[alsa-devel] Missing 4-channel support on older NVIDIA MCP HW

[Takashi Iwai]

At Mon, 30 Aug 2010 10:11:09 -0700,
Stephen Warren wrote:
> 
> 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 whether HD-audio codec can have a /dev/zero-kind of stream.
It's possible to set up but I don't think it's worth.
It'd be far easier to omit 4 and 6-channels for such a case.

> 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.

Do you mean for unsupported channels?

> 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.

It can be also masked.


Takashi