Re: [RFC 2/2] PM / OPP: extend DT parsing to allow voltage ranges

[Lucas Stach <l.stach@pengutronix.de>]

Am Dienstag, den 20.05.2014, 09:32 -0500 schrieb Nishanth Menon:
> On 05/20/2014 09:27 AM, Lucas Stach wrote:
> > Following the introduction of voltage ranges into OPP
> > we need a way to encode them in the device tree in a
> > similar fashion to the non-ranged versions.
> > 
> > To keep compatibility with old DTs the parsing function
> > is changed to understand both versions.
> > 
> > Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
> > ---
> >  Documentation/devicetree/bindings/power/opp.txt | 23 ++++++++++++++++++
> >  drivers/base/power/opp.c                        | 31 ++++++++++++++++++-------
> >  2 files changed, 46 insertions(+), 8 deletions(-)
> > 
> > diff --git a/Documentation/devicetree/bindings/power/opp.txt b/Documentation/devicetree/bindings/power/opp.txt
> > index 74499e5033fc..5b520ff321f5 100644
> > --- a/Documentation/devicetree/bindings/power/opp.txt
> > +++ b/Documentation/devicetree/bindings/power/opp.txt
> > @@ -10,6 +10,16 @@ Properties:
> >  	freq: clock frequency in kHz
> >  	vol: voltage in microvolt
> >  
> > +or
> > +
> > +- operating-points-range: An array of 4-tuple items, each item consisting
> > +  of a frequency and a related voltage range in the following form:
> > +  <freq min-vol-uV nom-vol-uV max-vol-uV>
> > +	freq: clock frequency in kHz
> > +	min-vol-uV: absolute minimum required voltage for this frequency
> > +	nom-vol-uV: nominal voltage for this frequency
> > +	max-vol-uV: absolute maximum allowed voltage for this frequency
> 
> And, Why cant we function at min-volt-uV? because PMIC cannot support
> it? then why add voltage tolerance? This is not clear in the dt
> description.
> 
The min-vol-uV is meant as the absolute minimum voltage where the device
is still able to work.
But still vendors are giving nominal voltages that should be met if
possible. So for example for a CPU device we might always want to try to
match the nominal voltage, but in case the regulator isn't able to
supply a this voltage it's still ok to use a voltage in the
min...nominal range.

Another case may be where we are scaling CPU voltage and have to
maintain a certain voltage difference between CPU and SoC power domain.
So there may be cases where we might decide that it's better (faster) to
scale CPU only to it's min voltage to avoid touching the SoC regulator.
> 
> 
> > +
> >  Examples:
> >  
> >  cpu@0 {
> > @@ -23,3 +33,16 @@ cpu@0 {
> >  		198000  850000
> >  	>;
> >  };
> > +
> > +cpu@0 {
> > +	compatible = "arm,cortex-a8";
> > +	reg = <0x0>;
> > +	operating-points-range = <
> > +		/*  kHz min(uV) nom(uV) max(uV) */
> > +		 166666  850000  900000 1400000
> > +		 400000  900000  950000 1400000
> > +		 800000 1050000 1100000 1400000
> > +		1000000 1200000 1250000 1400000
> > +		1200000 1300000 1350000 1400000
> > +	>;
> > +};
> 
> [...]
> 

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