Sensors reporting hot CPU in Maverick

My clunky old Lenovo has and Intel(R) Core(TM) Duo CPU (model 15) which coretemp in Lucid 10.04 assumed TjMax was 85 degrees C but now in Maverick 10.10 believes TjMax is 100 degrees C.

Kernel commit a321cedb12904114e2ba5041a3673ca24deb09c9 attempts to get TjMax from msr 0x1a2. If it fails to read this msr it defaults TjMax to 100 degrees C for CPU models 14, 15, 22 and 26, and one will see the following warning message:

[ 9.650025] coretemp coretemp.0: TjMax is assumed as 100 C!
[ 9.650322] coretemp coretemp.1: TjMax is assumed as 100 C!

For CPU models 23 and 28 (Atoms) TjMax will be 90 or 100 depending if it's a nettop or a netbook. Otherwise the patch will default TjMax to 100 degrees C.

One can check the value of TjMax using:

cat devices/platform/coretemp*/temp1_crit

Coretemp calculates the core temperature of the CPU by subtracting the thermal status from TjMax. Since the default has been increased from 85 to 100 degrees between Lucid and Maverick, the apparent core temperature now reads 15 degrees higher.

Now, if my machine really was running 15 degrees hotter between Lucid and Maverick I would see more power consumption. I checked the power consumption for Lucid and Maverick kernels on my Lenovo in idle and fully loaded CPU states with a power meter and observed that Maverick uses less power, so that's encouraging.

As for the correct value, why did the default change? Well, from what I can understand from several forums that discuss the setting of TjMax is that this is not well documented and not disclosed by Intel, hence the values are rule-of-thumb guesswork.

So, the bottom line is that if your CPU appears to run hot from the core temp readings between Lucid 10.04 and Maverick 10.10 first check to see if TjMax has changed on your hardware.