Date:	Wed, 30 Apr 2008 00:14:12 +0200
From:	Rudolf Marek <r.marek@...embler.cz>
To:	Jean Delvare <khali@...ux-fr.org>,
	Maxim Levitsky <maximlevitsky@...il.com>
CC:	trenn@...e.de, Kasper Sandberg <lkml@...anurb.dk>,
	Len Brown <lenb@...nel.org>, Matthew <jackdachef@...il.com>,
	linux-acpi@...r.kernel.org, torvalds@...ux-foundation.org,
	linux-kernel@...r.kernel.org, "Zhang, Rui" <rui.zhang@...el.com>
Subject: Re: Linux 2.6.25 (coretemp reads high temperatures)

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Hi all,

I already answered this thread while ago. I can just confirm what Jean told.

>>>> I confirm this.
>>>> I *know* that temperatures reported now are wrong.
> 
> And how do you know? The newly reported temperatures could be correct
> and the previous ones were incorrect (that's actually the case.) The
> thing is, the temperature is stored as a relative value in the CPU.
> Relative to what, depends on the CPU model, can be 85°C or 100°C. Up to
> kernel 2.6.24 we had a set of rules to find out, in 2.6.25 we have a
> presumably better heuristic. So some people have seen their CPU
> temperature climb by 15°C and others drop by 15°C, that's expected.

Yes exactly. I decided to move to 0-100C scale, and move the limit too.
Of course some users with too low jumped to better scale some like you seems to
complain now.

>>> i have watercooling, and well :P when i touch the "tube", its normal
>>> room temperature, and believe me, i would notice if it was 45.. this is
>>> with my cpu at idle - at full load on all 4 cores, temp2 says 35, and
>>> ~60 on coretemp, and THIS i would surely be able to notice over room
>>> temp :)
> 
> The coretemp driver reports the CPU _core_ temperature. That's not
> something you can touch, believe me (unless you are an electron.)
> 
> Also note that the CPU temperature reported by the IT8718F may or may
> not match the reality. To make sure, you'd need to know the type of
> thermal diode expected by the IT8718F, the type of thermal diode in
> your CPU, compute the correction factor if there is one. And you'd need
> to know where the thermal diode is exactly. It is most certainly built
> into the CPU, but some motherboard makers are doing weird things.
> 
> 22°C seems very low to me, even for water-cooling. Note that
> non-linearity of thermal diodes makes measurements inaccurate as they
> get away from the expected operating point. I guess that thermal diodes
> used in CPUs are calibrated for best results around the expected
> temperature when using air-cooling, rather than water-cooling.
> 
>>> any progress on this bug?
> 
> I still need to be convinced that there is a bug here.

It is not a bug, a max limit changed too, it is just matter how to scale it. The
temperature is non-physical so comparing it to physical temperature does not
make any sense. I'm sorry I did not invent this relative temp stuff - Complain
@intel. They have some calibration of TjMAX for mobiles, but this bit does not
work for desktops/servers. I tried really hard to get at LEAST some
documentation so the driver looks like it looks. And not
guessed/guessed/guessed/how it looked earlier.

> 
>>>> The reason is that bios did report same temperatures as coretemp in 2.6.24,
>>>> moreover some time ago I have run a cpu tool (don't remember its name) on windows

It was most likely coretemp - I'm in contact with the guy. We share infos.

>>>> temperature of both cores
>>>> (I had to run this on windows - intel haven't released 
>>>> drivers for their QST for temperature monitoring from bios - very sad)
>>>>
>>>> And the driver did say in kernel log that TJMAX is 85C
> 
> Which driver, which kernel? As I wrote above, the coretemp heuristic
> changed in kernel 2.6.25, so the fact that a previous kernel was
> reporting a different tjmax value is irrelevant. Unless you have
> additional documentation from Intel, I would tend to believe that the
> coretemp driver in 2.6.25 is correct. But feel free to report the exact
> CPU model you have (with CPUID info) to Rudolf, if he gets enough
> reports about a specific CPU model which most people believe gets the
> wrong tjmax, he can fix the driver.

Well again, I tried hard at Intel and I really could not get any info on some
calibration bit. The temperature is non-physical on arbitrary scale. I changed
that so for some people it jumped to 100C, for some it remained.

>>>> Lets at least make a kernel option to override tjmax?
> 
> That's a possibility for sure, but what we would really need is to
> adjust the coretemp driver heuristics to always get it right - if
> that's not already the case, that is. I'll let Rudolf decide anyway.

Well again, Intel swears there is no way how to get the TjMAX for
desktops/servers. It sucks but this is not my fault.

Thanks,
Rudolf






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