Those are just drivers to stuff that runs its own cpu and interfaces over some kind of serial port. Printers are a well known example of this. Also intel wireless nics with their firmware blobs.
Not are drivers are like that. For instance, drivers/input/serio/ps2-gpio.c is all about timing the right signals.
You are missing the point. The kernel is still abstracting over those GPIO timings so programs don't need to know the timings themselves. This is the benefit of using a platform. These low level things get abstracted away from what you have to do.
Oh, I see. You're piecing together the layers much like a cake.
The way I see it, everything is tied together as some kind of flow chart where different elements have different jobs. Linux is quite a small part of the system when compared to Google Chrome. Even if you were to invert the cake, as a whole it still wouldn't make sense to me to see it that way.
Hardware tends to have more distinct layering than the lalala-land of software where pretty much anything goes.
Not are drivers are like that. For instance, drivers/input/serio/ps2-gpio.c is all about timing the right signals.