hdaa driver documentation

I figured out how to get sound working on my MacPro5,1 (mid 2010). Here's that thread. Basically:
sudo vi /etc/sysctl.conf
dev.hdaa.0.gpio_config="0=set 1=set"
Part of this makes sense (which unit is the default), and part of it is magic(0=set 1=set). I gather this is turning on the first two gpio pins, but why this works is still pretty mysterious. I read in another thread here, about using hints such as:
and like any script-kiddie, I'm willing to give it a go, but I don't really have a clue as to what this does or how. I will gladly try it and if it works, use it, but I'd like to get a clue. Hence my latest question - where's the documentation, or put another way, how can I figure out what hints and what sysctls are applicable to my hardware?
A personal example/experience: as you already know, mixer(1) has lots of different options and you couldn't find any useful explanation about them in the man page. Options like vol, bass, treble, synth, pcm, speaker, line, mic, cd, mix, pcm2. For example I couldn't understand what is the difference between pcm and vol. Finally I've started to read its related source on /usr/src to get a clue what is going on, and I found some helpful hints. I can't remember exaclty where I was looking for, but you get the idea. I was looking into different places and reading different files such as: /usr/src/sys/sys/soundcard.h and /usr/src/usr.sbin/mixer/mixer.c so forth and so on.
[EDIT] using a combination of different tools for searching grep(1), sed(1), find(1), etc.