1.. SPDX-License-Identifier: GPL-2.0 2 3How to help improve kernel documentation 4======================================== 5 6Documentation is an important part of any software-development project. 7Good documentation helps to bring new developers in and helps established 8developers work more effectively. Without top-quality documentation, a lot 9of time is wasted in reverse-engineering the code and making avoidable 10mistakes. 11 12Unfortunately, the kernel's documentation currently falls far short of what 13it needs to be to support a project of this size and importance. 14 15This guide is for contributors who would like to improve that situation. 16Kernel documentation improvements can be made by developers at a variety of 17skill levels; they are a relatively easy way to learn the kernel process in 18general and find a place in the community. The bulk of what follows is the 19documentation maintainer's list of tasks that most urgently need to be 20done. 21 22The documentation TODO list 23--------------------------- 24 25There is an endless list of tasks that need to be carried out to get our 26documentation to where it should be. This list contains a number of 27important items, but is far from exhaustive; if you see a different way to 28improve the documentation, please do not hold back! 29 30Addressing warnings 31~~~~~~~~~~~~~~~~~~~ 32 33The documentation build currently spews out an unbelievable number of 34warnings. When you have that many, you might as well have none at all; 35people ignore them, and they will never notice when their work adds new 36ones. For this reason, eliminating warnings is one of the highest-priority 37tasks on the documentation TODO list. The task itself is reasonably 38straightforward, but it must be approached in the right way to be 39successful. 40 41Warnings issued by a compiler for C code can often be dismissed as false 42positives, leading to patches aimed at simply shutting the compiler up. 43Warnings from the documentation build almost always point at a real 44problem; making those warnings go away requires understanding the problem 45and fixing it at its source. For this reason, patches fixing documentation 46warnings should probably not say "fix a warning" in the changelog title; 47they should indicate the real problem that has been fixed. 48 49Another important point is that documentation warnings are often created by 50problems in kerneldoc comments in C code. While the documentation 51maintainer appreciates being copied on fixes for these warnings, the 52documentation tree is often not the right one to actually carry those 53fixes; they should go to the maintainer of the subsystem in question. 54 55For example, in a documentation build I grabbed a pair of warnings nearly 56at random:: 57 58 ./drivers/devfreq/devfreq.c:1818: warning: bad line: 59 - Resource-managed devfreq_register_notifier() 60 ./drivers/devfreq/devfreq.c:1854: warning: bad line: 61 - Resource-managed devfreq_unregister_notifier() 62 63(The lines were split for readability). 64 65A quick look at the source file named above turned up a couple of kerneldoc 66comments that look like this:: 67 68 /** 69 * devm_devfreq_register_notifier() 70 - Resource-managed devfreq_register_notifier() 71 * @dev: The devfreq user device. (parent of devfreq) 72 * @devfreq: The devfreq object. 73 * @nb: The notifier block to be unregistered. 74 * @list: DEVFREQ_TRANSITION_NOTIFIER. 75 */ 76 77The problem is the missing "*", which confuses the build system's 78simplistic idea of what C comment blocks look like. This problem had been 79present since that comment was added in 2016 — a full four years. Fixing 80it was a matter of adding the missing asterisks. A quick look at the 81history for that file showed what the normal format for subject lines is, 82and ``scripts/get_maintainer.pl`` told me who should receive it (pass paths to 83your patches as arguments to scripts/get_maintainer.pl). The resulting patch 84looked like this:: 85 86 [PATCH] PM / devfreq: Fix two malformed kerneldoc comments 87 88 Two kerneldoc comments in devfreq.c fail to adhere to the required format, 89 resulting in these doc-build warnings: 90 91 ./drivers/devfreq/devfreq.c:1818: warning: bad line: 92 - Resource-managed devfreq_register_notifier() 93 ./drivers/devfreq/devfreq.c:1854: warning: bad line: 94 - Resource-managed devfreq_unregister_notifier() 95 96 Add a couple of missing asterisks and make kerneldoc a little happier. 