"relevant" is a very subjective term
If you're interested in the hardware, you may want to read up on the kernel module code. d_uart and d_analogue are two good ones. Always make sure you read the code in the AM1808* folders, the other one is for an emulator.
Please note that not all of this code is well documented but d_uart.c contains details about the serial protocol used between the EV3 and the sensor. d_analogue.c has the code that is used to do the auto-id stuff for the sensors. It's not light reading
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