mt76: mt7915e: Fix degraded performance after temporary overheat
mt7915e registers a cooling_device with wrong semantics:
1. cooling_device expect that higher states values should cool more, but
mt7915e did the opposite... with the exception of state == 0, which
should "disable thermal management", but does not seem to have any
effect since the previous state is kept.
The result is that when the thermal zone heats up a bit and bumps the
cooling_device state from 0 to 1 to cool a bit, the performance is
destroyed, and when going back from 1 to 0, the performance stays bad.
2. Reading the cooling_device state does not always return the last
written state, but can return the actual hardware throttle state,
which is different.
This is a problem because the mt7915 firmware actually implement the
equivalent of a thermal zone with trip points. Setting the cooling
device state actually changes the throttles at each trip point, so the
following could occur if the first issue is fixed:
- thermal subsystem set state to 100% power (state=0)
- mt7915e driver set trip throttles to [100%, 50%, 25%, 12%]
- hardware heats up and decides to switch to 50% power
- thermal subsystem see that power is 50% (state=50), decide to increase
it to 60% (state=40) because the rest of the system is cool.
- mt7915e driver set trip throttle to [60%, 30%, 15%, 7%]
- hardware thus switches to 30% power
[race to the bottom continues...]
This patch corrects the semantics of the cooling_device to the one that
the thermal subsystem expect it.
Signed-off-by: Nicolas Cavallari <nicolas.cavallari@green-communications.fr>
Signed-off-by: Felix Fietkau <nbd@nbd.name>