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

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Message-ID: <20180530165937.GX6920@sirena.org.uk>
Date:   Wed, 30 May 2018 17:59:37 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Doug Anderson <dianders@...omium.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

On Wed, May 30, 2018 at 09:41:55AM -0700, Doug Anderson wrote:
> On Wed, May 30, 2018 at 9:36 AM, Mark Brown <broonie@...nel.org> wrote:

> > 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).

It'd only affect regulators that can vary in voltage and actually get
disabled which is a pretty small subset.  Most regulators are either
fixed voltage or always on.

> 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...

Even if the regulator lacks a cache we'd at least skip out on the write
when we're not changing voltage as we'd do a read/modify/update cycle
and stop at the modify.

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