Power measurement wrong when idle

[Marc SCHAEFER <schaefer@alphanet.ch>]

Hello,

I am using the apu2 embedded platform, which uses an amd64 AMD GX-412TC SOC,
stepping        : 1
microcode       : 0x7030105

With Debian bullseye, the power measurement when idle is very big, and wrong (>
80 .. 100 W). We have observed this behaviour on multiple systems.

The problem did not occur with Debian buster, does not occur with the
temperature sensor, and the power measurement goes back to apparently correct
values when the system is no longer idle.

It does not seem to be linked to amd64 specific firmwares.

The problem lies in the /sys/class/hwmon/hwmon0/power1_average not in the
lm-sensors package (direct reading the /sys files gives the same isue).

So it appears to be within the kernel: 4.19.0-22-amd64 seems ok and
5.10.0-18-amd64 is not.

Funnily, there does not seem to be relevant changes in the specific kernel
driver (fam15h_power).

Any idea what could lead to this strange behaviour?

Thank you for any ideas or pointers.

Examples:

When bullseye is idle, it's completely wrong (' are from me):

cat /sys/class/hwmon/hwmon0/power1_average
94'019'396

When bullseye has 100% CPU used (one core):
cat /sys/class/hwmon/hwmon0/power1_average
10'917'309

The only visible change is that hwmon1 and hwmon0 are interchanged:

bullseye:
   fam15h_power-pci-00c4
   Adapter: PCI adapter
   power1:       88.61 W  (interval =   0.01 s, crit =   6.00 W)
   
   k10temp-pci-00c3
   Adapter: PCI adapter
   temp1:        +54.5 C  (high = +70.0 C)
                          (crit = +105.0 C, hyst = +104.0 C)
   
buster:
   k10temp-pci-00c3
   Adapter: PCI adapter
   temp1:        +59.6°C  (high = +70.0°C)
                          (crit = +105.0°C, hyst = +104.0°C)
   
   fam15h_power-pci-00c4
   Adapter: PCI adapter
   power1:        8.00 W  (interval =   0.01 s, crit =   6.00 W)