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
