Asahi Linux speaker safety daemon

This is still very much a work in progress, is probably not "proper" Rust, and almost definitely makes competent developers extremely sad.

We currently rely on a local version of the alsa crate, pending the merge of bindings to snd_ctl_elem_value_{read,write} and snd_ctl_elem_set_id.

What works

• Parsing config file
• All borrows seem to work fine
• Volume getting/setting

Needs improvement

• Probably everything

Need to implement

• Daemonise and loop
• Threading (should probably make sure it works as intended first)
• Getting V/ISENSE (pending changes to the codec drivers, we have mock implementations)
• Actually fail safe (see below)

On failing safe

We need a way to guarantee safety on any fail condition. The TAS codecs have a safe mode which cuts all outputs down by 18 dB. This works out to being about half their full output capabiltiy. It might be worth having the macaudio driver start them explicitly in this mode, and only unlock full output capability with an IOCTL that can be sent by speakersafetyd when it's sure it has started correctly. We would then of course also need an IOCTL to do the opposite if we encounter a runtime error.

It was suggested by someone on IRC that this would be conducive to some sort of keepalive IOCTL, where the driver would automatically put the codecs into safe mode if it didn't hear from us for a while. This seems like it would suck to implement.

Like any SLA, it is likely that we will never be able to guarantee 100% safety for all nonstandard setups. The reference PipeWire DSP graph plus this should be enough for 99% of users, but I feel at some point those who insist on using Pulse or raw ALSA are just going to have to put up with a best effort service and accept the (small) risk of this failing.

Sundry

The alsa crate is Copyright (c) 2015-2021 David Henningsson, and other contributors. Redistributed under the MIT license.