[RFC PATCH 1/6] ALSA: compress: add Sample Rate Converter codec support

[Pierre-Louis Bossart]

> Why I use the metadata ioctl is because the ALSA controls are binding
> to the sound card.  What I want is the controls can be bound to
> snd_compr_stream, because the ASRC compress sound card can
> support multi instances ( the ASRC can support multi conversion in
> parallel).   The ALSA controls can't be used for this case,  the only
> choice in current compress API is metadata ioctl. 

I don't know if there is really a technical limitation for this, this is
for Jaroslav to comment. I am not sure why it would be a problem to e.g.
have a volume control prior to an encoder or after a decoder.

> And metadata
> ioctl can be called many times which can meet the ratio modifier
> requirement (ratio may be drift on the fly)

Interesting, that's yet another way of handling the drift with userspace
modifying the ratio dynamically. That's different to what I've seen before.

> And compress API uses codec as the unit for capability query and
> parameter setting,  So I think need to define "SND_AUDIOCODEC_SRC'
> and 'struct snd_dec_src',  for the 'snd_dec_src' just defined output
> format and output rate, channels definition just reuse the snd_codec.ch_in.

The capability query is an interesting point as well, it's not clear how
to expose to userspace what this specific implementation can do, while
at the same time *requiring* userpace to update the ratio dynamically.
For something like this to work, userspace needs to have pre-existing
information on how the SRC works.