ADC / DAC support for ARM SBCs

[<dhop%nwlink.com@localhost>]

Hi all,

I am interested in getting a DAC/ADC combo to work (full-duplex) with NetBSD
on ARM (RPi and Beaglebone Black, I've got a Boundary Nitrogen8M MINI on the
way).  I have the AudioInjector (wm8371), PiSound (PCM5102A / PCM1804) and
HiFiBerry DAC+ADC (PCM5122 / PCM1861) hats and the Bela cape for BBB.  USB
Class 1 would be a path forward but there are currently issues with the USB
audio stack on NetBSD that are a showstopper at the moment, and I'd like to
try a more performant bus path to dial down the latencies.

I'm coming in cold to this so bear with me and please correct any
misconceptions I've got, but I understand the GPIO is not an autoconfigure
bus in the sense that we can run MI drivers against it, is that right?  The
reason I believe this is because NetBSD has a wm8371 driver as
machine-dependent code within the Zaurus tree, and it's on a Zaurus GPIO
bus.

If I wanted to get the AudioInjector hat working, for example, I imagine
what I would need to do is port the Zaurus wm8371 driver over to the
Raspberry Pi-- in which case it would become just another machine dependent
driver for that platform.  GPIOs aren't standard pinouts and are
machine-dependent and variable even between platform iterations (viz. the
RPi UART / Bluetooth shenanigans), so it stands to reason there's no "MI
GPIO" bus per se, in the sense of PCI or Zorro III-- except that there *is*
one, and that's what I'm not clear about.

Is any of this correct, or am I way off?  Any good pointers for resources as
I begin my journey into NetBSD device driver hacking?

Thanks tons,
Dave