Addressing warnings¶

The documentation build currently spews out an unbelievable number ofwarnings. 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 newones. For this reason, eliminating warnings is one of the highest-prioritytasks on the documentation TODO list. The task itself is reasonablystraightforward, but it must be approached in the right way to besuccessful.

Warnings issued by a compiler for C code can often be dismissed as falsepositives, leading to patches aimed at simply shutting the compiler up.Warnings from the documentation build almost always point at a realproblem; making those warnings go away requires understanding the problemand fixing it at its source. For this reason, patches fixing documentationwarnings 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 byproblems in kerneldoc comments in C code. While the documentationmaintainer appreciates being copied on fixes for these warnings, thedocumentation tree is often not the right one to actually carry thosefixes; they should go to the maintainer of the subsystem in question.

For example, in a documentation build I grabbed a pair of warnings nearlyat 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 kerneldoccomments 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’ssimplistic idea of what C comment blocks look like. This problem had beenpresent since that comment was added in 2016 — a full four years. Fixingit was a matter of adding the missing asterisks. A quick look at thehistory for that file showed what the normal format for subject lines is,andscripts/get_maintainer.pl told me who should receive it. Theresulting patch looked like this:

[PATCH] PM / devfreq: Fix two malformed kerneldoc commentsTwo 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.cindex 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 thatsomebody 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 diginto it.

Other fixes will take longer, especially those relating to structuremembers or function parameters that lack documentation. In such cases, itis necessary to work out what the role of those members or parameters isand describe them correctly. Overall, this task gets a little tedious attimes, but it’s highly important. If we can actually eliminate warningsfrom the documentation build, then we can start expecting developers toavoid adding new ones.

Languishing kerneldoc comments¶

Developers are encouraged to write kerneldoc comments for their code, butmany of those comments are never pulled into the docs build. That makesthis information harder to find and, for example, makes Sphinx unable togenerate links to that documentation. Addingkernel-doc directives tothe documentation to bring those comments in can help the community derivethe full value of the work that has gone into creating them.

Thescripts/find-unused-docs.sh tool can be used to find theseoverlooked comments.

Note that the most value comes from pulling in the documentation forexported functions and data structures. Many subsystems also havekerneldoc comments for internal use; those should not be pulled into thedocumentation build unless they are placed in a document that isspecifically aimed at developers working within the relevant subsystem.

Typo fixes¶

Fixing typographical or formatting errors in the documentation is a quickway to figure out how to create and send patches, and it is a usefulservice. I am always willing to accept such patches. That said, once youhave fixed a few, please consider moving on to more advanced tasks, leavingsome typos for the next beginner to address.

Please note that some things arenot typos and should not be “fixed”:

• Both American and British English spellings are allowed within thekernel documentation. There is no need to fix one by replacing it withthe other.
• The question of whether a period should be followed by one or two spacesis not to be debated in the context of kernel documentation. Otherareas of rational disagreement, such as the “Oxford comma”, are alsooff-topic here.

As with any patch to any project, please consider whether your change isreally making things better.

Ancient documentation¶

Some kernel documentation is current, maintained, and useful. Somedocumentation is … not. Dusty, old, and inaccurate documentation canmislead readers and casts doubt on our documentation as a whole. Anythingthat can be done to address such problems is more than welcome.

Whenever you are working with a document, please consider whether it iscurrent, whether it needs updating, or whether it should perhaps be removedaltogether. There are a number of warning signs that you can pay attentionto 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 documentationcurrent, adding whatever information is needed. Such work often requiresthe cooperation of developers familiar with the subsystem in question, ofcourse. Developers are often more than willing to cooperate with peopleworking to improve the documentation when asked nicely, and when theiranswers are listened to and acted upon.

Some documentation is beyond hope; we occasionally find documents thatrefer to code that was removed from the kernel long ago, for example.There is surprising resistance to removing obsolete documentation, but weshould do that anyway. Extra cruft in our documentation helps nobody.

In cases where there is perhaps some useful information in a badly outdateddocument, and you are unable to update it, the best thing to do may be toadd 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 thedocument may lead them astray.

Documentation coherency¶

The old-timers around here will remember the Linux books that showed up onthe shelves in the 1990s. They were simply collections of documentationfiles scrounged from various locations on the net. The books have (mostly)improved since then, but the kernel’s documentation is still mostly builton that model. It is thousands of files, almost each of which was writtenin isolation from all of the others. We don’t have a coherent body ofkernel documentation; we have thousands of individual documents.

We have been trying to improve the situation through the creation ofa set of “books” that group documentation for specific readers. Theseinclude:

• The Linux kernel user’s and administrator’s guide
• Core API Documentation
• The Linux driver implementer’s API guide
• The Linux kernel user-space API guide

As well as this book on documentation itself.

Moving documents into the appropriate books is an important task and needsto continue. There are a couple of challenges associated with this work,though. Moving documentation files creates short-term pain for the peoplewho work with those files; they are understandably unenthusiastic aboutsuch changes. Usually the case can be made to move a document once; wereally don’t want to keep shifting them around, though.

Even when all documents are in the right place, though, we have onlymanaged to turn a big pile into a group of smaller piles. The work oftrying to knit all of those documents together into a single whole has notyet 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 weonce did. But it could still use a lot of improvement; Donald Knuth andEdward Tufte would be unimpressed. That requires tweaking our stylesheetsto create more typographically sound, accessible, and readable output.

Be warned: if you take on this task you are heading into classic bikeshedterritory. Expect a lot of opinions and discussion for even relativelyobvious 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 andPython skills. The Sphinx toolchain is relatively small and wellcontained; it is easy to add to a development system. But building PDF orEPUB output requires installing LaTeX, which is anything but small or wellcontained. 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 towork with that project to make rst2pdf work with the kernel documentationbuild, the world would be eternally grateful.

Write more documentation¶

Naturally, there are massive parts of the kernel that are severelyunderdocumented. If you have the knowledge to document a specific kernelsubsystem and the desire to do so, please do not hesitate to do somewriting and contribute the result to the kernel. Untold numbers of kerneldevelopers and users will thank you.