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1 3: EARLY-STAGE PLANNING
3 When contemplating a Linux kernel development project, it can be tempting
4 to jump right in and start coding. As with any significant project,
5 though, much of the groundwork for success is best laid before the first
6 line of code is written. Some time spent in early planning and
7 communication can save far more time later on.
10 3.1: SPECIFYING THE PROBLEM
12 Like any engineering project, a successful kernel enhancement starts with a
13 clear description of the problem to be solved. In some cases, this step is
14 easy: when a driver is needed for a specific piece of hardware, for
15 example. In others, though, it is tempting to confuse the real problem
16 with the proposed solution, and that can lead to difficulties.
18 Consider an example: some years ago, developers working with Linux audio
19 sought a way to run applications without dropouts or other artifacts caused
20 by excessive latency in the system. The solution they arrived at was a
21 kernel module intended to hook into the Linux Security Module (LSM)
22 framework; this module could be configured to give specific applications
23 access to the realtime scheduler. This module was implemented and sent to
24 the linux-kernel mailing list, where it immediately ran into problems.
26 To the audio developers, this security module was sufficient to solve their
27 immediate problem. To the wider kernel community, though, it was seen as a
28 misuse of the LSM framework (which is not intended to confer privileges
29 onto processes which they would not otherwise have) and a risk to system
30 stability. Their preferred solutions involved realtime scheduling access
31 via the rlimit mechanism for the short term, and ongoing latency reduction
32 work in the long term.
34 The audio community, however, could not see past the particular solution
35 they had implemented; they were unwilling to accept alternatives. The
36 resulting disagreement left those developers feeling disillusioned with the
37 entire kernel development process; one of them went back to an audio list
38 and posted this:
40 There are a number of very good Linux kernel developers, but they
41 tend to get outshouted by a large crowd of arrogant fools. Trying
42 to communicate user requirements to these people is a waste of
43 time. They are much too "intelligent" to listen to lesser mortals.
45 (http://lwn.net/Articles/131776/).
47 The reality of the situation was different; the kernel developers were far
48 more concerned about system stability, long-term maintenance, and finding
49 the right solution to the problem than they were with a specific module.
50 The moral of the story is to focus on the problem - not a specific solution
51 - and to discuss it with the development community before investing in the
52 creation of a body of code.
54 So, when contemplating a kernel development project, one should obtain
55 answers to a short set of questions:
57 - What, exactly, is the problem which needs to be solved?
59 - Who are the users affected by this problem? Which use cases should the
60 solution address?
62 - How does the kernel fall short in addressing that problem now?
64 Only then does it make sense to start considering possible solutions.
67 3.2: EARLY DISCUSSION
69 When planning a kernel development project, it makes great sense to hold
70 discussions with the community before launching into implementation. Early
71 communication can save time and trouble in a number of ways:
73 - It may well be that the problem is addressed by the kernel in ways which
74 you have not understood. The Linux kernel is large and has a number of
75 features and capabilities which are not immediately obvious. Not all
76 kernel capabilities are documented as well as one might like, and it is
77 easy to miss things. Your author has seen the posting of a complete
78 driver which duplicated an existing driver that the new author had been
79 unaware of. Code which reinvents existing wheels is not only wasteful;
80 it will also not be accepted into the mainline kernel.
82 - There may be elements of the proposed solution which will not be
83 acceptable for mainline merging. It is better to find out about
84 problems like this before writing the code.
86 - It's entirely possible that other developers have thought about the
87 problem; they may have ideas for a better solution, and may be willing
88 to help in the creation of that solution.
90 Years of experience with the kernel development community have taught a
91 clear lesson: kernel code which is designed and developed behind closed
92 doors invariably has problems which are only revealed when the code is
93 released into the community. Sometimes these problems are severe,
94 requiring months or years of effort before the code can be brought up to
95 the kernel community's standards. Some examples include:
97 - The Devicescape network stack was designed and implemented for
98 single-processor systems. It could not be merged into the mainline
99 until it was made suitable for multiprocessor systems. Retrofitting
100 locking and such into code is a difficult task; as a result, the merging
101 of this code (now called mac80211) was delayed for over a year.
103 - The Reiser4 filesystem included a number of capabilities which, in the
104 core kernel developers' opinion, should have been implemented in the
105 virtual filesystem layer instead. It also included features which could
106 not easily be implemented without exposing the system to user-caused
107 deadlocks. The late revelation of these problems - and refusal to
108 address some of them - has caused Reiser4 to stay out of the mainline
109 kernel.
