Matthew Garrett (mjg59) wrote,
Matthew Garrett

Thinkpad mixers

Older (up to the *60 series) Thinkpads have a slightly odd mixer setup. There's a mixer stage that sits between the software controllable mixer and the speakers. This is controlled by the firmware in response to the volume keys being hit and can't be prevented by the OS. This is unfortunate, since users are quite enthusiastic about seeing an on-screen display when they hit various keys and if we do the obvious thing and map the volume keys to KEY_VOLUMEUP and KEY_VOLUMEDOWN then they get the OSD but also get their software-controlled mixer changed at the same time as their firmware-controlled mixer. This causes boundless confusion.

Lorne Applebaum recently sent a patch that added an ALSA control to represent the Thinkpad mixer. This means that we can now read and control the Thinkpad-specific hardware without userspace needing any special knowledge. I've gone a little further and sent a followup patch that sends ALSA notifications on button presses. Lennart suggested that it would also be helpful to export dB values and wrote a nice app that measures them - that means that Pulseaudio will be able to set the mixer volume correctly by default.

The only missing part of the puzzle now is getting the OSD back. We still can't map the keys to KEY_VOLUMEUP and KEY_VOLUMEDOWN because that'll still result in two mixers being changed at once. We can't add new keys to indicate that it's a notification event rather than a command, since X can't deal with keycodes greater than 255 (actually 243, since it adds 8 to every kernel keycode for historical hilarity) and we've run out of space. We can't hook off the ALSA notifications because that'll result in the OSD popping up in response to someone dragging a slider in a volume control application. The easiest thing might be to add another ALSA notification that's only triggered if the notification was generated in the kernel and then have userspace listen for that.

Newer Thinkpads appear to have done away with this mixer setup entirely and just send standard volume events. This is a Good Thing.

In other news, I'm off on holiday to Australia for a couple of weeks so may be a bit slow in replying to things.
Tags: advogato, fedora

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