linux-kernel - Re: [PATCH v3 1/2] regulator: dt-bindings: add QCOM RPMh regulator bindings

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Date:   Wed, 30 May 2018 09:41:55 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Mark Brown <broonie@...nel.org>
Cc:     David Collins <collinsd@...eaurora.org>,
        Liam Girdwood <lgirdwood@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        linux-arm-msm@...r.kernel.org,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        devicetree@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Rajendra Nayak <rnayak@...eaurora.org>,
        Stephen Boyd <sboyd@...nel.org>
Subject: Re: [PATCH v3 1/2] regulator: dt-bindings: add QCOM RPMh regulator bindings

Hi,

On Wed, May 30, 2018 at 9:36 AM, Mark Brown <broonie@...nel.org> wrote:
> On Wed, May 30, 2018 at 09:31:55AM -0700, Doug Anderson wrote:
>> On Wed, May 30, 2018 at 9:13 AM, Mark Brown <broonie@...nel.org> wrote:
>
>> > If we're just going to use the most recently set voltage then hopefully
>> > the hardware already knew that, and it might not be the lowest available
>> > voltage if the last consumer to get turned off was holding the voltage
>> > higher.
>
>> To circle back to the original point: the problem is that (IMHO) the
>> hardware is doing the wrong thing by still counting Linux's vote for a
>> voltage even though Linux also voted that the regulator should be
>> disabled.  So basically we're working around the hardware by
>> pretending to vote for a dummy lower voltage whenever Linux wants the
>> regulator disabled.  From Linux's point of view everything works as
>> normal--we just tell the hardware a falsehood so it doesn't count our
>> vote for a voltage while the regulator is disabled.
>
> Yeah, and I don't think that's unreasonable for the core to do - just
> drop the voltage to the constraint minimum after it has turned off the
> regulator, then recheck and raise if needed before it enables again.

Would it do this for all regulators, though?  Most regulators are
hooked up over a slow i2c bus, so that means that every regulator
disable would now do an extra i2c transfer even though for all
regulators (other than RPMh) the voltage of a regulator doesn't matter
when it's been "disabled' (from Linux's point of view).

Hrmmm, I suppose maybe it'd be OK though because for most regulators
we'd use "regcache" and most regulators that we enabled/disable a lot
are not expected to change voltage in Linux (so the regcache write
would hopefully be a noop), so maybe it wouldn't be _that_
inefficient...

-Doug