After working through block devices, I ended up going even further afield: Network card drivers.
My, Donald Becker was busy here. His code is nice and readable. One fun side effect is it means that a significant proportion of the kernel is copyrighted by the US Govt., represented by the Director of the NSA. Take that, conspiracy theorists!
The 8390 was one popular chip in the '90s. I read the code associated with this particular chip, and then all the card drivers based around it (largely varying in autodetection and how the buffers are mapped). Once I understood the pattern, the various other non-8390 drivers made sense.
This is not the most glamarous of directories, but it makes a nice change from the filesystem-y stuff.