[alsa-devel] How to correct params_format on DPCM ?

[Kuninori Morimoto]

Hi Mark

> > 	rsnd:   SNDRV_PCM_FMTBIT_S24_LE | SNDRV_PCM_FMTBIT_S16_LE
> > 	ak4642: SNDRV_PCM_FMTBIT_S16_LE
(snip)
> I'm not sure what exactly was intended here but this seems to need
> fixing in the core - I think the assumption in the DPCM code is that
> the sort of hardware that uses it will have rewriting support that makes
> the back end constraints irrelevant to the front end.  I think what we
> want here is some way to signal to the framework that the front end
> constraints aren't used and it should use the back end ones instead -
> perhaps just omit the constraints that aren't rewritten and then have
> the framework take those from the backend instead.

Now I'm debuging about this issue, but it is very complex.
I need your help. where should I check/debug ?

My problem is...

In non-DPCM case

cpu  : rsnd  : SNDRV_PCM_FMTBIT_S16_LE | SNDRV_PCM_FMTBIT_S24_LE
codec: ak4642: SNDRV_PCM_FMTBIT_S16_LE
-----------------------------------------
 SNDRV_PCM_FMTBIT_S16_LE is used, this is OK

In DPCM case

FE cpu  : rsnd  : SNDRV_PCM_FMTBIT_S16_LE | SNDRV_PCM_FMTBIT_S24_LE
FE codec: dummy : SNDRV_PCM_FMTBIT_S16_LE | SNDRV_PCM_FMTBIT_S24_LE | ...

BE cpu  : dummy : SNDRV_PCM_FMTBIT_S16_LE | SNDRV_PCM_FMTBIT_S24_LE | ...
BE codec: ak4642: SNDRV_PCM_FMTBIT_S16_LE

SNDRV_PCM_FMTBIT_S24_LE is used on FE,
SNDRV_PCM_FMTBIT_S16_LE is used on BE.
this means FE send S24 data, but BE can't use it.

Which function/method controls format ?
I guess it is doning some kind of

	format = cpu->formats & codec->formats;

DPCM case need to care about FE/BE, and maybe this is lost item.

	format = fe_cpu->formats & fe_codec->formats &
	         be_cpu->formats & be_codec->formats;

I guess it is done before hw_param() (?),
or snd_soc_instantiate_card() timing. but I couldn't find it.
It is very confusable/complex. which function is doing this ?

Best regards
---
Kuninori Morimoto