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