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To: 9front@9front.org
From: "chötrin." <chotrin@posteo.net>
Date: Sun, 25 Apr 2021 22:37:53 +0000
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Subject: Re: [9front] Getting Started with Kernel Development?
Reply-To: 9front@9front.org
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Quoth cinap_lenrek@felloff.net:
> As far as i know, the stock raspberry pi is not using I2S.

I've run Raspbian on another RPi and used the onboard sound with it,
but not sure about how it's handled under the hood.  I *suspect* it's
using I2S.  At least, that's what it looks like from this ALSA driver
source:

https://github.com/raspberrypi/linux/blob/rpi-5.10.y/sound/soc/bcm/bcm283=
5-i2s.c

There's some discussion on the RPi forums that hints at how the
onboard sound works:

https://www.raspberrypi.org/forums/viewtopic.php?t=3D59823

And the BCM spec I mentioned is here (warning: huge PDF):

https://cs140e.sergio.bz/docs/BCM2837-ARM-Peripherals.pdf

The relevant parts start on page 119.  Apparently the register base
address moved between BCM2835 and BCM2837.

Anyway, this may be a fool's errand to get working-- you could very
well be correct that they're doing some really janky stuff between the
Broadcom chip and the audio out pins.  I haven't dug deep enough to
stare at any schematics or anything like that yet.

Still, I'd like to give it a go.  I'd like to dig into the guts of
Plan 9 and I'd like to get audio working without tacking on an
external DAC (although, that'd probably sound WAY better).

I'm *thinking* that I'll use some external thumbdrive or be dropping
stuff into the RPi's FAT partition depending on how accessible the
bitbanging is from userspace.  I'm new to Plan 9, so I'm probably
pretty far off the mark.

Quoth cinap_lenrek@felloff.net:
> The GPU has firmware code to drive that GPIO pin and also send
> audio over HDMI, but todo that you have to talk to the videocore
> firmware thru a complicated software media streaming interface.

Oh yeah, I saw that, also.  I think I'll pass on trying to do that for
the time being.  :^)

ch=C3=B6trin.