[RFC 4/8] regulator: core: Check enabling bypass respects constraints

[Mark Brown]

On Tue, Mar 28, 2017 at 10:49:55PM +0300, Leonard Crestez wrote:
> On Tue, 2017-03-28 at 17:47 +0100, Mark Brown wrote:

> > To repeat what I said previously the whole point of bypassing is to not
> > do regulation and generally the constraints in the unregulated idle case
> > are substantially more relaxed.  This would break use cases relying on
> > the existing behaviour which wouldn't expect to affect the parent
> > voltage at all, either stopping things working or making them less
> > efficient by needlessly regulating the voltage down which defeats the
> > main point of bypassing.

> So what you want is to prevent voltage changes unless strictly
> required, even lowering? What I want is to get the minimum voltage in
> the SOC because that's where power is being consumed.

So your end goal here is to bypass a regulator which is forced into your
system design by being integrated into the SoC which isn't able to
regulate down to a low enough voltage for your use case?  I guess one
question is if you need to use the regulator at all?

> It's not at all obvious that in bypass mode the output constraints of a
> regulator need not be respected by the core. I expected the opposite,
> this is something that should be documented.

SubmittingPatches...  Bear in mind that most regulators are fixed
voltage in a given system so bypass would be very difficult to use if it
tried to pass the constraints upstream.

> But if the bypassed regulator has a downstream consumer then it's
> requirements should definitely still be met in bypass mode, right? I
> could set my maximum voltage directly from cpufreq in that case.

What we're interpreting bypass mode as meaning is "stop regulating".
There will still be some limits but we're expecting them to be enforced
in the extremes of the constraints in the parent regulators.

> Or should a bypassed regulator ignore all other requests? One of the
> behaviors that this patch series relies on is that calling set_voltage
> on a bypassed regulator propagates this request to the supply and picks
> the minimum voltage there. An alternative implementation would be to

Yes, the expectation is that if anything is being changed it won't have
any effect until regulation is reenabled but we're not particularly
expecting much activity on bypassed regulators.

> call set_voltage directly on the supply regulator by changing the
> "{arm,soc,pu}-supply" references in DT to point to the PMIC instead.
> Would that be better?

That's more what's expected here.

> Both approaches work. Relying on propagation feels like it is the
> "right way" to handle this, even if it's harder to get right and the
> regulator core does not entirely support it. But it's possible that
> this is based on a misunderstanding of what "bypass" is actually
> supposed to do.

Another option would be to add a regulator configuration which allowed
the regulator to transparently go into bypass mode if the parent could
do things directly, only enabling regulation if the parent couldn't
support.  That would mean you'd loose the supply cleanup function which
is typically part of why there are LDOs in the system but it sounds like
you're OK with that in at least your use case.

We could also perhaps have a way to set the regulator into permanent
bypass mode from the constraints for use cases that just never need it
and then remap the supply relationship at probe time (which avoids any
special runtime casing) for something even simpler.
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