[S] Change in coreboot[main]: mb/google/rex/var/rex: Configure PL4 to 64W if battery status is invalid

[Tim Van Patten (Code Review)]

Attention is currently required from: Brandon Breitenstein, Caveh Jalali, Eran Mitrani, Hannah Williams, Jamie Ryu, Kapil Porwal, Subrata Banik, Sukumar Ghorai, Tarun.

Tim Van Patten has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/78322?usp=email )

Change subject: mb/google/rex/var/rex: Configure PL4 to 64W if battery status is invalid
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Patch Set 5:

(1 comment)

File src/mainboard/google/rex/variants/baseboard/rex/ramstage.c:

https://review.coreboot.org/c/coreboot/+/78322/comment/28f1abf6_db4e629b :
PS5, Line 48: battery_status
> isn't the battery cut-off or no battery is the major concern for us?

A battery that is present but has 0% charge is effectively useless, meaning it should be treated the same as no battery, because it can't supply any of the extra power that the SOC is requiring.

The other thing to take into consideration is that battery cut-off and charge are dynamic and can change after the device has booted.

Are these power limits undone when battery cutoff is disabled, so the SOC can utilize its full power and performance?

Sending a host command every boot to handle the <0.01% of the time the battery is not present, is wasteful and will impact everyone's boot time to handle the very rare case when the battery is not present. It should be avoided, since it's possible to be interrupt driven (via ACPI) instead.

Most all of the discussion here is resolved by following what's already implemented for AMD. See: https://b.corp.google.com/issues/296952944#comment101