Linux Device Drivers, Second Edition

Chapter 11. kmod and Advanced Modularization

In this second part of the book, we discuss more advanced topics than we’ve seen up to now. Once again, we start with modularization.

The introduction to modularization in Chapter 2 was only part of the story; the kernel and the modutils package support some advanced features that are more complex than we needed earlier to get a basic driver up and running. The features that we talk about in this chapter include the kmod process and version support inside modules (a facility meant to save you from recompiling your modules each time you upgrade your kernel). We also touch on how to run user-space helper programs from within kernel code.

The implementation of demand loading of modules has changed significantly over time. This chapter discusses the 2.4 implementation, as usual. The sample code works, as far as possible, on the 2.0 and 2.2 kernels as well; we cover the differences at the end of the chapter.

Loading Modules on Demand

To make it easier for users to load and unload modules, to avoid wasting kernel memory by keeping drivers in core when they are not in use, and to allow the creation of “generic” kernels that can support a wide variety of hardware, Linux offers support for automatic loading and unloading of modules. To exploit this feature, you need to enable kmod support when you configure the kernel before you compile it; most kernels from distributors come with kmod enabled. This ability to request additional modules ...