[lm-sensors] Accuracy of "via_cputemp" Module in LM_Sensors

[Jeff Rickman <jrickman@myamigos.us>]

Hi all,

I am trying to track down a temperature discrepancy on a Jetway 
J7F4K1G5D "Versa" motherboard. This board has a VIA C-7 D cpu (25W 
version). The operating system is Fedora Core 14 i386, kernel 
2.6.35.10-74.fc14.i686.PAE. I am running this version of "sensors": 
sensors version 3.2.0 with libsensors version 3.2.0

LM_Sensors loads the following modules:
acpitz-virtual-0
f71805f-isa-0290 (using acpi_enforce_resources=lax)
via_cputemp-isa-0000

I understand the risk of using "lax", but there is no ACPI version of 
the "f71805f" module and Jetway ACPI grabs 0x295-0x296 from the IO range 
0x290-0x297.

Back to my question....

When I boot this machine I can read the System and CPU temperatures from 
the boot screen. I can also go into the BIOS and read the same values in 
the "PC Health" screen as seen on the boot screen. The boot values 
typically show as 12C-16C for CPU temp and 27C-30C for System temp. 
Others have posted comments elsewhere saying this VIA CPU does not run 
warm, especially when idle, but that temperature range is approximately 
the ambient range for the room where that machine is located. I thought 
pushing electrons through silicon generated some heat?

Once FC14 is booted, I can run the "sensors" command and see the 
following values:

[root@XX ~]# sensors
acpitz-virtual-0
Adapter: Virtual device
temp1:       +11.0°C  (crit = +60.0°C)

f71805f-isa-0290
Adapter: ISA adapter
[...voltages and fan info removed...]
System Temp: +11.0°C  (high = +60.0°C, hyst = +49.0°C)  sensor = thermal 
diode
CPU Temp:    +27.0°C  (high = +60.0°C, hyst =  +0.0°C)  sensor = thermal 
diode

via_cputemp-isa-0000
Adapter: ISA adapter
Core 0:      +27.0°C

I modified my "/etc/sensors.d/local.conf" file to match the outputs 
shown in "f71805f-isa-0290" to those seen in "via_cputemp-isa-0000" and 
"acpitz-virtual-0" since there are no corresponding values in my 
"/etc/sensors3.conf". In this case "temp1" is mapped to "System Temp" 
and "temp2" is mapped to "CPU Temp" in my "local.conf" file.

After studying the code posted on "lm-sensors.org" for the standalone 
"via_cputemp" module, I think the "via_cputemp" module pulls it's 
temperature value from a MSR. I did not have the source code handy for 
my distribution when I did this research, but does one expect 
differences on what value is being accessed (doubt it)? Perhaps the 
value being accessed is achieved through a different mechanism? If it 
matters I will do the research in the FC14 source code.

Every time I run "sensors", the value shown in "via_cputemp" is within 
+-2C or less of the "CPU Temp" value reported by "f71805f". The same 
(+-2C or less) can be said for the "acpitz" value and the "System Temp" 
value from "f71805f". Numbers that are that close on a consistent basis 
are a coincidence that is too good to be true, which is why I suspect 
Jetway has what could be a simple "label swap error" in their BIOS.

If the MSR value read by "via_cputemp" is correct, and I think it is, 
then I think Jetway has their BIOS labels (what they display on screen) 
switched for "CPU temp" and "System Temp". I have posted a message to 
Jetway tech support asking them about this matter. Hopefully they will 
respond in a timely manner.

Does my logic and understanding of the "via_cputemp" code make sense, or 
have I gotten myself turned around the wrong way on this issue?

Thank you.

Jeff


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