Linux Device Drivers, Second Edition

Initialization and Shutdown

As already mentioned, init_module registers any facility offered by the module. By facility, we mean a new functionality, be it a whole driver or a new software abstraction, that can be accessed by an application.

Modules can register many different types of facilities; for each facility, there is a specific kernel function that accomplishes this registration. The arguments passed to the kernel registration functions are usually a pointer to a data structure describing the new facility and the name of the facility being registered. The data structure usually embeds pointers to module functions, which is how functions in the module body get called.

The items that can be registered exceed the list of device types mentioned in Chapter 1. They include serial ports, miscellaneous devices, /proc files, executable domains, and line disciplines. Many of those registrable items support functions that aren’t directly related to hardware but remain in the “software abstractions” field. Those items can be registered because they are integrated into the driver’s functionality anyway (like /proc files and line disciplines for example).

There are other facilities that can be registered as add-ons for certain drivers, but their use is so specific that it’s not worth talking about them; they use the stacking technique, as described earlier in Section 2.3. If you want to probe further, you can grep for EXPORT_SYMBOL in the kernel sources and find the entry ...