Linux Kernel checkpatch.pl mentorship

Checkpatch - What is it?

checkpatch.pl is a trivial style checker for patches sent to the linux kernel.

Why is it needed you say?
I did an analysis on checkpatch warnings on about 50k commits from v5.4.
(Find the full report here )

The top errors were then generated via the following shell command:

cat checkpatch_out_50k.txt | grep -oP "(?:WARNING|ERROR|CHECK):(\w+)" \
| sort | uniq -c | sort -nr  | head -n6

As we can see, the number of stylistically incorrect bits still continue to exist and are still merged into the kernel code. To make the kernel code more stylistically coherent, we can:

• Create cleanup patches for the existing code
• Make sure new patches are stylistically consistent

Both of these are where checkpatch comes handy.

Checkpatch in the patch submission process

Before sending out the patches to the mailing list, it is recommended to run checkpatch over the patches.

In general, a Kernel developer would:

• Create the patch with his proposed changes.
• Build check the patch and lookout for compiler warnings.
• Run additional tools such as sparse, KASAN, etc.
• Run checkpatch.pl over the patches to eliminate all trivial style violations.
• Send out the patch to the maintainers and mailing list.

It is to be noted that checkpatch is not always right. And at times the author’s judgement should take precedence over checkpatch.

Drawbacks of checkpatch

Checkpatch is only a static trivial style checker. It does not have the capabilities of a tool using a full-fledged C parser, such as clang-format . In this sense it is limited in what it can do.

Still, it is highly valuable as a style checker tool and can ease the work of maintainers and increase the chances of the patch being accepted by the community.

What did we do to improve it?

Over the first three months of my mentorship period, I primarily focused on eliminating false-positives from checkpatch as well as adding new rules.

Consider the following patch commit log:

#include asm/bitsperlong.h avoid further potential userspace pollution
by moving the #define of SHIFT_PER_LONG to bitops.h which is not
exported to userspace.

A kernel developer in 2020 would not see any checkpatch warning.

A kernel developer in 2021 would see the following warning:

WARNING: Commit log lines starting with '#' are dropped by git as comments
#82: 
#include asm/bitsperlong.h avoid further potential userspace pollution

During the last three months I worked on two major features:

• Addition of a verbose mode to checkpatch
• Addition of an external documentation to checkpatch

For the documentation we decided to document all checkpatch message types, their meanings and how to resolve them. This is still a work in progress. Contributions are definitely welcome.

Verbose Mode

This was one of the new additions we did to checkpatch.

One can run checkpatch like this:

./scripts/checkpatch.pl -v test.patch

All verbose descriptions can be seen by pairing the verbose option with –list-types:

./scripts/checkpatch.pl --list-types -v

Checkpatch Documentation

The checkpatch documentation is at Documentation/dev-tools/checkpatch.rst in the kernel repository.

To build the documentation make sure you have configured sphynx either in the system or in a python virtualenv. Next the kernel doc can be built using:

make htmldocs

How to contribute to the documentation?

The documentation is still a work in progress. Feel free to send the patches to the kernel-doc mailing list, and add me (Dwaipayan Ray dwaipayanray1@gmail.com) and Lukas (Lukas Bulwahn lukas.bulwahn@gmail.com) for reviewing it.

The documentation is written in rst syntax. Refer here for a quick guide.

Before sending out the patch, please always:

• checkpatch.pl your changes
• build check your patch (in this case check the generated html docs)
• spell check your patch (this can be easily done through checkpatch using –codespell flag if you already have the codespell dictionary)

Acknowledgements

During my mentorship period I learned a lot from my mentors. I would like to thank Lukas for his constant support as my mentor, Joe for his guidance with checkpatch and everyone in the community for being such amazing people.

The kernel contribution process may look old but it works. At the beginning when I started sending my initial patches, it looked like a tiring and unnecessary process. I was more accustomed to creating pull requests rather than sending patches over mail.

Slowly I found out that scripts like get_maintainer.pl actually lets you easily find the people associated with that particular code. After finding those people it’s very easy to send the patches to the list and the associated people. This follows a period of extensive review which is very useful for having good code. I have got lots of reviews from people in the community and those patches are successfully merged into the mainline.

So I was really amazed by the process. What is even more amazing is that the newcomers won’t get lost in this extensive process. There are always people who help and encourage others to create good patches. And that makes me glad to say that I too am a part of this community.