Re: [alsa-devel] [PATCH] ASoC: bdw-rt5677: channel constraint support

[Pierre-Louis Bossart]

On 9/12/19 1:00 AM, Lu, Brent wrote:
> > > The story is Chrome has a tool called alsa_conformance_test which runs
> > > capture or playback against a PCM port with all possible
> > > configurations (channel, format, rate) then measure if the sample rate
> > > is correct. Since the channel max number reported is 4, it tests the
> > > 4-channel 48K capture and reports the actual sample rate is 24000
> > > instead of 48000. That's the reason we want to add a constraint in
> > > machine driver to avoid user space programs trying to do 4 channel
> > recording since this machine does not support it in the beginning.
> >
> > ok, that helps get context, thanks for the details.
> >
> > I would have expected some error to be returned if there's a front-end
> > opened with 4 channels and the back-end only supports two. Adding the
> > constraint seems like a work-around to avoid dealing with the mismatch
> > between FE and BE. I don't understand DPCM enough to suggest an
> > alternative though. Ranjani, can you help on this one?
> >
> > And even if we agree with this solution, it'd be nice to apply it for the
> > Broadwell machine driver for consistency.
>
> It's not only the mismatch but also the design limitation. According to the
> information from google, the board (samus) only uses two microphone so
> 3 or 4 channel recording are not supported. That's the reason we leverage
> the constraint from other machine driver (like kbl_da7219_max98357a.c)
> to remove the 3 and 4 channel recording option.

The design limitation is already handled by the fact that the SSP 
operates in 2ch mode, so it's a different case from KBL where indeed the 
DMIC-based back-end can support 4 channels.

> The difference after the constraint is implemented is that the
> snd_pcm_hw_params_set_channels() function will return error (Invalid
> argument) when channel number is 3 or 4 so the application knows the
> configuration is not supported.

I get the error, I am just wondering if the fix is at the right 
location. I'll look into it, give me until tomorrow.