[BPI-R3] Add passive cooling

Banana Pi Router design BPI-R3/R3 Mini(MT7986)
1

@frank-w @dangowrt @sam33 (Authors of mt7986a-bananapi-bpi-r3.dts)

Looking at the BPI-R3:

At the moment there is only an active cooling device defined. Because there is no variable voltage regulator for the CPU, there is no cpu-frequency scaling defined, unlike the BPI-R64.

So when there is no fan installed, or it fails (and no 4th pin to tell us it isn’t rotating), there is no cooling-map that will somehow throttle the cpu when the cpu gets hotter and hotter.

I’m not sure if the mediatek drver does something with THERMAL_TRIP_HOT, but I can’t find it if I have a brief look…

Anyway, may I suggest adding thermal-idle as cooling-device, like so?

Just wrote this down to elaborate, without actually testing it yet

&cpu0 {
	cpu0_therm: thermal-idle {
		#cooling-cells = <2>;
		duration-us = <10000>;
		exit-latency-us = <500>;
	};
};

&cpu1 {
	cpu1_therm: thermal-idle {
		#cooling-cells = <2>;
		duration-us = <10000>;
		exit-latency-us = <500>;
	};
};

&cpu2 {
	cpu2_therm: thermal-idle {
		#cooling-cells = <2>;
		duration-us = <10000>;
		exit-latency-us = <500>;
	};
};

&cpu3 {
	cpu3_therm: thermal-idle {
		#cooling-cells = <2>;
		duration-us = <10000>;
		exit-latency-us = <500>;
	};
};

&cpu_thermal {
	trips {
		cpu_trip_passive_idle: passive_idle {
			temperature = <100000>;
			hysteresis = <2000>;
			type = "passive";
		};
	};

	cooling-maps {
		map-cpu-passive_idle {
			trip = <&cpu_trip_passive_idle>;
			cooling-device = <&cpu0_therm 0 15>,
							 <&cpu1_therm 0 15>,
							 <&cpu2_therm 0 15>,
							 <&cpu3_therm 0 15>;
		};
	};
};

As found in: thermal-idle.yaml - Documentation/devicetree/bindings/thermal/thermal-idle.yaml - Linux source code (v6.9) - Bootlin