CPU Thermal Protection on Astra Machina

Astra Machina uses the Linux Kernel’s Thermal Framework to limit CPU speed to prevent the CPU from overheating. Astra Machina defines two level protection policy.

• If the CPU reaches temperatures greater than 105°C, the thermal framework will trigger a cpu_alert protection. This will cause the CPUFreq subsystem to reduce the clock frequency of the CPU to reduce the temperature.

• If the CPU reaches a temperature greater than 110°C, the thermal framework will trigger a cpu_critical protection. This will cause the kernel to shutdown the device to prevent overheating.

Device Tree Configuration

The Thermal Zones and trips are defined in platform dts files located in build-sl1680/workspace/sources/linux-syna/arch/arm64/boot/dts/synaptics.

	SL1620	SL1640	SL1680
DTS	myna2.dtsi	platypus.dtsi	dolphin.dtsi

Documentation on Thermal Zones in Device Tree can be found here.

Userspace Controls

The cpu_alert and cpu_critical temperature thresholds can be adjusted using the thermal sysfs interface.

Set the cpu_alert threshold by writing a value to /sys/class/thermal/thermal_zone0/trip_point_0_temp. To set the cpu_critical threshold, write a value to /sys/class/thermal/thermal_zone0/trip_point_1_temp.

The temperature value is in millidegrees Celsius. This example sets the threshold to 30°C.

echo 30000 > /sys/class/thermal/thermal_zone0/trip_point_0_temp

To test that the thermal policy is working you can check the current CPU frequency, set the 30°C threshold, and then check the current CPU frequency again. Since, 30°C is a low threshold, the current CPU frequency should be reduced.

cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq
echo 30000 > /sys/class/thermal/thermal_zone0/trip_point_0_temp
cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq