[thedance]

You write the software in such a way that instead of just reading and writing registers or memory you exercise some set of functions. In normal operation you pass the driver a set of real functions that read and write real registers. In testing you pass functions that do other things. This makes it quite easy to exercise the features of the hardware that are rarely seen in the wild. For example most IO adapters and NICs have some kind of signal that they are overheating. Most Linux drivers simply ignore or malfunction when these conditions are raised, because the author of the driver never got a chance to manually exercise that feature.

This is basic design for unit testing but it's impossible in Linux because Linux lacks a zero-cost abstraction that would let you mock out a device. C only has costly abstractions such as tables of function pointers.

[vbezhenar]

You can compile object file in isolation and provide mocked implementations for all imported functions.