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 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.
Regards, Brent