[lm-sensors] RE: vt8231.c

Roger Lucas roger at planbit.co.uk
Tue Nov 15 12:13:53 CET 2005

Hi Jean,

> -----Original Message-----
> From: Jean Delvare [mailto:khali at linux-fr.org]
> Sent: 15 November 2005 07:13
> To: Roger Lucas
> Cc: Knut Petersen; LM Sensors; Mark Studebaker
> Subject: Re: [lm-sensors] RE: vt8231.c
> Hi Roger,
> Once again, sorry for the delay... :(
> > > temp3 is a bit special, it is diode-based. The conversion which is
> > > currently done in userspace should be moved into the driver, so that
> the
> > > exported value is a temperature rather than a random register value.
> > > Other temps are thermistor based, a part of the conversion has to be
> > > moved into the driver so that we export the voltage value at the chip
> > > pin. The thermistor bridge computation (voltag -> temperature) belongs
> > > to userspace because different thermistors can be used on different
> > > motherboards. The same is done for the PC87366 temp4-6 already.
> >
> > We know what the datasheet and porting guide say about the temperatures,
> but
> > I am not sure that this matches reality.  On my VIA EPIA 500
> motherboard,
> > the CPU is a VIA EPIA 5000 and not an Intel CPU.  I don't doubt that the
> > sensor scale is correct for an Intel CPU, but the VIA EPIA CPU is
> different.
> > If I use the scaling values in the porting guide then my temperatures
> are
> > over 30C off.
> >
> > For reference,
> > 	VIA's equation:  REG = TEMP * 0.9686 + 65
> > 	My EPIA Mobo:    REG = TEMP * 0.7809 + 45
> Good point. Do these constant have any kind of physical meaning? Is the
> information publicly available? How did you come up with the correct
> one for your VIA CPU?

We have been running some thermal tests on the VIA board and the values from
LM-Sensors were significantly off.  We therefore ran our own thermal cycling
tests with a probe attached to the center of the heatsink directly above the
CPU.  We were then able to obtain a set of measurements for the probe
temperature versus the register value.  This gave us a very linear response
with the above equation.  We did not get the curve information from VIA.

We have another sample of the board here now, so we will be repeating the
measurements to ensure that the VIA results are predictable from board to

> What would the formula required for VIA CPUs look like if the Intel one
> was integrated into the driver as I first proposed?

It would still be a simple y=mx+c style graph but just with different

> > We can put the VIA official Intel scaling into the driver, but this
> would be
> > very misleading as the VT8231 is used on the VIA EPIA platform which is
> in
> > widespread use, and all users of that board would therefore need to know
> > that additional scaling was required.  If they didn't then they would be
> > panicking when they see the CPU temperature (since the motherboard is
> only
> > passively cooled).
> Well, whatever we do, the VIA CPU users will have an incorrect reading
> at first and will have to edit some lines in their configuration file
> to get the proper reading. So this point doesn't really weight in
> either direction. All it really means is that we will have to improve
> the vt8231's sensors.conf.eg section to make the point clear.

I agree that whatever we do, the VIA users must apply some kind of equation
to get their correct temperature, but I feel that the safest approach is
"first do no wrong".  

I am extremely uncomfortable with the situation where we release a driver
that we know will give incorrect information under some conditions.  I feel
that it is much better to return the result from the driver as a register
value and put the text below:

# If you have an Intel CPU, then uncomment the line below
# temp0_input = (@-65)/0.9686, (@*0.9686)+65

# If you have an VIA EPIA CPU, then uncomment the line below
# temp0_input = (@-45)/0.7809, (@*0.7809)+45

This way, an incorrect value is never returned and the user must correctly
set the board configuration before a temperature is returned.

> Thanks,
> --
> Jean Delvare

Thanks for the work you are doing on reviewing this driver. 

Best regards,


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