Issues using simple-audio-card driver with Xilinx Audio Formatter

[Robert Hancock]

On Fri, 2021-07-09 at 19:02 +0100, Mark Brown wrote:
> On Fri, Jul 09, 2021 at 05:05:47PM +0000, Robert Hancock wrote:
> > On Fri, 2021-07-09 at 13:38 +0100, Mark Brown wrote:
> > > For something like this I think the driver should be able to figure out
> > > the ratio based on the configured MCLK and sample rate.  For the most
> > > part set_clkdiv() should be a legacy thing, it's very manual and hard to
> > > see why a system would do something different to the obvious ratio
> > > usually.
> > Possibly the I2S transmitter should be implementing set_sysclk rather than
> > set_clkdiv then? simple-audio-card seems like it would already propagate
> > that
> > through into the CPU DAI in asoc_simple_hw_params and then it could figure
> > out
> > the right divider value to use.
> 
> I think so.
> 
> > The tricky part is that the Audio Formatter (used as the "plat" component
> > here)
> > also needs to know what the mclk-fs value is. (I really don't know why it
> > cares, I would think it would just push data to the output stream as fast
> > as it
> > was accepted, but indeed it does have a register to set the sample rate to
> > MCLK
> > divider, and if it's not set properly the I2S transmitter downstream seems
> > to
> > constantly get underruns.) I'm not sure if there's any mechanism for it to
> > determine the value right now, or if something new would need to be added?
> 
> Given that it knows the MCLK if set_sysclk() is used and it knows the
> sample rate it should just be able to calculate the ratio?

I see that snd_soc_component_driver has a set_sysclk callback as well, so that
allows the formatter to handle setting the divider. However, right now with
simple-audio-card that callback is not being invoked on the formatter, though
it is on the I2S transmitter.

I'm thinking something needs to be added to asoc_simple_hw_params to call
snd_soc_component_set_sysclk on the platform component(s) like it calls
snd_soc_dai_set_sysclk for the codec DAI and CPU DAI.

Not sure exactly how that should be done though - we could use 
for_each_rtd_components to iterate through all of the components and call
snd_soc_component_set_sysclk on all of them, though that would also potentially
duplicate some settings already done by the snd_soc_dai_set_sysclk calls on the
CPU and codec DAIs. I'm not sure if that really hurts anything though?

-- 
Robert Hancock
Senior Hardware Designer, Calian Advanced Technologies
www.calian.com