97 98 Signed-off-by: Jonathan Corbet <corbet@lwn.net> 99 --- 100 drivers/devfreq/devfreq.c | 4 ++-- 101 1 file changed, 2 insertions(+), 2 deletions(-) 102 103 diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c 104 index 57f6944d65a6..00c9b80b3d33 100644 105 --- a/drivers/devfreq/devfreq.c 106 +++ b/drivers/devfreq/devfreq.c 107 @@ -1814,7 +1814,7 @@ static void devm_devfreq_notifier_release(struct device *dev, void *res) 108 109 /** 110 * devm_devfreq_register_notifier() 111 - - Resource-managed devfreq_register_notifier() 112 + * - Resource-managed devfreq_register_notifier() 113 * @dev: The devfreq user device. (parent of devfreq) 114 * @devfreq: The devfreq object. 115 * @nb: The notifier block to be unregistered. 116 @@ -1850,7 +1850,7 @@ EXPORT_SYMBOL(devm_devfreq_register_notifier); 117 118 /** 119 * devm_devfreq_unregister_notifier() 120 - - Resource-managed devfreq_unregister_notifier() 121 + * - Resource-managed devfreq_unregister_notifier() 122 * @dev: The devfreq user device. (parent of devfreq) 123 * @devfreq: The devfreq object. 124 * @nb: The notifier block to be unregistered. 125 -- 126 2.24.1 127 128The entire process only took a few minutes. Of course, I then found that 129somebody else had fixed it in a separate tree, highlighting another lesson: 130always check linux-next to see if a problem has been fixed before you dig 131into it. 132 133Other fixes will take longer, especially those relating to structure 134members or function parameters that lack documentation. In such cases, it 135is necessary to work out what the role of those members or parameters is 136and describe them correctly. Overall, this task gets a little tedious at 137times, but it's highly important. If we can actually eliminate warnings 138from the documentation build, then we can start expecting developers to 139avoid adding new ones. 140 141Languishing kerneldoc comments 142~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 143 144Developers are encouraged to write kerneldoc comments for their code, but 145many of those comments are never pulled into the docs build. That makes 146this information harder to find and, for example, makes Sphinx unable to 147generate links to that documentation. Adding ``kernel-doc`` directives to 148the documentation to bring those comments in can help the community derive 149the full value of the work that has gone into creating them. 150 151The ``scripts/find-unused-docs.sh`` tool can be used to find these 152overlooked comments. 153 154Note that the most value comes from pulling in the documentation for 155exported functions and data structures. Many subsystems also have 156kerneldoc comments for internal use; those should not be pulled into the 157documentation build unless they are placed in a document that is 158specifically aimed at developers working within the relevant subsystem. 159 160 161Typo fixes 162~~~~~~~~~~ 163 164Fixing typographical or formatting errors in the documentation is a quick 165way to figure out how to create and send patches, and it is a useful 166service. I am always willing to accept such patches. That said, once you 167have fixed a few, please consider moving on to more advanced tasks, leaving 168some typos for the next beginner to address. 169 170Please note that some things are *not* typos and should not be "fixed": 171 172 - Both American and British English spellings are allowed within the 173 kernel documentation. There is no need to fix one by replacing it with 174 the other. 175 176 - The question of whether a period should be followed by one or two spaces 177 is not to be debated in the context of kernel documentation. Other 178 areas of rational disagreement, such as the "Oxford comma", are also 179 off-topic here. 180 181As with any patch to any project, please consider whether your change is 182really making things better. 183 184Ancient documentation 185~~~~~~~~~~~~~~~~~~~~~ 186 187Some kernel documentation is current, maintained, and useful. Some 188documentation is ... not. Dusty, old, and inaccurate documentation can 189mislead readers and casts doubt on our documentation as a whole. Anything 190that can be done to address such problems is more than welcome. 191 192Whenever you are working with a document, please consider whether it is 193current, whether it needs updating, or whether it should perhaps be removed 194altogether. There are a number of warning signs that you can pay attention 195to here: 196 197 - References to 2.x kernels 198 - Pointers to SourceForge repositories 199 - Nothing but typo fixes in the history for several years 200 - Discussion of pre-Git workflows 201 202The best thing to do, of course, would be to bring the documentation 203current, adding whatever information is needed. Such work often requires 204the cooperation of developers familiar with the subsystem in question, of 205course. Developers are often more than willing to cooperate with people 206working to improve the documentation when asked nicely, and when their 207answers are listened to and acted upon. 208 209Some documentation is beyond hope; we occasionally find documents that 210refer to code that was removed from the kernel long ago, for example. 