The Kernel Hacker's Guide to Source Code Control

Many issues involved with Linux kernel development are different from traditional software development processes. When working on a portion of the kernel (or a specific driver), you need to 1) stay aware of changes that are happening to other portions of the kernel with which you interact, 2) constantly apply your changes to the moving target of a fast-based kernel development release schedule, 3) resolve any merge conflicts between changes you have made and changes made by other people and 4) be able to export your changes in a format others can use easily.

For a number of years, I developed and maintained the USB to serial port drivers and then eventually took over maintaining all of the USB code in the kernel. In this article, I explain some of the tools I used in the past to do this work and show how some new tools have enhanced my ability to keep on top of changes in the kernel and let me do my job with less effort.

One of the most common methods of doing kernel work is to use the patch and diff programs. You can use this and no other type of source-code control system to do kernel development. One way is to use two different directory trees: a “clean” one and a “working” one. The clean tree is a released kernel version, while the working one is based on the same released kernel version, but contains your modifications. Then you can use patch and diff to extract your changes and forward port these changes to a new kernel release. For example, let's start off with a clean 2.4.18 kernel (available at www.kernel.org/pub/linux/kernel/v2.4/linux-2.4.18.tar.gz) in our working directory:

$ ls
linux-2.4.18.tar.gz

Uncompress this kernel, and then rename the created directory, which will be called “linux” to something that makes sense:

$ tar -zxf linux-2.4.18.tar.gz
$ mv linux linux-2.4.18
$ ls
linux-2.4.18  linux-2.4.18.tar.gz

$ tar -zxf linux-2.4.18.tar.gz
$ mv linux linux-2.4.18-greg
$ ls
linux-2.4.18  linux-2.4.18-greg  linux-2.4.18.tar.gz

To create a patch, use the following command:

$ diff -Naur -X dontdiff \
linux-2.4.18 linux-2.4.18-greg/ > my_patch

This creates a file called my_patch that contains the difference between your work and a clean 2.4.18 kernel tree. This patch then can be sent to other people via e-mail.

If a new kernel version is released, and you want to forward port your changes to the new version, you need to try to apply your generated patch onto a clean kernel version. This can be done in the following steps:

$ cd linux-2.4.18
$ patch -p1 < ../patch-2.4.19
$ cd ..
$ mv linux-2.4.18 linux-2.4.19

$ cd linux-2.4.18-greg
$ patch -p1 -R < ../my_patch
$ patch -p1 < ../patch-2.4.19
$ cd ..
$ mv linux-2.4.18-greg linux-2.4.19-greg

$ cd linux-2.4.19-greg
$ patch -p1 < ../my_patch

If your patch does not apply cleanly, resolve all of the conflicts that are created (patch will tell you about these, leaving behind .rej and .orig files for you to compare and fix up manually using your favorite editor). This merge process can be the most difficult part if you have made changes to portions of the source tree that have been changed by other people.

If you use this development process, I highly recommend getting the excellent patchutils set of programs (found at cyberelk.net/tim/patchutils). These programs enable you to manipulate text patches easily in all sorts of useful ways, and they have saved kernel developers many hours of tedious work.

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