Letux Kernel

Issue 928: omap3: thermal madness

Reported by Nikolaus Schaller, Sep 24, 2019

cat /sys/devices/virtual/thermal/thermal_zone0/temp 
seems to report the value from the previous measurement and not a 
current value.

This has the strange effect that after boot or after longer time of 
idle, you get a value that may be more than 10°C apart from the next 
read.

It is not clear if this is a bug in the bandgap ADC or in the driver 
or somewhere else because it is not seen on am335x or omap5.

One indication is that we can see the effect also when using devmem2 
to directly read the bandgap register (0x48002524).

Some ideas are that the timeout in the driver which waits for SOC 
and EOCZ bits is too short and therefore reads too early.

BTW: it is not clear how often the govenor reads the bandgap sensor 
for thermal management. If it does regularily it may hide this 
effect from the user.

So tests should be run with bandgap based thermal management 
disabled.

Created: 26 days 10 hours ago by Nikolaus Schaller

Status: New

Labels:
Type:Defect
Priority:Medium