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

[Takashi Iwai]

At Tue, 31 Aug 2010 08:41:53 -0700,
Stephen Warren wrote:
> 
> Takashi Iwai wrote:
> > 
> > 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?
> 
> Oops, yes.
> 
> Do you think that's the correct fix?
> 
> If the driver errors out the open, will a plug: (or other non hw:) ALSA device
> dynamically "upsample" a 4-channel stream to the next supported (8-channel), or
> will the user application have to handle this?

alsa-lib can do make working (e.g. via upmix plugin).  But other audio
backend can do it by itself.
IMO, telling the truth is a better approach.  It is 4-channel, after
all.

> > > 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.
> 
> Are you saying the APIs already support a mask for channel count, and I just
> didn't find it, or that the APIUs should be updated to report a mask?

You can pass the list as hw_constraint.  Look for drivers using
struct snd_pcm_hw_constraint_list.


thanks,

Takashi