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.. SPDX-License-Identifier: GPL-2.0
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How to help improve kernel documentation
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========================================
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Documentation is an important part of any software-development project.
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Good documentation helps to bring new developers in and helps established
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developers work more effectively. Without top-quality documentation, a lot
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of time is wasted in reverse-engineering the code and making avoidable
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mistakes.
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Unfortunately, the kernel's documentation currently falls far short of what
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it needs to be to support a project of this size and importance.
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This guide is for contributors who would like to improve that situation.
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Kernel documentation improvements can be made by developers at a variety of
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skill levels; they are a relatively easy way to learn the kernel process in
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general and find a place in the community. The bulk of what follows is the
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documentation maintainer's list of tasks that most urgently need to be
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done.
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The documentation TODO list
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---------------------------
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There is an endless list of tasks that need to be carried out to get our
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documentation to where it should be. This list contains a number of
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important items, but is far from exhaustive; if you see a different way to
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improve the documentation, please do not hold back!
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Addressing warnings
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~~~~~~~~~~~~~~~~~~~
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The documentation build currently spews out an unbelievable number of
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warnings. When you have that many, you might as well have none at all;
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people ignore them, and they will never notice when their work adds new
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ones. For this reason, eliminating warnings is one of the highest-priority
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tasks on the documentation TODO list. The task itself is reasonably
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straightforward, but it must be approached in the right way to be
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successful.
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Warnings issued by a compiler for C code can often be dismissed as false
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positives, leading to patches aimed at simply shutting the compiler up.
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Warnings from the documentation build almost always point at a real
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problem; making those warnings go away requires understanding the problem
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and fixing it at its source. For this reason, patches fixing documentation
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warnings should probably not say "fix a warning" in the changelog title;
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they should indicate the real problem that has been fixed.
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Another important point is that documentation warnings are often created by
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problems in kerneldoc comments in C code. While the documentation
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maintainer appreciates being copied on fixes for these warnings, the
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documentation tree is often not the right one to actually carry those
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fixes; they should go to the maintainer of the subsystem in question.
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For example, in a documentation build I grabbed a pair of warnings nearly
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at random::
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./drivers/devfreq/devfreq.c:1818: warning: bad line:
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- Resource-managed devfreq_register_notifier()
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./drivers/devfreq/devfreq.c:1854: warning: bad line:
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- Resource-managed devfreq_unregister_notifier()
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(The lines were split for readability).
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A quick look at the source file named above turned up a couple of kerneldoc
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comments that look like this::
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/**
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* devm_devfreq_register_notifier()
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- Resource-managed devfreq_register_notifier()
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* @dev: The devfreq user device. (parent of devfreq)
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* @devfreq: The devfreq object.
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* @nb: The notifier block to be unregistered.
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* @list: DEVFREQ_TRANSITION_NOTIFIER.
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*/
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The problem is the missing "*", which confuses the build system's
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simplistic idea of what C comment blocks look like. This problem had been
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present since that comment was added in 2016 — a full four years. Fixing
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it was a matter of adding the missing asterisks. A quick look at the
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history for that file showed what the normal format for subject lines is,
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and ``scripts/get_maintainer.pl`` told me who should receive it. The
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resulting patch looked like this::
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[PATCH] PM / devfreq: Fix two malformed kerneldoc comments
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Two kerneldoc comments in devfreq.c fail to adhere to the required format,
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resulting in these doc-build warnings:
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./drivers/devfreq/devfreq.c:1818: warning: bad line:
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- Resource-managed devfreq_register_notifier()
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./drivers/devfreq/devfreq.c:1854: warning: bad line:
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- Resource-managed devfreq_unregister_notifier()
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Add a couple of missing asterisks and make kerneldoc a little happier.
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Signed-off-by: Jonathan Corbet <corbet@lwn.net>
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---
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drivers/devfreq/devfreq.c | 4 ++--
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1 file changed, 2 insertions(+), 2 deletions(-)
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diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c
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index 57f6944d65a6..00c9b80b3d33 100644
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--- a/drivers/devfreq/devfreq.c
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+++ b/drivers/devfreq/devfreq.c
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@@ -1814,7 +1814,7 @@ static void devm_devfreq_notifier_release(struct device *dev, void *res)
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/**
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* devm_devfreq_register_notifier()
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- - Resource-managed devfreq_register_notifier()
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+ * - Resource-managed devfreq_register_notifier()
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* @dev: The devfreq user device. (parent of devfreq)
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* @devfreq: The devfreq object.
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* @nb: The notifier block to be unregistered.
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@@ -1850,7 +1850,7 @@ EXPORT_SYMBOL(devm_devfreq_register_notifier);
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/**
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* devm_devfreq_unregister_notifier()
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- - Resource-managed devfreq_unregister_notifier()
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+ * - Resource-managed devfreq_unregister_notifier()
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* @dev: The devfreq user device. (parent of devfreq)
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* @devfreq: The devfreq object.
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* @nb: The notifier block to be unregistered.
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--
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2.24.1
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The entire process only took a few minutes. Of course, I then found that
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somebody else had fixed it in a separate tree, highlighting another lesson:
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always check linux-next to see if a problem has been fixed before you dig
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into it.
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Other fixes will take longer, especially those relating to structure
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members or function parameters that lack documentation. In such cases, it
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is necessary to work out what the role of those members or parameters is
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and describe them correctly. Overall, this task gets a little tedious at
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times, but it's highly important. If we can actually eliminate warnings
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from the documentation build, then we can start expecting developers to
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avoid adding new ones.
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Languishing kerneldoc comments
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Developers are encouraged to write kerneldoc comments for their code, but
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many of those comments are never pulled into the docs build. That makes
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this information harder to find and, for example, makes Sphinx unable to
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generate links to that documentation. Adding ``kernel-doc`` directives to
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the documentation to bring those comments in can help the community derive
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the full value of the work that has gone into creating them.
