[alsa-devel] Exposing the ThinkPad HW mute switch to ALSA?

[Takashi Iwai]

At Tue, 7 Oct 2014 07:44:35 -0700,
Andy Lutomirski wrote:
> 
> On Oct 7, 2014 6:20 AM, "Henrique de Moraes Holschuh" <hmh at hmh.eng.br> wrote:
> >
> > On Tue, 07 Oct 2014, Takashi Iwai wrote:
> > > At Tue, 7 Oct 2014 17:25:13 +1100,
> > > Grant Diffey wrote:
> > > > so I have a really dumb question.
> > > >
> > > > Why can't the controls be mapped as HCI rather than a soundcard? given
> > > > there's no audio production capability here only a control capability.
> > > >
> > > > Also this seems to be how they end up functioning on a modern system they
> > > > get bound to pulse which remaps them to whatever's 'active'
> > > >
> > > > Why does everyone say that this is a soundcard when it's really a keyboard
> > > > (from my pov)
> > >
> > > thinkpad_acpi actually mutes the sound on some models, at least the
> > > old ones, independently from the sound chip mute state.  That's why it
> > > was implemented as the sound card object initially.
> >
> > That's how the hardware works, really.  One AC97 audio channel is routed to
> > a digital volume chip, and from there to the console headphone jack, and to
> > the built-in speakers.  line-out is not routed through this chip, and AFAIK
> > MIC/line-in is not routed through either this chip _or_ a separate mute
> > gate, either.
> >
> > The EC controls this digital volume chip directly, it is independent from
> > the AC97 mixer.  The default behaviour cannot be changed in earlier
> > thinkpads, and trying to change it in later thinkpads (they're supposed to
> > have a legacy mode that is the same as the older models) doesn't work very
> > well.
> >
> > Line-out and MIC are controlled only by the AC97 mixer in the older models
> > with digital volume control.
> >
> > In newer-but-not-newest thinkpads, there are two mute gates, one for MIC,
> > and another for the speakers/headphones.  The EC controls those.  All volume
> > control is in the HDA mixer.
> >
> > In the newest thinkpads, I think there's only a MIC mute gate controlled by
> > the EC, and everything else is in the HDA mixer.
> >
> > And depeding on the EC operating mode and thinkpad model, the volume hotkeys
> > might interact with the mute state (unmuting on first press when it is
> > mute).
> >
> 
> On newish thinkpads (at least X60-X220, and I think this works on the
> latest models as well), we can disable all of this and make the mute
> button be a regular button if we want.  Getting the mute LED to DTRT
> might be tricky, but modern userspace can probably do it.
> 
> This is almost certainly the approach that does something sensible
> with the least code written.  We would just check for HAUM/SAUM on
> startup and, if available, set SAUM to "software control only",
> force-unmute the mute circuit, force the volume to full, and disable
> the fake soundcard.

That sounds sensible.  But, we'll still need to handle the mute LED,
like we do for the mic-mute LED now?


Takashi