It looks like even though some NVIDIA MCPs have min/max channel of 2/8, not all HW supports the intermediate # channels (4 and 6) over HDMI. Various combinations are supported on various HW: 2, 2/8, 2/6/8, 2/4/6/8.
At present, when an application uses an unsupported number of channels, playback appears to operate correctly, but the HW doesn't actually send the audio data over HDMI.
Is it possible for patch_nvhdmi.c to simply program codec the HW in 8-channel mode even though the controller is only sending 4-/6-channel data? I'm not sure if this would cause the codec to get out of sync with the controller's data stream. Would the controller end up grabbing 8 channels worth of data from the stream at a time, have no way to synchronize to the start of each sample, and hence end up packing e.g. 2 complete 4 channel samples into a single 8 channel sample?
If not, it seems that patch_nvhdmi.c should be modified so that each codec's _open() function returns an error for unsupported rates. I can code that up if we need.
Unfortunately, it doesn't look like it's possible for applications to query this information from ALSA, since a hw_params_t exposes just a min/max channel count rather than a mask.