I'm working on a kernel-level fixup for a set of hardware which uses a separate DAC for the "headphones" output. So far, I've gotten everything but the volume control working correctly. "PCM Playback Volume" controls the headphones volume since the same digital output drives both SPDIF and the analog headphones jack.
So, "PCM Playback Volume" should always equal "Master Playback Volume" when headphones are plugged in. (You can't insert both an SPDIF and headphones at the same time anyway, since they use the same jack, so there's no downside there.)
Here's where I could really use some help from people who know ALSA plumbing better than I do. It seems to me this should be possible by overriding the "Master Playback Volume" put function with one that finds the "PCM Playback Volume" control and sets the volume for both controls.
For some reason, I can't seem to access the PCM control from my master put function. Attempts to find it via snd_ctl_find_id or snd_ctl_find_numid result in kernel oops. There might be something simple I'm missing, or perhaps there is a better way to link the two volume controls?