linux-kernel - Re: [PATCH V4 1/6] dt-bindings: regulator: Add "regulator-min-dropout-voltage-microvolt"

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Message-ID: <Ya97cnuwM+MuNMg3@sirena.org.uk>
Date:   Tue, 7 Dec 2021 15:19:14 +0000
From:   Mark Brown <broonie@...nel.org>
To:     "Satya Priya Kakitapalli (Temp)" <quic_c_skakit@...cinc.com>
Cc:     Bjorn Andersson <bjorn.andersson@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Liam Girdwood <lgirdwood@...il.com>, swboyd@...omium.org,
        collinsd@...eaurora.org, subbaram@...eaurora.org,
        Das Srinagesh <gurus@...eaurora.org>,
        linux-arm-msm@...r.kernel.org, Lee Jones <lee.jones@...aro.org>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V4 1/6] dt-bindings: regulator: Add
 "regulator-min-dropout-voltage-microvolt"

On Tue, Dec 07, 2021 at 08:36:11PM +0530, Satya Priya Kakitapalli (Temp) wrote:
> On 12/6/2021 11:55 PM, Mark Brown wrote:
> > On Mon, Dec 06, 2021 at 06:33:26PM +0530, Satya Priya Kakitapalli (Temp) wrote:

> > > The min-dropout value (headroom) varies with boards, that's why we have a DT
> > > property for it. We overwrite the default value in driver with actual value
> > > read from DT

> > Interesting.  How exactly does that end up happening - presumably other
> > systems are going to run into it?

> The parent supplies such as "vdd-l1-l2" are coming from other pmic
> regulators, which are shared supplies with other subsystems like BT, Display
> etc, they vary between boards as per requirements, so we cannot expect these
> to be fixed and so are the headroom values. We get the headroom values from
> PMIC systems team for every target.

I don't think you're talking about the thing the code is saying it's
describing here.  The regulator API is referring to the minimum droput
voltage that individual regulators require, that is how much higher the
input to a single regulator must be than the voltage being output by
that regulator.  We absolutely can and do expect this to be board
independent, it's a function of the design of the regulator.  Sharing
the input supply has no impact on this, the input voltage that the
regulator needs just get fed into the requiremnts on the supply voltage.

If there is a board specific constraint on the minimum voltage that a
given supply can have then that should be expressed using the normal
constraint mechanism, that's nothing to do with the headroom that the
regulators require to operate though.

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