[alsa-devel] [PATCH] ASoC: ssm2602: Fix ADC powerup sequencing

[Mark Brown]

On Fri, May 25, 2018 at 05:18:09PM +0200, Philipp Zabel wrote:
> On Fri, 2018-05-25 at 15:52 +0100, Mark Brown wrote:
> > On Fri, May 25, 2018 at 01:42:53PM +0200, Marco Felsch wrote:

> > > Also the formula for the delay time (t = C × 25,000/3.5) depends only on
> > > the capacity size.

> > Why not just have the user specify the capacitance of the capacitor on
> > the rail which they can directly read from the schematic rather than
> > forcing them to do the calcualtion?  That seems a bit clearer and more
> > user friendly (plus if someone decides the spec was wrong it's easier to
> > roll out fixes).

> The exact capacitance may not be known or vary above the nominal value
> because of cheap components, and the formula from the datasheet is just
> a guideline.

That variability is going to apply just as much to the charge time
calculations/measurements as it is to the initial capacitance value -
the results are going to be very much garbage in, garbage out.

> I'd expect the usual method to set this delay to be semi-empirical:
> "start from the value calculated from datasheet and schematics and then
> increase until no more audio artifacts on a representative sample of
> boards".
> I think it is be better to specify a delay that works than a bogus
> capacitance value that happens to correspond to a delay that works.

If this is varying so drastically per board/system that it's relevant
then we're already into problematic territory.  For most devices we just
have a number for the part, not something that varies so wildly that
each system needs to configure it.
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