Re: [PATCH] ARM: dts: imx6sx-sdb: Remove cpufreq OPP override

[Leonard Crestez <leonard.crestez@nxp.com>]

On Wed, 2017-05-03 at 17:59 +0200, Marek Vasut wrote:
> On 05/03/2017 04:58 PM, Leonard Crestez wrote:
> > On Wed, 2017-05-03 at 16:26 +0200, Marek Vasut wrote:
> > > 2) It actually fixes a problem with the voltage rails such that the DVFS
> > >    works without leaving the system in unstable or dead state. You do
> > >    need the second part of my patch if you drop the OPP hackery, without
> > >    it the power framework cannot correctly configure the core voltages,
> > >    so the patch from Leonard makes things worse.
> > No, I think there is a misunderstanding here. The second part of your
> > patch will cause cpufreq poking at LDOs to indirectly adjust the input
> > from the PMIC to the minimum required (this is LDO target +
> > min_dropout_uv). Without it by default VDD_ARM_SOC_IN will remain fixed
> > as 1375mV from boot.

> Who sets / guarantees that default value for ARM and SOC rails ?

I think it's from the PMIC hardware itself (but maybe uboot plays with
it). VDD_ARM_SOC_IN on this board is tied to SW1AB from MMPF0200:

http://www.nxp.com/assets/documents/data/en/data-sheets/MMPF0200.pdf

It seems reasonable to rely on such voltages set externally.

> With the OPP override in place, there's at least the guarantee that both
> rails will have the same voltage requirement. If you remove the OPP
> override without modeling the actual regulator wiring, the guarantee is
> gone.

The imx6sx chip has internal LDO_ARM and LDO_SOC regulators which can
generate separate voltages for arm/soc. The fact that these regulators
share the same supply is only an issue when they are set in bypass
mode.

However the boot issues happen on REV C but apparently not REV B of the
board. I don't have a good explanation for this so maybe I am missing
something.

-- 
Regards,
Leonard