[Sound-open-firmware] mixer and formats question

[Pierre-Louis Bossart]

Hi,
we have a rather odd issue [1] with mixer/formats, where somehow an 
error is thrown by firmware when we play an S24/32_LE PCM stream through 
a mixer and into a DAI configured in S16_LE (because of codec restrictions)

I could use feedback on the following points:

a) was it really a design decision to assume that all sources to the 
mixer share the same format?

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.

c) if no, same question "how is the format of the mixer defined"? we 
would need to have the ability to set a format for the mixer so that it 
can convert on the fly, but we don't seem to set formats anywhere except 
for PCM streams and DAIs.

Thanks,
-Pierre

[1] https://github.com/thesofproject/sof/pull/3341