Re: [PATCH v4 2/4] ASoC: qcom: sc8280xp: limit speaker volumes

[Johan Hovold <johan@xxxxxxxxxx>]

On Fri, Jan 19, 2024 at 12:24:18PM +0100, Johan Hovold wrote:
> The UCM configuration for the Lenovo ThinkPad X13s has up until now
> been setting the speaker PA volume to the minimum -3 dB when enabling
> the speakers, but this does not prevent the user from increasing the
> volume further.
> 
> Limit the digital gain and PA volumes to a combined -3 dB in the machine
> driver to reduce the risk of speaker damage until we have active speaker
> protection in place (or higher safe levels have been established).
> 
> Note that the PA volume limit cannot be set lower than 0 dB or
> PulseAudio gets confused when the first 16 levels all map to -3 dB.

I tracked the down the root cause for this, which appears to be a bug
(feature) in pulseaudio that causes it to reject the dB range if the
maximum is negative:

	https://gitlab.freedesktop.org/pulseaudio/pulseaudio/-/merge_requests/447

This happened to work with v3 which limited the PA volume to the single
lowest setting, but would similarly break if anyone wants to set a -1.5
dB limit.

Johan