[Sound-open-firmware] mixer and formats question

[Liam Girdwood]

On Wed, 2020-08-26 at 10:38 -0500, Pierre-Louis Bossart wrote:
> > Yes. Mixer only mixes same format today, however the PCM converter
> > can
> > be invoked here on the pipeline (but I dont think the mixer does
> > this
> > today).
> > 
> > > b) if yes, how is the format for a mixer defined? We typically
> > > convert
> > > using volume elements, but somehow a mixer would need to be set-
> > > up
> > > with
> > > a predefined format (i.e. not propagated by hw_params()) if we
> > > want
> > > the
> > > volume elements to know when/what to convert. I see in my
> > > experiments
> > > that the format for the mixer sources and sinks seems to be
> > > changed
> > > dynamically, which doesn't seem quite right to me.
> > > 
> > 
> > It could be the PCM converter being invoked by host or dai
> > component
> > depending on which PCM is opened in which order etc ?.
> 
> Yes, the volume can convert, but the question is to what?

I think this is no longer true. Volume used to convert but I think this
was moved to pcm_converter.c 

> 
> take a topology like this
> 
> PCM1 (S32_LE) -> volume1----v
>                              --- mixer (S??) -- volume -- DAI
> (S16_LE)
> PCM2 (S16_LE) -> volume2 -- ^
> 
> currently we only set the formats for the PCM devices and the DAIs.
> It's my understanding that the format used by the mixer would be 
> propagated in the hw_params stage.
> 
> That's really bad! This would mean the mixer format would be
> different 
> depending on which of PCM1 or PCM2 is used first.
> 
Yes, today first user sets the format.

> I think if we want a consistent result we *have* to define the
> format 
> used by the mixer with e.g. a token passed to firmware with the
> config IPC.
> 
> A mux is probably similar btw.
> 

Yes, that's also the intention of topology 2. Each state of the
pipeline can have a predefined immutable format/rate/etc.

Liam