Sometimes coretemp fails on Atom CPU

[khali]

We had several reports in the past 18 months that the coretemp driver is sometimes unable to get temperature readings from Atom CPU:

 Atom N330
 Atom D510
 Atom D525
 Atom D525
 Atom D525

[khali]

Tested on an Atom D525:

$ sudo rdmsr -p 0 -x 0x19c
8600000
$ sudo rdmsr -p 1 -x 0x19c
8630000
$ sudo rdmsr -p 2 -x 0x19c
8630000
$ sudo rdmsr -p 3 -x 0x19c
8600000

The valid reading bit (31) is indeed never set, which is why the coretemp driver returns an error (and in turn "sensors" returns N/A.)

The readings themselves don't look good anyway. 0x60 and 0x63 would translate to offset values of 96 and 99, respectively. With a critical limit at 100°C, that would lead to readings of 4°C and 1°C, respectively, which is obviously incorrect.

I am wondering if maybe the valid reading bit gets cleared when the measured temperature is too low, because it is known that the reliability and accuracy of the digital thermal sensor get very poor when too far below the critical limit.