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The ``scripts/find-unused-docs.sh`` tool can be used to find these
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overlooked comments.
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Note that the most value comes from pulling in the documentation for
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exported functions and data structures. Many subsystems also have
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kerneldoc comments for internal use; those should not be pulled into the
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documentation build unless they are placed in a document that is
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specifically aimed at developers working within the relevant subsystem.
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Typo fixes
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~~~~~~~~~~
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Fixing typographical or formatting errors in the documentation is a quick
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way to figure out how to create and send patches, and it is a useful
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service. I am always willing to accept such patches. That said, once you
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have fixed a few, please consider moving on to more advanced tasks, leaving
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some typos for the next beginner to address.
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Please note that some things are *not* typos and should not be "fixed":
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- Both American and British English spellings are allowed within the
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kernel documentation. There is no need to fix one by replacing it with
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the other.
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- The question of whether a period should be followed by one or two spaces
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is not to be debated in the context of kernel documentation. Other
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areas of rational disagreement, such as the "Oxford comma", are also
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off-topic here.
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As with any patch to any project, please consider whether your change is
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really making things better.
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Ancient documentation
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~~~~~~~~~~~~~~~~~~~~~
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Some kernel documentation is current, maintained, and useful. Some
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documentation is ... not. Dusty, old, and inaccurate documentation can
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mislead readers and casts doubt on our documentation as a whole. Anything
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that can be done to address such problems is more than welcome.
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Whenever you are working with a document, please consider whether it is
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current, whether it needs updating, or whether it should perhaps be removed
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altogether. There are a number of warning signs that you can pay attention
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to here:
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- References to 2.x kernels
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- Pointers to SourceForge repositories
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- Nothing but typo fixes in the history for several years
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- Discussion of pre-Git workflows
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The best thing to do, of course, would be to bring the documentation
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current, adding whatever information is needed. Such work often requires
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the cooperation of developers familiar with the subsystem in question, of
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course. Developers are often more than willing to cooperate with people
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working to improve the documentation when asked nicely, and when their
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answers are listened to and acted upon.
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Some documentation is beyond hope; we occasionally find documents that
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refer to code that was removed from the kernel long ago, for example.
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There is surprising resistance to removing obsolete documentation, but we
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should do that anyway. Extra cruft in our documentation helps nobody.
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In cases where there is perhaps some useful information in a badly outdated
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document, and you are unable to update it, the best thing to do may be to
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add a warning at the beginning. The following text is recommended::
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.. warning ::
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This document is outdated and in need of attention. Please use
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this information with caution, and please consider sending patches
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to update it.
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That way, at least our long-suffering readers have been warned that the
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document may lead them astray.
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Documentation coherency
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~~~~~~~~~~~~~~~~~~~~~~~
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The old-timers around here will remember the Linux books that showed up on
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the shelves in the 1990s. They were simply collections of documentation
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files scrounged from various locations on the net. The books have (mostly)
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improved since then, but the kernel's documentation is still mostly built
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on that model. It is thousands of files, almost each of which was written
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in isolation from all of the others. We don't have a coherent body of
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kernel documentation; we have thousands of individual documents.
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We have been trying to improve the situation through the creation of
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a set of "books" that group documentation for specific readers. These
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include:
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- Documentation/admin-guide/index.rst
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- Documentation/core-api/index.rst
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- Documentation/driver-api/index.rst
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- Documentation/userspace-api/index.rst
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As well as this book on documentation itself.
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Moving documents into the appropriate books is an important task and needs
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to continue. There are a couple of challenges associated with this work,
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though. Moving documentation files creates short-term pain for the people
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who work with those files; they are understandably unenthusiastic about
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such changes. Usually the case can be made to move a document once; we
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really don't want to keep shifting them around, though.
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Even when all documents are in the right place, though, we have only
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managed to turn a big pile into a group of smaller piles. The work of
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trying to knit all of those documents together into a single whole has not
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yet begun. If you have bright ideas on how we could proceed on that front,
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we would be more than happy to hear them.
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Stylesheet improvements
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~~~~~~~~~~~~~~~~~~~~~~~
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With the adoption of Sphinx we have much nicer-looking HTML output than we
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once did. But it could still use a lot of improvement; Donald Knuth and
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Edward Tufte would be unimpressed. That requires tweaking our stylesheets
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to create more typographically sound, accessible, and readable output.
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Be warned: if you take on this task you are heading into classic bikeshed
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territory. Expect a lot of opinions and discussion for even relatively
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obvious changes. That is, alas, the nature of the world we live in.
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Non-LaTeX PDF build
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~~~~~~~~~~~~~~~~~~~
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This is a decidedly nontrivial task for somebody with a lot of time and
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Python skills. The Sphinx toolchain is relatively small and well
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contained; it is easy to add to a development system. But building PDF or
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EPUB output requires installing LaTeX, which is anything but small or well
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contained. That would be a nice thing to eliminate.
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The original hope had been to use the rst2pdf tool (https://rst2pdf.org/)
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for PDF generation, but it turned out to not be up to the task.
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Development work on rst2pdf seems to have picked up again in recent times,
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though, which is a hopeful sign. If a suitably motivated developer were to
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work with that project to make rst2pdf work with the kernel documentation
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build, the world would be eternally grateful.
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Write more documentation
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~~~~~~~~~~~~~~~~~~~~~~~~
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Naturally, there are massive parts of the kernel that are severely
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underdocumented. If you have the knowledge to document a specific kernel
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subsystem and the desire to do so, please do not hesitate to do some
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writing and contribute the result to the kernel. Untold numbers of kernel
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developers and users will thank you.
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