Reverse engineering GPL code

Suppose you want to reverse-engineer a GPL'd driver whose original documentation is , as usual, missing. Comments are rather scarce and useless. How do you proceed if your goal is to obtain an equivalent working driver not covered by the GPL?

You said it's a driver, so it works with hardware.

Just write your code to do the same thing with the hardware, but with different code. (If it's GNU bloat, it should be really easy to do this.)