According to the github readme, the guide has been around since 2001 but it seems fairly up to date.. There are numerous discussion going on the issue tracker,

According to the github readme, the guide has been around since 2001 but it seems fairly up to date.. There are numerous discussion going on the issue tracker, about adding rust chapter, targeting linux 6.1, etc.. Very cool to see github issues and pull request actively used to keep a guide up to date. The guide will walk you through the basic of a kernel module. Exposing virtual files under /proc or /sys or messing around with system calls. It will fall short of helping you with the nitty gritty of writing an hardware driver though. The problem with writing drivers is that you are interfacing with a real-world machine for which you will have very little information about. Pick a device similar to the one you want to support, but that is already supported in linux. Go look at its source code. This will tell you far more about the work involved. You might realize that there is simply not enough information available to support the driver.