Am Mittwoch, den 22.03.2017, 19:48 +0200 schrieb Leonard Crestez:

On Wed, 2017-03-22 at 18:09 +0100, Lucas Stach wrote:

quoted

Am Mittwoch, den 22.03.2017, 18:53 +0200 schrieb Leonard Crestez:

quoted

Several imx6* socs have three built-in regulators LDO_ARM LDO_SOC and
LDO_PU used to control internal chip voltages. "ldo-bypass" mode refers
to placing these regulators in bypass mode and controlling voltages from
an external power management chip instead. This is intended to save
power at the expense of an extra PMIC on the board.

The voltages for these regulators are controlled from the imxq6 cpufreq
driver so it makes sense to also control LDO bypass from here. The gpc
driver also fetches a reference to LDO_PU and uses it to gate power to
graphics blocks.

The LDO regulator has a minimum dropout voltage of 125mv so enabling
bypass mode will raise the effective voltage by that amount. We need set
the minimum frequency first to avoid overvolting when enabling LDO
bypass.

The binding is an u32 fsl,ldo-bypass in the gpc node because that's how
it was defined in the freescale vendor tree for a long time and
compatibility is desirable. Otherwise it would be a bool.

Some versions of u-boot shipped by freescale check this same property
and set regulators in bypass mode before linux actually starts so we
check for that scenario as well and finish early.

I've not looked at the patch at all, but this feels like the wrong
location to implement this. Using bypass mode or not should really be a
internal decision of the regulator driver, influenced by a DT property
to allow bypass mode.

The regulator driver can also implement the correct sequencing of first
lowering external voltage to min + dropout, then going into bypass mode,
then lower the external voltage by the amount of the dropout. I don't
see why we need a frequency switch for this at all.

Because minimum voltages are dictated by core frequency. At high frequency
the (minimum voltage for frequency + dropout) is too high and would go beyond
the maximum of 1300 mv when bypass is enabled. It doesn't actually instantly
break, this is based on the "operating ranges" from this document:

http://www.nxp.com/assets/documents/data/en/data-sheets/IMX6DQCEC.pdf

Urgh, yes, this is unfortunate.

quoted

Implementing this in the consumers seems like the wrong spot.

It doesn't belong in drivers for individual regulators either, it's a piece
of board-level global configuration.

The configuration weather to use the bypass or not is board level, I
agree. But the decision to bypass the regulator should be done either at
the regulator driver level or even in the regulator framework. That is
the point where you are able to match up the constraints of the
consumers with the ability to get the correct voltage from the supplying
regulator.

Regards,
Lucas