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

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Message-ID: <23a47965-4ea9-5f6c-7e3c-27f5bd35f5b7@quicinc.com>
Date:   Wed, 8 Dec 2021 16:56:48 -0800
From:   David Collins <quic_collinsd@...cinc.com>
To:     Mark Brown <broonie@...nel.org>,
        "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>,
        <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 12/7/21 7:19 AM, Mark Brown wrote:
> 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.

The PM8008 LDOs are low noise LDOs intended to supply noise sensitive
camera sensor hardware.  They can maintain output regulation with a
fixed headroom voltage.  However, in order to guarantee high PSRR, the
headroom voltage must be scaled according to the peak load expected from
the each LDO on a given board.  Thus, we included support for a DT
property to specify the headroom per LDO to meet noise requirements
across boards.

As a minor note the PM8008 chip package has a single pin to supply LDOs
1 and 2 along with a single pin for LDOs 3 and 4.  That is why
vdd_l1_l2-supply is specified instead of vdd_l1-supply and vdd_l2-supply.

Take care,
David