Message-Id: <1209514274.27499.12.camel@localhost>
Date:	Wed, 30 Apr 2008 02:11:14 +0200
From:	Kasper Sandberg <lkml@...anurb.dk>
To:	Rudolf Marek <r.marek@...embler.cz>
Cc:	Jean Delvare <khali@...ux-fr.org>,
	Maxim Levitsky <maximlevitsky@...il.com>, trenn@...e.de,
	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)

On Wed, 2008-04-30 at 00:14 +0200, Rudolf Marek wrote:
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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.

So, im confused.. The reason for this is that the internal sensor is
operating on some sort of weird scale, and thus when you interpolate it
into "your" scale, it doesent quite come out in the actual degrees
celcius the cpu temperature really is?

so if i understand this correctly, the coretemp output does NOT
represent the actual deg celcius temperature my cpu is?

> 
> >>>> 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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