Add a document on how to contribute to the documentation

This is mostly a collection of thoughts for how people who want to help out
can make the docs better.  Hopefully the world will respond with a flurry
of useful patches.
Acked-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/doc-guide/contributing.rst

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

Documentation/doc-guide/index.rst

@@ -10,6 +10,7 @@ How to write kernel documentation
    sphinx
    kernel-doc
    parse-headers
+   contributing
 
 .. only::  subproject and html