Messages in this thread | | | Subject | Re: [alsa-devel] [PATCH] ASoC: bdw-rt5677: channel constraint support | From | Pierre-Louis Bossart <> | Date | Thu, 12 Sep 2019 08:06:35 -0500 |
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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.
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