Contributing to the Linux Kernel

To make changes in the kernel, you need to know all about the diff and patch commands.
Maintenance of Source Trees

Getting back to the point of using diff and cousins as a sort of bare-bones revision-control system, I should mention there is obviously no one right or wrong way to maintain separate branches of a source tree. Low-level tools like these provide you with the framework to define the way you work, rather than forcing you into using one method or another. My suggestions here are only suggestions, and have been useful to me in the past when writing patches for open-source projects. However, I haven't exactly clocked the man-hours of Alan Cox working on the kernel or any other project, so there may be a better way. Please feel free to e-mail me at the address listed below with your thoughts.

When dealing with source trees, my general rule is always to maintain more than one. Unlike CVS or other revision-control systems, there is often no going back when you make a mistake. It is very easy to add instability to a stable patch, and no easy way to roll back your changes to a “last good” configuration. For example, let's imagine I have a source tree for a hypothetical project called “Foogram”. If I were maintaining this with CVS, I would need only one tree and a CVS server (which obviously maintains multiple trees). Since we don't have the advantage of a separate server with the bare-metal approach, I would generally have at least two directories for the project: foogram and foogram-last. (These are my personal naming conventions.) The last-released version would be in foogram, it's known to be stable and it's what I have to generate all my patches against when I want to distribute them. The foogram-last directory would contain my latest changes. This two-tiered approach is often effective enough for general use.

However, many projects actually get too complicated for this approach. I have been known to create a third (or fourth or fifth) directory which contains the latest changes relative to foogram-last. For our purposes, I'll call this directory foogram-work. It often contains unstable and recent changes I've made to my source trees, which I want to keep separate from my more stable patches. It's important to keep a baseline directory to generate patches against, but this tends to lead to a rapidly expanding number of directories that other directories are relative against, and a mess for patching the manual way. In this example, I would try to maintain foogram-last as the baseline for the foogram-work directory by making sure I merge forward stable changes as I make them. If this were to become over-complicated, I would have to create a copy of foogram-last, called foogram-stable, which contained the copy of foogram-last that foogram-work was drawn against, so I could use created patches between the last and the stable directory to apply against the working directory in order to keep it up to date.

Confused yet? I sure am! Obviously, that's an extreme example, and many programmers will naturally simplify the process based on their own needs. Many projects do not require that level of complexity from individual developers. If you are reading this and still getting anything useful from it, you probably won't need to go to that complicated extreme to develop your own open-source modification patches, and instead will use the easy method.

The easy method is how it's possible to get away with keeping only one tree around, and this has worked for me on nearly every light project where I needed to modify only one or two source files as part of my patch. In that case, I recommend just making a copy of the source files you are editing, with a different extension (such as .old); and in the main tree, creating a diff that way using individual file patches. These small patches can be concatenated together when distributed to project maintainers. When doing it this way, you should be careful to run diff from the root directory of the source tree, so that the patch will be able to figure out later where the changed files were, especially if they were in different directories.

Conclusion

Much of the information here can be extracted from info pages and common sense. However, I hope that by documenting my own experiences with open-source projects and patches, I can encourage that small percentage of you who have the skill and desire to program the kernel, but have not chosen to do so. In the future, I hope to cover some of the other facets of kernel development that may be turning developers away, and I would be interested in hearing the reasons you might be nervous about lending your brain to some of these projects. Until then, happy hacking.

email: jpranevich@lycos.com

Joseph Pranevich (jpranevich@lycos.com) is an avid Linux geek, and while not working for Lycos, enjoys writing (all kinds) and working with a number of open-source projects.

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