linux-kernel - Re: [PATCH v4 4/4] regulator: Prevent falling too fast

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Date:   Mon, 12 Dec 2016 13:15:02 -0800
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Mark Brown <broonie@...nel.org>
Cc:     Doug Anderson <dianders@...omium.org>,
        Liam Girdwood <lgirdwood@...il.com>,
        Brian Norris <briannorris@...omium.org>,
        Javier Martinez Canillas <javier@...hile0.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>
Subject: Re: [PATCH v4 4/4] regulator: Prevent falling too fast

El Fri, Oct 28, 2016 at 07:15:21PM +0100 Mark Brown ha dit:

> On Mon, Sep 26, 2016 at 10:41:59AM -0700, Doug Anderson wrote:
> 
> > I guess I think of the whole network of components as the PWM
> > regulator and not the individual discreet BUCK.  I'm also not quite
> > sure how you would model it as you're asking.  I suppose you could say
> > that all of the resistors / capacitors / inductors end up producing a
> > voltage and this voltage is an input to the BUCK.  ...then the BUCK
> 
> Yes, that's what's happening.
> 
> > I know for sure that our EEs have massively modified the behavior of
> > the whole thing by just changing the resistors / capacitors /
> > inductors, changing the undershoot, OVP issue, voltage ranges, default
> > voltage, etc.  That's what leads me to believe it's not so separable.
> 
> What you're describing to me is a discrete DCDC that has an input
> voltage that sets the output voltage which happens to be set with a PWM.
> It's of course going to be the case that the passives are important to
> the system performance but it seems we have two bits here - the PWM
> regulator providing an input to the DCDC and the DCDC itself which is
> sensitive to rate changes.

I experimented a bit with this. Besides the question of how to model
the passives I wonder how the two regulators would interact. The
correct thing seems to be to specify the input regulator as a supply
of the DCDC. dcdc->set_voltage breaks down a voltage transition into
steps (if needed) and calls regulator_set_voltage(supply) for each
step. The problem with that is that regulator_set_voltage(dcdc)
acquires the supply lock(s), later regulator_set_voltage(supply) tries
to acquire its own lock which is already held. This can be worked
around by only using the supply regulator in the DCDC, but not
specify it as a supply. However this seems more a hack than a proper
solution.

Am I missing something obvious here or approaching this from a wrong
angle?

Thanks

Matthias