Re: [PATCH v2 4/7] power: supply: bq24190_charger: Never reset the charger chip

[Tony Lindgren <tony@atomide.com>]

* Hans de Goede <hdegoede@redhat.com> [170322 07:57]:
> Resetting the charger should never be necessary it should always have
> sane values programmed. If it is running with invalid values while we
> are not running (system turned off or suspended) there is a big problem
> as that may lead to overcharging the battery.

And some systems may only configure it in the bootloader with no
configuration passed to the kernel at all.

> The reset in suspend() is meant to put the charger back into default
> mode, but this is not necessary and not a good idea. If the charger has
> been programmed with a higher max charge_current / charge_voltage then
> putting it back in default-mode will reset those to the safe power-on
> defaults, leading to slower charging, or charging to a lower voltage
> (and thus not using the full capacity) while suspended which is
> undesirable. Reprogramming the max charge_current / charge_voltage
> after the reset will not help here as that will put the charger back
> in host mode and start the i2c watchdog if the host then does not do
> anything for 40s (iow if we're suspended for more then 40s) the watchdog
> expires resetting the device to default-mode, including resetting all
> the registers to there safe power-on defaults. So the only way to keep
> using custom charge settings while suspending is to keep the charger in
> its normal running state with the i2c watchdog disabled. This is fine
> as the charger will still automatically switch from constant current
> to constant voltage and stop charging when the battery is full.
> 
> Besides never being necessary resetting the charger also causes problems
> on systems where the charge voltage limit is set higher then the reset
> value, if this is the case and the charger is reset while charging and
> the battery voltage is between the 2 voltages, then about half the time
> the charger gets confused and claims to be charging (REG08 contains 0x64)
> but in reality the charger has decoupled itself from VBUS (Q1 off) and
> is drawing 0A from VBUS, leaving the system running from the battery.

We do have cases where Linux kernel is the bootloader though using
kexec. And in those cases we may need to reset, so I wonder if the
reset should just be optional based on a proper device tree
(or platform_data) configuration?

Regards,

Tony