Very Low Temperature Reading for graphics driver using lm-sensors

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.

A lot of info is from this Arch forum entry and this Arch doc page (hat off to Arch for the quality of documentation and for having it up-to-date...).