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3 Early-stage planning
4 ====================
6 When contemplating a Linux kernel development project, it can be tempting
7 to jump right in and start coding. As with any significant project,
8 though, much of the groundwork for success is best laid before the first
9 line of code is written. Some time spent in early planning and
10 communication can save far more time later on.
13 Specifying the problem
14 ----------------------
16 Like any engineering project, a successful kernel enhancement starts with a
17 clear description of the problem to be solved. In some cases, this step is
18 easy: when a driver is needed for a specific piece of hardware, for
19 example. In others, though, it is tempting to confuse the real problem
20 with the proposed solution, and that can lead to difficulties.
22 Consider an example: some years ago, developers working with Linux audio
23 sought a way to run applications without dropouts or other artifacts caused
24 by excessive latency in the system. The solution they arrived at was a
25 kernel module intended to hook into the Linux Security Module (LSM)
26 framework; this module could be configured to give specific applications
27 access to the realtime scheduler. This module was implemented and sent to
28 the linux-kernel mailing list, where it immediately ran into problems.
30 To the audio developers, this security module was sufficient to solve their
31 immediate problem. To the wider kernel community, though, it was seen as a
32 misuse of the LSM framework (which is not intended to confer privileges
33 onto processes which they would not otherwise have) and a risk to system
34 stability. Their preferred solutions involved realtime scheduling access
35 via the rlimit mechanism for the short term, and ongoing latency reduction
36 work in the long term.
38 The audio community, however, could not see past the particular solution
39 they had implemented; they were unwilling to accept alternatives. The
40 resulting disagreement left those developers feeling disillusioned with the
41 entire kernel development process; one of them went back to an audio list
42 and posted this:
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.
49 (http://lwn.net/Articles/131776/).
51 The reality of the situation was different; the kernel developers were far
52 more concerned about system stability, long-term maintenance, and finding
53 the right solution to the problem than they were with a specific module.
54 The 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
56 creation of a body of code.
58 So, when contemplating a kernel development project, one should obtain
59 answers to a short set of questions:
61 - What, exactly, is the problem which needs to be solved?
63 - Who are the users affected by this problem? Which use cases should the
64 solution address?
66 - How does the kernel fall short in addressing that problem now?
68 Only then does it make sense to start considering possible solutions.
71 Early discussion
72 ----------------
74 When planning a kernel development project, it makes great sense to hold
75 discussions with the community before launching into implementation. Early
76 communication can save time and trouble in a number of ways:
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.
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.
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.
95 Years of experience with the kernel development community have taught a
96 clear lesson: kernel code which is designed and developed behind closed
97 doors invariably has problems which are only revealed when the code is
98 released into the community. Sometimes these problems are severe,
99 requiring months or years of effort before the code can be brought up to
100 the kernel community's standards. Some examples include:
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.
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.
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.
121 In each of these cases, a great deal of pain and extra work could have been
122 avoided with some early discussion with the kernel developers.
125 Who do you talk to?
126 -------------------
128 When developers decide to take their plans public, the next question will
129 be: where do we start? The answer is to find the right mailing list(s) and
130 the right maintainer. For mailing lists, the best approach is to look in
131 the MAINTAINERS file for a relevant place to post. If there is a suitable
132 subsystem list, posting there is often preferable to posting on
133 linux-kernel; you are more likely to reach developers with expertise in the
134 relevant subsystem and the environment may be more supportive.
136 Finding maintainers can be a bit harder. Again, the MAINTAINERS file is
137 the place to start. That file tends to not always be up to date, though,
138 and not all subsystems are represented there. The person listed in the
139 MAINTAINERS file may, in fact, not be the person who is actually acting in
140 that role currently. So, when there is doubt about who to contact, a
141 useful trick is to use git (and "git log" in particular) to see who is
142 currently active within the subsystem of interest. Look at who is writing
143 patches, and who, if anybody, is attaching Signed-off-by lines to those
144 patches. Those are the people who will be best placed to help with a new
145 development project.
147 The task of finding the right maintainer is sometimes challenging enough
148 that the kernel developers have added a script to ease the process:
150 ::
152 .../scripts/get_maintainer.pl
154 This script will return the current maintainer(s) for a given file or
155 directory when given the "-f" option. If passed a patch on the
156 command line, it will list the maintainers who should probably receive
157 copies of the patch. There are a number of options regulating how hard
158 get_maintainer.pl will search for maintainers; please be careful about
159 using the more aggressive options as you may end up including developers
160 who have no real interest in the code you are modifying.
162 If all else fails, talking to Andrew Morton can be an effective way to
163 track down a maintainer for a specific piece of code.
166 When to post?
167 -------------
169 If possible, posting your plans during the early stages can only be
170 helpful. Describe the problem being solved and any plans that have been
171 made on how the implementation will be done. Any information you can
172 provide can help the development community provide useful input on the
173 project.
175 One discouraging thing which can happen at this stage is not a hostile
176 reaction, but, instead, little or no reaction at all. The sad truth of the
177 matter is (1) kernel developers tend to be busy, (2) there is no shortage
178 of people with grand plans and little code (or even prospect of code) to
179 back them up, and (3) nobody is obligated to review or comment on ideas
180 posted by others. Beyond that, high-level designs often hide problems
181 which are only revealed when somebody actually tries to implement those
182 designs; for that reason, kernel developers would rather see the code.
184 If a request-for-comments posting yields little in the way of comments, do
185 not assume that it means there is no interest in the project.
186 Unfortunately, you also cannot assume that there are no problems with your
187 idea. The best thing to do in this situation is to proceed, keeping the
188 community informed as you go.
191 Getting official buy-in
192 -----------------------
194 If your work is being done in a corporate environment - as most Linux
195 kernel work is - you must, obviously, have permission from suitably
196 empowered managers before you can post your company's plans or code to a
197 public mailing list. The posting of code which has not been cleared for
198 release under a GPL-compatible license can be especially problematic; the
199 sooner that a company's management and legal staff can agree on the posting
200 of a kernel development project, the better off everybody involved will be.
202 Some readers may be thinking at this point that their kernel work is
203 intended to support a product which does not yet have an officially
204 acknowledged existence. Revealing their employer's plans on a public
205 mailing list may not be a viable option. In cases like this, it is worth
206 considering whether the secrecy is really necessary; there is often no real
207 need to keep development plans behind closed doors.
209 That said, there are also cases where a company legitimately cannot
210 disclose its plans early in the development process. Companies with
211 experienced kernel developers may choose to proceed in an open-loop manner
212 on the assumption that they will be able to avoid serious integration
213 problems later. For companies without that sort of in-house expertise, the
214 best option is often to hire an outside developer to review the plans under
215 a non-disclosure agreement. The Linux Foundation operates an NDA program
216 designed to help with this sort of situation; more information can be found
217 at:
219 http://www.linuxfoundation.org/en/NDA_program
221 This kind of review is often enough to avoid serious problems later on
222 without requiring public disclosure of the project.