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, 11:09 -0500 schrieb Nishanth Menon:
> On Tue, May 20, 2014 at 10:48 AM, Lucas Stach <l.stach@pengutronix.de> wrote:
> > Am Dienstag, den 20.05.2014, 10:29 -0500 schrieb Nishanth Menon:
> >> On Tue, May 20, 2014 at 10:23 AM, Nishanth Menon <nm@ti.com> wrote:
> 
> [...]
> >> If you need to handle PMIC accuracy, use voltage-tolerance.
> >>
> > And that's exactly one of the issues of the current binding, the
> > tolerance isn't stable, but rather per OPP.
> >
> > As an example look at two operating points of the i.MX5:
> >
> > /*  kHz min(uV) nom(uV) max(uV) */
> >  166666  850000  900000 1400000
> > 1200000 1300000 1350000 1400000
> >
> > So for the 1,2GHz OPP the voltage tolerance is just 3,7% as anything
> > bigger would exceed the absolute maximum rating, but for the 166MHz OPP
> > it is already 64,7%.
> >
> > I agree that I could just cap the rail at 1,4V and give a max voltage
> > equal to that as max_volt to the regulator API, but actually the device
> > doesn't even know it's tolerance as it has no max defined. It makes
> > having a max_volt in the regulator API kind of senseless, as connected
> > devices don't know their maximum rating.
> 
> Is'nt that(voltage-tolerance binding) a different problem that needs a
> different solution rather than creating a new binding for OPP - which
> actually makes no description sense (since the max is absolute max)?
> 
Ok, I agree that having a nominal voltage isn't really required, so this
leaves us with min...max, where min is already there in the OPP
framework.

The question here is: are there any devices out there where we could
imagine a frequency dependent max-voltage? I could very well image such
a thing for CPU devices, where thermal constraints would force us to
reduce the regulator voltage to allow for a higher frequency. If any
device on the same rail needs a higher voltage this would prevent using
the higher OPP.

I have never seen such a device in practice, so maybe just having a
global max-voltage property is the right thing. I'll have to think about
this.

> How can we address that? should we add some logic that says:
> 
> if voltage tolerance == 0, then try min_uV to max_opp_uV?
> OR
> introduce a new binding that says absolute-max-voltage (this is what I
> was planning on doing with voltage domain btw).. and ensure that that
> is used?
> 
> Personally, I never liked voltage-tolerance binding precisely for the
> same reason as you bring up.. but that is a ABI now and we need to see
> if the requirements can match with it, or define a new one keeping the
> old ABI alive..
> 
I would like to keep the focus on new drivers using OPPs. How to fit in
drivers with existing bindings is to be defined later. I'm working on a
new cpufreq driver and would like to work out a solution that avoids
using this wobbly voltage-tolerance.

Regards,
Lucas
-- 
Pengutronix e.K.             | Lucas Stach                 |
Industrial Linux Solutions   | http://www.pengutronix.de/  |