[alsa-devel] Adding support for Android's Wired Audio Headset Specification v1.1

[Mads Lønsethagen]

Posting this to alsa-devel without being actually sure it belongs here, 
so please do tell if it belongs elsewhere. But here goes:

If you have e.g. a Bose QC25 headset (wired, not bluetooth) in Linux, 
you can't get the volume buttons nor the play/pause button on the 
headphone cable itself to work. As far as I know, these buttons works in 
both Android and Windows 10, so it would be nice if they would work with 
plain Linux too.

This StackExchange thread discusses how it works: 
https://electronics.stackexchange.com/questions/242883/how-do-media-buttons-on-a-plain-wired-headset-work

As one poster on StackExchange mentions, Android has a defined standard 
on these kind of buttons: 
https://source.android.com/devices/accessories/headset/plug-headset-spec

Since these kind of button presses must be decoded from impedance 
changes in the 3.5mm jack connector, does it belong in  alsa somewhere? 
Bose QC25 buttons works on Intel HDA cards on new Dell XPS laptops, so 
I'm guessing it should be possible within snd-hda-intel...

- Mads