Re: [PATCH] ALSA: hda: Refactor calculating SDnFMT according to specification

[Takashi Iwai <tiwai@suse.de>]

On Mon, 24 Aug 2020 14:16:26 +0200,
Kai Vehmanen wrote:
> 
> Hey,
> 
> On Mon, 24 Aug 2020, Pawel Harlozinski wrote:
> 
> > Set SDnFMT depending on which format was given, as maxbps only describes container size.
> 
> hmm, I'm not entirely sure that is correct. Usage may be a bit varied, but 
> most places in existing code, "maxbps" is treated as number of significant 
> bits, not the container size. E.g. in hdac_hda.c:
> 
> »       if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK)
> »       »       maxbps = dai->driver->playback.sig_bits;
> »       else
> »       »       maxbps = dai->driver->capture.sig_bits;
> 
> It would seem "maxbps" is a bit superfluous given the same information can 
> be relayed in "format" as well. But currently it's still used. E.g. if you 
> look at snd_hdac_query_supported_pcm(), if codec reports 24bit support, 
> format is always set to SNDRV_PCM_FMTBIT_S32_LE, even if only 24bit are 
> valid. So snd_pcm_format_width() will not return the expected significant 
> bits info, but you have to use "maxbps". So original code seems correct 
> (or at least you'd need to update both places).

Hm, we need to check the call pattern, then.  The maxbps passed to
this function was supposed to be the value obtained from
snd_hdac_query_supported_pcm(), i.e. the codec capability.

But, basically this patch wouldn't change any practical behavior.  In
the current code, snd_pcm_format_width() can be never 20 or 24,
because the 24 and 24bit supports are also with SNDRV_PCM_FMT_S32_LE.
That is, the cases 20 and 24 there are superfluous from the
beginning (although the checks of maxbps are still needed).

Instead, what we could improve is:
- Set up the proper msbits hw_constraint to reflect the maxbps value
- Choose the right AC_FMT_BITS_* depending on the hw_params msbits

We may change the query function not to return a single maxbps value
but rather storing the raw PCM parameter value (AC_SUPPCM_*), and pass
it at re-encoding the format value, too, if we want to make things
perfect.


thanks,

Takashi