111 - The AppArmor security module made use of internal virtual filesystem
112 data structures in ways which were considered to be unsafe and
113 unreliable. This concern (among others) kept AppArmor out of the
114 mainline for years.
116 In each of these cases, a great deal of pain and extra work could have been
117 avoided with some early discussion with the kernel developers.
120 3.3: WHO DO YOU TALK TO?
122 When developers decide to take their plans public, the next question will
123 be: where do we start? The answer is to find the right mailing list(s) and
124 the right maintainer. For mailing lists, the best approach is to look in
125 the MAINTAINERS file for a relevant place to post. If there is a suitable
126 subsystem list, posting there is often preferable to posting on
127 linux-kernel; you are more likely to reach developers with expertise in the
128 relevant subsystem and the environment may be more supportive.
130 Finding maintainers can be a bit harder. Again, the MAINTAINERS file is
131 the place to start. That file tends to not always be up to date, though,
132 and not all subsystems are represented there. The person listed in the
133 MAINTAINERS file may, in fact, not be the person who is actually acting in
134 that role currently. So, when there is doubt about who to contact, a
135 useful trick is to use git (and "git log" in particular) to see who is
136 currently active within the subsystem of interest. Look at who is writing
137 patches, and who, if anybody, is attaching Signed-off-by lines to those
138 patches. Those are the people who will be best placed to help with a new
139 development project.
141 The task of finding the right maintainer is sometimes challenging enough
142 that the kernel developers have added a script to ease the process:
144 .../scripts/get_maintainer.pl
146 This script will return the current maintainer(s) for a given file or
147 directory when given the "-f" option. If passed a patch on the
148 command line, it will list the maintainers who should probably receive
149 copies of the patch. There are a number of options regulating how hard
150 get_maintainer.pl will search for maintainers; please be careful about
151 using the more aggressive options as you may end up including developers
152 who have no real interest in the code you are modifying.
154 If all else fails, talking to Andrew Morton can be an effective way to
155 track down a maintainer for a specific piece of code.
158 3.4: WHEN TO POST?
160 If possible, posting your plans during the early stages can only be
161 helpful. Describe the problem being solved and any plans that have been
162 made on how the implementation will be done. Any information you can
163 provide can help the development community provide useful input on the
164 project.
166 One discouraging thing which can happen at this stage is not a hostile
167 reaction, but, instead, little or no reaction at all. The sad truth of the
168 matter is (1) kernel developers tend to be busy, (2) there is no shortage
169 of people with grand plans and little code (or even prospect of code) to
170 back them up, and (3) nobody is obligated to review or comment on ideas
171 posted by others. Beyond that, high-level designs often hide problems
172 which are only reviewed when somebody actually tries to implement those
173 designs; for that reason, kernel developers would rather see the code.
175 If a request-for-comments posting yields little in the way of comments, do
176 not assume that it means there is no interest in the project.
177 Unfortunately, you also cannot assume that there are no problems with your
178 idea. The best thing to do in this situation is to proceed, keeping the
179 community informed as you go.
182 3.5: GETTING OFFICIAL BUY-IN
184 If your work is being done in a corporate environment - as most Linux
185 kernel work is - you must, obviously, have permission from suitably
186 empowered managers before you can post your company's plans or code to a
187 public mailing list. The posting of code which has not been cleared for
188 release under a GPL-compatible license can be especially problematic; the
189 sooner that a company's management and legal staff can agree on the posting
190 of a kernel development project, the better off everybody involved will be.
192 Some readers may be thinking at this point that their kernel work is
193 intended to support a product which does not yet have an officially
194 acknowledged existence. Revealing their employer's plans on a public
195 mailing list may not be a viable option. In cases like this, it is worth
196 considering whether the secrecy is really necessary; there is often no real
197 need to keep development plans behind closed doors.
199 That said, there are also cases where a company legitimately cannot
200 disclose its plans early in the development process. Companies with
201 experienced kernel developers may choose to proceed in an open-loop manner
202 on the assumption that they will be able to avoid serious integration
203 problems later. For companies without that sort of in-house expertise, the
204 best option is often to hire an outside developer to review the plans under
205 a non-disclosure agreement. The Linux Foundation operates an NDA program
206 designed to help with this sort of situation; more information can be found
207 at:
209 http://www.linuxfoundation.org/en/NDA_program
211 This kind of review is often enough to avoid serious problems later on
212 without requiring public disclosure of the project.