u-boot low-battery handling

[Mike Montour]

Andy Green wrote:

> Weurgh it's a scary mess in there -- not your patches the whole U-Boot
> power thing.

I agree there's some nasty stuff, but I think that a lot of it comes 
from the hardware and is not u-boot's fault (like the PMU's choice of 
which registers are preserved in "save" mode, and which ones reset to 
mask-programmed defaults). My head hurts after reading through that 
PCF50633 manual.

> Maybe the best way to come at this is going to be move GTA02 to Qi which
> has no power handling and fix GTA02 at the same time we allegedly solve
> the GTA03 booting-under-100mA thing.  At that point, the bootloader has
> no part in charging action and it just waits for kernel to handle it (as
> it has to anyway).

I haven't looked at Qi yet - maybe that's a project for next weekend.

I don't know if you'll be able to put all of the PMU control logic into 
Linux. It seems that it might still make sense to have code in the 
earliest bootloader for cases like deeply-discharged batteries (e.g. 
turn on the charger, flash an LED, and wait for the voltage to reach 
some threshold before powering up the glamo, SD card, and whatever else 
is needed to boot Linux).