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SubjectRe: [PATCH v7 1/6] DONOTMERGE: arm64: dts: ti: Add TI TPS65219 PMIC support for AM642 SK board.
On 18:22-20221215, Mark Brown wrote:
> On Thu, Dec 15, 2022 at 11:54:11AM -0600, Nishanth Menon wrote:
> > On 16:09-20221215, Mark Brown wrote:
> > > That proposal looks really non-idiomatic and quite unusual, if there's a
> > > fixed voltage supply to the LDO I'd expect to see it modeled as a fixed
> > > voltage regulator. I'm not sure what the use of bypass here is trying
> > > to accomplish TBH.
> > The problem is this - the default NVM in the PMIC is setup such that
> > VSET value =3.3v and bypass bit set (makes sense since the vin=3.3v).
> This implies no voltage drop over the LDO? Sounds a bit suspect.

Not the choice I'd probably have made ;)

> > Now the constraint is bypass bit cannot be changed without the LDO
> > being switched off.
> > regulator-allow-bypass property allows us to control bypass bit, but we
> > should'nt toggle it when LDO is active. Not providing the property
> > implies the bit wont be toggled by regulator core either.
> > What we need is a scheme that will disable the bypass bit with the
> > intent of operating the LDO with just the vset field. I did'nt find it
> > possible atm.. unless I am mistaken..
> Can the consumer just disable the supply as part of startup? Though
> that's starting to feel rather board specific. There's not really a

Yeah - this happens to be SDcard supply (at least in my case).. I'd
rather not change the mmc host or core layer to handle a case where
LDO happened to be in bypass. it is a regulator driver's problem, IMHO
how to provide the stated voltage OR fail to transition the voltage.

In this driver's case, it happily accepts and set the VSET voltage - for
example to 1.8V, but then, since the bypass bit is set, well, voltage
sticks around at 3.3v.

> good place to put a board specific setup process like that in the kernel
> at the minute, you'd ideally want the firmware to leave the device at
> least disabled if not actually out of bypass on startup so we don't have
> to deal with this. Ugh...

Yeah - that would be the other option - I could plug this bypass clear
in the u-boot or someplace early so that the LDO behaves

Also the reason why I did'nt send the mentioned patch (or the like
upstream and the patch was done just a couple of days back) were the following
a) Why would'nt we handle the case where bypass bit
is set AND voltage change implies bypass bit needs to be disabled? (i
would expect it to fail but if i did provide regulator-allow-bypass,
then if bypass is set AND requested-voltage != vin-supply, then i'd
have expected framework to probably disable bypass and switch voltage
to new voltage - which this driver, based on it's constraint will say
"nope, cant do" - but that would be better than silently telling me
all good, setting vset and leaving the bypass bit on.)
b) If I wanted the LDO to poweroff the bypass bit at start (define the
startup hardware condition), I dont seem to have a description for
that either.

Nishanth Menon
Key (0xDDB5849D1736249D) / Fingerprint: F8A2 8693 54EB 8232 17A3 1A34 DDB5 849D 1736 249D

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