I recently installed Ubuntu 14.04 and got the package lm-sensors. I mainly did this to monitor if my laptop was overheating. Each time I run it, it shows a very low temperature reading (the same every time) of -128.0 degrees celcius. Why does this happen?
I think it is supposed to be like that. New kernel 3.13 has an advanced dynamic power management of the ATI card with the radeon driver. In normal conditions the ATI discrete is off:
[:~/Pictures/2014] 1 % sudo cat /sys/kernel/debug/vgaswitcheroo/switch 0:DIS: :DynOff:0000:01:00.0 1:IGD:+:Pwr:0000:00:02.0
so when we read the sensors:
[:~/Pictures/2014] % sensors [radeon-pci-0100 Adapter: PCI adapter temp1: -128.0°C (crit = +120.0°C, hyst = +90.0°C)...]
The sensor read nothing --- the card is off, its sensor too.
If you start the card (see for example this blog entry), for example by opening another windows and starting in it:
DRI_PRIME=1 glxgears -info
and check again, you'll have:
[:~/Pictures/2014] % sudo cat /sys/kernel/debug/vgaswitcheroo/switch 0:DIS: :DynPwr:0000:01:00.0 1:IGD:+:Pwr:0000:00:02.0
the card is ON and...
[:~/Pictures/2014] % sensors radeon-pci-0100 Adapter: PCI adapter temp1: +65.0°C (crit = +120.0°C, hyst = +90.0°C)
...the sensor too.