[pulseaudio-discuss] Dealing with the ThinkPad hardware mixer

[Andrew Lutomirski]

Most ThinkPad laptops have an extra mixer that has nothing to do with
the HDA audio hardware.  The kernel can talk to it through the
embedded controller and, if it's muted, then there's no sound even if
the normal controls are all set on.

These laptops have three special buttons.  The mute button either
mutes the mixer or toggles the mute state.  The volume up button
unmutes the mixer (if muted) and sends KEY_VOLUMEUP.  The volume down
button unmutes the mixer and sends KEY_VOLUMEDOWN.

On very new models, there's a hardware mute light that indicates the
status of the hardware mute.

The thinkpad-acpi driver currently exposes an ALSA mixer and the
alsamixer utility can talk to it.  But PulseAudio ignores it
completely.

What is the kernel supposed to do to get PulseAudio to DTRT?  On some,
but not (I think) all, models, we can disable hardware mixer control
and make all of the buttons act like ordinary buttons, but that will
cause the light (if present) to malfunction, and I don't know if all
models can do this.  So as it stands, PulseAudio would have to
understand that the mixer is special and watch for ALSA change
notifications.

I'm willing to change the thinkpad-acpi driver to make it work better,
but I know nothing at all about PulseAudio internals.  What should I
do?

(There's an added complexity.  Really old models have mute and volume.
 I don't own one, so I don't know much about how they work.)

--Andy