[Sound-open-firmware] mixer and formats question

[Guennadi Liakhovetski]

On Wed, Aug 26, 2020 at 09:18:57PM +0100, Liam Girdwood wrote:
> 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.

déjà vu :-)

This was one of the reasons to develop the media driver subsystem with 
the user-space subdevice API VIDEO_V4L2_SUBDEV_API and since many years 
audio wanted to move over to using the media framework too. This might 
be a long shot, but maybe we should consider that at some point, maybe 
even sooner than later.

Thanks
Guennadi

> > 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
> 
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