If that were the Windows source, you wouldn't have been able to find it. Parts of the "driver" would be found in seventeen different systems, using seventeen different styles of coding, using macros that look the same but act differently in different files, leaving the driver as nothing more than a side-effect of the whole.

... I might not miss that codebase.

I was working with network drivers and the core network stack. I've heard bad things about questionable device driver implementations, I could only imagine what you saw.

I did an internship where I had to write fake network devices to test some server platform firmware configuration. I spent a bunch of time digging around the core PCIe code and Linux network stack. The first few weeks were a bit overwhelming but by the end of the internship I was really impressed with how clean and relatively understandable the interactions between userland, kernel space, and hardware operations were. It really made me aware about the importance of deliberately designed API surfaces that are critical to millions of users.

I also enjoy C programming, and it was interesting to see how the developers built out such a complex system without the more friendly programming language structures that I was first exposed to in school (mostly C++ and Java).