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