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

[Pierre-Louis Bossart]

> > I also don't see any case where we support 4 channels in any broadwell
> > machine driver?
> It's the bdw-rt5650.c which only exists in chrome's 3.14 branch supporting Buddy
> project. They submitted the machine driver but not yet merged.
>
> https://patchwork.kernel.org/patch/11050985/
>
> > So again can you point me to an issue or existing backport that requires the
> > patch below. Not trying to be obtuse but we should only change older
> > platforms when there is evidence that a change is needed.
> 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.