Part 2 of this series outlines a method for starting to learn the process of Linux kernel development. As stated in part 1, this is but one method. The aim of this post is to illuminate a pathway starting at the point when you have had your first patch merged into the mainline. If you have not had your first patch merged you may like to read part 1 of this series.


Learning Linux kernel development consists of a number of conceptual pieces. One way of discussing them is;

  1. Understanding how the kernel works.
  2. Understanding what kernel code does.
  3. Understanding how to change kernel code.

The first in this list takes years. The Linux kernel, like any operating system, is a large complex software system. Understanding complex systems takes time and effort.

The second is more tangible, in order to be able to understand kernel code you simply need experience reading and writing kernel code. Circular argument I know, but let’s move onto number three of our list for the moment and return to two shortly.

In theory, one can get the kernel source code and hack away merrily in the privacy of ones local machine. That kind of defeats the purpose though. Open source is about collaboration, we want to learn how to work on the kernel so we can contribute to, and collaborate with, the kernel development community. If we want to be able to understand what kernel code does we need to read [a lot of] code and write code. And as we have established, if we are going to write kernel code we want to be able to contribute the code back to the community. It is therefore essential that we first become comfortable with the process of changing the source and having our changes merged back into the mainline.

Fortunately, and in contrast to popular opinion, the Linux kernel is extremely open and welcoming to new developers. There is a complete system in place that provides new developers with a place to learn the kernel development process. The aim of this blog post is to shed some light on this system and outline one method of using this system to learn the Linux kernel development process.

First Patch Series

At this stage, it is assumed you have a development environment set up that is amenable to kernel development. You have read all the documentation, within the kernel tree, in Documentation/process/*. You have subscribed to the kernel newbies mailing list and also the device driver development mailing list. You are sorting your incoming email into mailboxes so that mailing list mail can be viewed separately from other mail. Finally, you have had a patch merged into Greg Kroah-Hartman’s staging tree. More specifically, into the staging-testing branch of that tree. If you are not at this stage you may like to read part 1 of this series.

Thus far, most of the information in this blog series is accessible online without too much difficulty. Primarily on the kernel newbies web site. What follows was, at the time of writing (or more accurately, six months ago when I was at this stage) not easily garnered from the web.

We will be working exclusively patching code within drivers/staging. While trivial patches to the kernel proper can at times get through the merge process it is, in my opinion, disrespectful of the kernel maintainers time to attempt such changes. Please learn from my mistakes. Pushing cleanup patches into subsystems outside of staging benefits the kernel very little. Doing so trades your education for a non-trivial amount of effort on the part of the maintainer. There is a system in place for you to learn and that system is the staging directory (and the machinery in place around it).

On Motivation

Hackers tend to like working on interesting problems. It is of little use outlining a method for learning kernel development if it is not interesting. Now arises then the question of motivation. We must at this stage select some code to work on. Within drivers/staging there are a number of drivers. They are at various stages of development. Some are on their way into the kernel, some may be on their way out of the kernel and some may be stuck because of some design issue i.e implementing an old API. Also the drivers may vary in how complete or how functional they are.

There are [at least] three things that may be considered in selecting a driver to work on.

  1. Does the driver present easily accessible code modifications.
  2. Does the driver belong to a subsystem of interest.
  3. Is hardware for the driver accessible.

These three questions are personal and you may or may not be able to answer them. Also, there may be a trade off between them. You may like to keep them in mind as you navigate your way around staging. You need not confine yourself to a single driver, however, I found it more engaging to do so.

On Hardware

It is not necessary in order to complete the steps in this blog post to have access to real hardware. It is however necessary if you intend to continue down the path of writing a functioning device driver. Doing so is often stated as a stepping stone to kernel development. A web search on the driver name will usually turn up some information on the hardware that the driver is intended to run on. While you may not like the idea of spending $200 on a piece of hardware that you don’t need if you are going to spend 3 months writing a device driver then the investment in your education is well worth it. You may also like to consider the machine that the device requires, for example if you intend working on a driver for an SDIO device you will need a machine that has an SD card reader that supports the SDIO protocol.

Getting Started

We will be using to find some code to work on. If you are still looking for a driver to work on you may like to run checkpatch on the whole staging directory and pick a driver that throws a lot of warnings.

scripts/ -f --terse --strict drivers/staging/*/*.c

You may like to set up a shell alias for checkpatch

alias checkpatch='/path/to/kernel/tree/scripts/ -f'

You need the -f option to have checkpatch work on source files, as apposed to unified-diff formatted patches.

At this stage we have a driver we are going to work on (henceforth referred to as your driver). We have the output from checkpatch providing ideas for code to work on. Let us now start with the actual steps intended for this blog post.