211There is surprising resistance to removing obsolete documentation, but we 212should do that anyway. Extra cruft in our documentation helps nobody. 213 214In cases where there is perhaps some useful information in a badly outdated 215document, and you are unable to update it, the best thing to do may be to 216add a warning at the beginning. The following text is recommended:: 217 218 .. warning :: 219 This document is outdated and in need of attention. Please use 220 this information with caution, and please consider sending patches 221 to update it. 222 223That way, at least our long-suffering readers have been warned that the 224document may lead them astray. 225 226Documentation coherency 227~~~~~~~~~~~~~~~~~~~~~~~ 228 229The old-timers around here will remember the Linux books that showed up on 230the shelves in the 1990s. They were simply collections of documentation 231files scrounged from various locations on the net. The books have (mostly) 232improved since then, but the kernel's documentation is still mostly built 233on that model. It is thousands of files, almost each of which was written 234in isolation from all of the others. We don't have a coherent body of 235kernel documentation; we have thousands of individual documents. 236 237We have been trying to improve the situation through the creation of 238a set of "books" that group documentation for specific readers. These 239include: 240 241 - Documentation/admin-guide/index.rst 242 - Documentation/core-api/index.rst 243 - Documentation/driver-api/index.rst 244 - Documentation/userspace-api/index.rst 245 246As well as this book on documentation itself. 247 248Moving documents into the appropriate books is an important task and needs 249to continue. There are a couple of challenges associated with this work, 250though. Moving documentation files creates short-term pain for the people 251who work with those files; they are understandably unenthusiastic about 252such changes. Usually the case can be made to move a document once; we 253really don't want to keep shifting them around, though. 254 255Even when all documents are in the right place, though, we have only 256managed to turn a big pile into a group of smaller piles. The work of 257trying to knit all of those documents together into a single whole has not 258yet begun. If you have bright ideas on how we could proceed on that front, 259we would be more than happy to hear them. 260 261Stylesheet improvements 262~~~~~~~~~~~~~~~~~~~~~~~ 263 264With the adoption of Sphinx we have much nicer-looking HTML output than we 265once did. But it could still use a lot of improvement; Donald Knuth and 266Edward Tufte would be unimpressed. That requires tweaking our stylesheets 267to create more typographically sound, accessible, and readable output. 268 269Be warned: if you take on this task you are heading into classic bikeshed 270territory. Expect a lot of opinions and discussion for even relatively 271obvious changes. That is, alas, the nature of the world we live in. 272 273Non-LaTeX PDF build 274~~~~~~~~~~~~~~~~~~~ 275 276This is a decidedly nontrivial task for somebody with a lot of time and 277Python skills. The Sphinx toolchain is relatively small and well 278contained; it is easy to add to a development system. But building PDF or 279EPUB output requires installing LaTeX, which is anything but small or well 280contained. That would be a nice thing to eliminate. 281 282The original hope had been to use the rst2pdf tool (https://rst2pdf.org/) 283for PDF generation, but it turned out to not be up to the task. 284Development work on rst2pdf seems to have picked up again in recent times, 285though, which is a hopeful sign. If a suitably motivated developer were to 286work with that project to make rst2pdf work with the kernel documentation 287build, the world would be eternally grateful. 288 289Write more documentation 290~~~~~~~~~~~~~~~~~~~~~~~~ 291 292Naturally, there are massive parts of the kernel that are severely 293underdocumented. If you have the knowledge to document a specific kernel 294subsystem and the desire to do so, please do not hesitate to do some 295writing and contribute the result to the kernel. Untold numbers of kernel 296developers and users will thank you. 297