TL;DR: Does anyone know what the correct Tctl_max value for the GX-412TC is?
APU2/3 CPU temperatures seem a popular topic for discussion but there doesn't appear to have been any discussion of what the reported processor temperature is actually measuring. According to https://support.amd.com/TechDocs/52740_16h_Models_30h-3Fh_BKDG.pdf (BIOS and Kernel Developer's Guide), section 2.10, the value reported "is a temperature on its own scale" and "specifies the processor temperature relative to the maximum operating temperature". In particular, the maximum operating temperature (specified as 90°C in https://www.amd.com/Documents/AMDGSeriesSOCProductBrief.pdf) is reported as Tctl_max and Tctl_max "is specified in the power and thermal data sheet". Unfortunately, I have been unable to locate that datasheet for the GX-412TC used in the APU2/3. For that matter, it's not even clear that the step size is 0.125K, though I haven't found any code that assumes anything else.
Does anyone know what the correct Tctl_max value for the GX-412TC is?
I've done some calculations and, assuming the step size is 0.125K, I measure the die-to-case resistance as about 5K/W. If Tctl_max = 0x2D0 (ie 90°C) that implies an idle CPU power dissipation of about 4.5W - which seems very high since the entire APU3C2 draws about 5.3W when idle. This suggests to me that Tctl_max is actually somewhat higher than 0x2D0, I suspect it's more like 0x320-0x348.