1. Pick your Fixes

Using the above alias run checkpatch on the source (or header) files within your driver.

checkpatch --strict --terse --show-types *.c

Henceforth, for brevity, we will refer to all checkpatch outputs i.e ERROR, WARNING, CHECK as ‘warnings’.

As you did when creating your first patch, have a look at the different types of warnings (--show-types). Read the code around the line in question. At this stage there is little hope that you will understand what the whole driver is doing. We are aiming, however, to fix warnings for which we do understand the surrounding code. At a minimum you should understand what the surrounding block of code does.

Pick three warning types that you feel you understand. For each warning type fix all instances of that warning type within the driver, both header files and source files. Once you have fixed all instances of a single warning type commit your changes. You read part 1 right? You read Documentation/process/submitting-patches.rst right? You read the section 2) Describe your changes right? If you want to have any hope of getting real code changes into the kernel you need to be able to describe your changes. The place to do this is the git log message. The time to learn how to do this well is now. Even though your initial changes will be very simple you should describe them fully and thoroughly as outline in the above mention kernel document, specifically section 2.

At the risk of repeating myself, no one has to respond to your email/patches when doing kernel development. If you want your patches to be taken seriously you must present them in the correct manner. The more you display that you understand the process and correctly adhere to it the better your chances of success in kernel development.

Please use correct, both grammatically and technically, descriptions of your code changes. Use the correct format for the git log message and for the summary phrase. The summary phrase will be used in the subject line when your patch is in email form. This information is contained in section 14) The canonical patch format of submitting-patches.rst. Also, every other patch on the device drivers mailing list should have the same form. Newbie mistakes not-withstanding. There is a strict form for a reason, you will have better success if you follow it.

2. Check your Fixes

At this stage you have three commits in your git history and you have been working on a branch created off of staging-next. Now we are going to try to eliminate mistakes.

On Making Mistakes in Public

Kernel development is done in the open. I believe that one of the reasons people are so intimidated by LKML and kernel development in general is that making mistakes in public hurts. It hurts bad. There is nothing you can read that is going to make it hurt less when (and you will) you write some brain dead comment in an email on a kernel mailing list. Or when you breach some simple protocol that you well know, making you look like a complete goose. Mistakes happen, mistakes are tolerated. Repeated mistakes, especially after being corrected, are not so well tolerated. And I personally do not see why they should be. We are striving for excellence, mediocrity has no place here. Continually striving for excellence is one of the reasons open source software is such a beautiful pursuit.

1. Read the diff for each commit.

Patches submitted to kernel mailing lists are typically reviewed by reading the diff. At first this may seem difficult since there are some peculiarities to the diff format but if you check each of your patches first by reading the diff you will get good at catching mistakes. Here is a shell function that you may find useful

    git-log-patch() {
        local commit

        if (( $# < 1 )); then
            commit="HEAD~"          # default to last commit

        git log --color=always --patch --reverse "${commit}".. | less 

git-log-patch accepts as argument the commit prior to that which you wish to view, as usual this can be a hash or HEAD~. It then displays each commit in succession in patch format. You may also find this shell function useful when viewing your git index

git-log-abbrev () {
    # number of commits from index to view, default 1
    local COMMITS=${1:-20} 

    git log -$COMMITS --pretty=oneline --abbrev-commit --reverse

If you find mistakes in one of the commits then you may like to rebase in order to fix.

    git rebase -i HEAD~~~

Rebasing takes practice, that is one reason why we are doing a three patch series. Rebasing a 20 patch series is more prone to error. You will need to rebase to edit the commit log as well as to make code changes. It is very easy to mix up your commits, changes from one commit find their way into the next commit if you do not use the correct git commands when adding files and continuing the rebase. Any time you rebase I strongly suggest re-reading the whole series. Catching your mistakes yourself is much easier on the ego than submitting patches with blatant errors. See ‘On Making Mistakes in Public’ above.

3. Create the Patch Set

At this stage you have three commits in your git index. Each commit does one thing and one thing only, in this case each commit fixes one checkpatch warning type. Your git log clearly states the reason for this patch (checkpatch emits warning XYZ) and clearly states how the patch goes about fixing the issue. Now we can create the patch series (or patch set).

git format-patch -3 -o tmp/path/to/patches --cover-letter

This will output 4 files to the specified path. You can check each file at this stage for correctness also. We can now turn our attention to the cover letter. A cover letter is not required but it is not hard to create and makes the patch set a little more tidy. The content of the cover letter can range from describing and justifying every patch to briefly outlining the whole series. For a checkpatch series like this a quick line or two saying what the series does is sufficient. The cover letter is also where you may like to indicate the level of testing carried out. Let’s go right ahead and assume you have not tested your changes. This leads on to the next step in catching mistakes

Verify your Patches Build

Each and every patch that is applied to the Linux kernel must leave the kernel in a sane state. This means that each of your patches must apply and build. There are a few reasons for this. It aids bisecting and bug hunting. Not necessarily every patch in a series will be applied, a maintainer may pick out a subset of the patches to apply. And finally down stream [distribution] developers may cherry pick patches to apply to their kernels.

Patches to staging that do not apply are caught by Greg’s robots. You get an email saying so, no hard feelings. Patches that do not build however are viewed much more dimly. Again, please learn from my mistakes. Apply each patch individually and build the kernel for each patch before submitting a patch series. Here is a link to a script I wrote to carry out this task.

If you have access to other machines then it is a nice idea to apply and build your patch set on different architectures. Particularly Power PC so I read somewhere in the kernel docs.

Back to the cover letter, you may like to add a line at the end stating

Code is untested. Builds on x86_64 and PowerPC.

3. Submit the Patch Set

As for your single patch previously the patch recipients can be garnered from the TODO file within the driver or by running the script on a patch or file. If you manually fill the To: and Cc: header fields within the cover letter then you can use the following git command to send the patch set.

git send-email --to-cover --cc-cover path/to/dir/*.patch

Once again, you may like to send the series to yourself first to check it for mistakes.

Now wait. You will get a response from either a reviewer, Greg’s robots, the kbuild test robot, or from Greg himself. If anyone makes suggestions, thank them for their input and follow their suggestions. Do not submit a second version (see below) without seeing to each and every suggestion made by a reviewer.

4. Submitting Patch Set Version 2

If changes to your patch set are necessary you will need to submit a second (and third …) version. You need to deal with all comments to your patch on the mailing list. Greg receives a lot of patches each day, and response or contention typically pushes a patch to the bottom of the list of patches he manages, to get back to the top of the list (and be merged) you need to incorporate any reviewer suggestions into a new version. At this early stage you are most likely going to need to incorporate the suggestion as apposed to arguing your point.

If you get more than one suggestion you can implement all the changes at once. You do so by rebasing your patch set so as to finish up, as before, with just three patches. Once you are happy with the new series you must re-submit it. There are two things to note when doing so. The new subject for the patch set will be the same as the old except it will include [PATCH v2] in the subject. You can generate this using

git format-patch -3 --subject-prefix='PATCH v2'

Your cover letter will be the same as for the previous version except you need to add a section at the bottom that outlines the change history. Let’s assume you are up to version 3, your cover letter may include something like

v2 -> v3
- Fix whitespace issue as suggested by review. 

v1 -> v2
- Use u8 instead of uint8.

If you are working on a single patch instead of a series, this change history is still required. The place to put it is below the ---- within the patch. Things below the dashed line are not included in the git index when the patch is applied.

On Arguing with Reviewers

TL;DR; Don’t argue with reviewers.

Linux kernel maintainers and patch reviewers are in short supply, do not waste there valuable time arguing with them. At this early stage it is highly unlikely that you know more about kernel development than the developer reviewing your patch. Sure, reviewers make mistakes, sometimes email is misinterpreted. In this case by all means re-state your position in a different, more thoroughly articulated, manner. If the reviewer repeats the original sentiment then please accept that you are probably wrong. There may be something that you have not yet learned that justifies the reviewers position. A reviewer is not required to spell out exactly why something is wrong, they are reviewers not tutors. If you ask for pointers as to where you can learn more about why you are wrong you will do much better than attempting to prove you are correct. And if finally you still do not agree with the reviewers position just drop the patch and move onto something else, you are fixing staging bugs here not re-writing the scheduler. You are supposed to be learning, forge another patch set on something different and come back to the contentious issue when you have learned some more.

The more respectful of other developers time and efforts you are the more help you will receive. It is not magic, just simple human interactions.

Lather, Rinse, Repeat

Above I have attempted to outline a method for learning the Linux kernel development process while controlling some of the complexity that is inherent in learning a new complex software system. The issues touched on are issues I came up against while learning. You may well come up against different issues, each of us is different with different skill sets and personalities. One thing is for sure, you will not learn the process without repeating it many times. So go ahead, enjoy yourself, fix all the checkpatch warnings in that driver. If there are not enough there, pick another driver and learn it also. Once you are comfortable with the process and getting more comfortable with kernel code you may like to try fixing Sparse warnings. Pass the C=2 option to your build command to run Sparse.

make -j9 C=2 M=drivers/staging/FOO

Final Tips

Here are a couple of final tips you may find useful

Good luck