimes these problems are severe,
99requiring months or years of effort before the code can be brought up to
100the kernel community's standards.  Some examples include:
101
102 - The Devicescape network stack was designed and implemented for
103   single-processor systems.  It could not be merged into the mainline
104   until it was made suitable for multiprocessor systems.  Retrofitting
105   locking and such into code is a difficult task; as a result, the merging
106   of this code (now called mac80211) was delayed for over a year.
107
108 - The Reiser4 filesystem included a number of capabilities which, in the
109   core kernel developers' opinion, should have been implemented in the
110   virtual filesystem layer instead.  It also included features which could
111   not easily be implemented without exposing the system to user-caused
112   deadlocks.  The late revelation of these problems - and refusal to
113   address some of them - has caused Reiser4 to stay out of the mainline
114   kernel.
115
116 - The AppArmor security module made use of internal virtual filesystem
117   data structures in ways which were considered to be unsafe and
118   unreliable.  This concern (among others) kept AppArmor out of the
119   mainline for years.
120
121In each of these cases, a great deal of pain and extra work could have been
122avoided with some early discussion with the kernel developers.
123
124
125Who do you talk to?
126-------------------
127
128When developers decide to take their plans public, the next question will
129be: where do we start?  The answer is to find the right mailing list(s) and
130the right maintainer.  For mailing lists, the best approach is to look in
131the MAINTAINERS file for a relevant place to post.  If there is a suitable
132subsystem list, posting there is often preferable to posting on
133linux-kernel; you are more likely to reach developers with expertise in the
134relevant subsystem and the environment may be more supportive.
135
136Finding maintainers can be a bit harder.  Again, the MAINTAINERS file is
137the place to start.  That file tends to not always be up to date, though,
138and not all subsystems are represented there.  The person listed in the
139MAINTAINERS file may, in fact, not be the person who is actually acting in
140that role currently.  So, when there is doubt about who to contact, a
141useful trick is to use git (and "git log" in particular) to see who is
142currently active within the subsystem of interest.  Look at who is writing
143patches, and who, if anybody, is attaching Signed-off-by lines to those
144patches.  Those are the people who will be best placed to help with a new
145development project.
146
147The task of finding the right maintainer is sometimes challenging enough
148that the kernel developers have added a script to ease the process:
149
150::
151
152	.../scripts/get_maintainer.pl
153
154This script will return the current maintainer(s) for a given file or
155directory when given the "-f" option.  If passed a patch on the
156command line, it will list the maintainers who should probably receive
157copies of the patch.  There are a number of options regulating how hard
158get_maintainer.pl will search for maintainers; please be careful about
159using the more aggressive options as you may end up including developers
160who have no real interest in the code you are modifying.
161
162If all else fails, talking to Andrew Morton can be an effective way to
163track down a maintainer for a specific piece of code.
164
165
166When to post?
167-------------
168
169If possible, posting your plans during the early stages can only be
170helpful.  Describe the problem being solved and any plans that have been
171made on how the implementation will be done.  Any information you can
172provide can help the development community provide useful input on the
173project.
174
175One discouraging thing which can happen at this stage is not a hostile
176reaction, but, instead, little or no reaction at all.  The sad truth of the
177matter is (1) kernel developers tend to be busy, (2) there is no shortage
178of people with grand plans and little code (or even prospect of code) to
179back them up, and (3) nobody is obligated to review or comment on ideas
180posted by others.  Beyond that, high-level designs often hide problems
181which are only revealed when somebody actually tries to implement those
182designs; for that reason, kernel developers would rather see the code.
183
184If a request-for-comments posting yields little in the way of comments, do
185not assume that it means there is no interest in the project.
186Unfortunately, you also cannot assume that there are no problems with your
187idea.  The best thing to do in this situation is to proceed, keeping the
188community informed as you go.
189
190
191Getting official buy-in
192-----------------------
193
194If your work is being done in a corporate environment - as most Linux
195kernel work is - you must, obviously, have permission from suitably
196empowered managers before you can post your company's plans or code to a
197public mailing list.  The posting of code which has not been cleared for
198release under a GPL-compatible license can be especially problematic; the
199sooner that a company's management and legal staff can agree on the posting
200of a kernel development project, the better off everybody involved will be.
201
202Some readers may be thinking at this point that their kernel work is
203intended to support a product which does not yet have an officially
204acknowledged existence.  Revealing their employer's plans on a public
205mailing list may not be a viable option.  In cases like this, it is worth
206considering whether the secrecy is really necessary; there is often no real
207need to keep development plans behind closed doors.
208
209That said, there are also cases where a company legitimately cannot
210disclose its plans early in the development process.  Companies with
211experienced kernel developers may choose to proceed in an open-loop manner
212on the assumption that they will be able to avoid serious integration
213problems later.  For companies without that sort of in-house expertise, the
214best option is often to hire an outside developer to review the plans under
215a non-disclosure agreement.  The Linux Foundation operates an NDA program
216designed to help with this sort of situation; more information can be found
217at:
218
219    http://www.linuxfoundation.org/en/NDA_program
220
221This kind of review is often enough to avoid serious problems later on
222without requiring public disclosure of the project.
223