[Sound-open-firmware] mixer and formats question

[Liam Girdwood]

On Tue, 2020-08-25 at 17:57 -0500, Pierre-Louis Bossart wrote:
> 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?
> 

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 ?.

> 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.

Liam
> 
> Thanks,
> -Pierre
> 
> [1] https://github.com/thesofproject/sof/pull/3341
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