Based on kernel version 4.1. Page generated on 2015-06-28 12:08 EST.
1 3: EARLY-STAGE PLANNING 2 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. 8 9 10 3.1: SPECIFYING THE PROBLEM 11 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. 17 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. 25 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. 33 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: 39 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. 44 45 (http://lwn.net/Articles/131776/). 46 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. 53 54 So, when contemplating a kernel development project, one should obtain 55 answers to a short set of questions: 56 57 - What, exactly, is the problem which needs to be solved? 58 59 - Who are the users affected by this problem? Which use cases should the 60 solution address? 61 62 - How does the kernel fall short in addressing that problem now? 63 64 Only then does it make sense to start considering possible solutions. 65 66 67 3.2: EARLY DISCUSSION 68 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: 72 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. 81 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. 85 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. 89 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: 96 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. 102 103 - 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. 110 111 - 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. 115 116 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. 118 119 120 3.3: WHO DO YOU TALK TO? 121 122 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. 129 130 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. 140 141 The task of finding the right maintainer is sometimes challenging enough 142 that the kernel developers have added a script to ease the process: 143 144 .../scripts/get_maintainer.pl 145 146 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. 153 154 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. 156 157 158 3.4: WHEN TO POST? 159 160 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. 165 166 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. 174 175 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. 180 181 182 3.5: GETTING OFFICIAL BUY-IN 183 184 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. 191 192 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. 198 199 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: 208 209 http://www.linuxfoundation.org/en/NDA_program 210 211 This kind of review is often enough to avoid serious problems later on 212 without requiring public disclosure of the project.