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Date: Thu, 27 Mar 2003 12:16:38 -0500
From: "Mark M. Hoffman" <mhoffman@lightlink.com>
To: Jean Delvare <khali@linux-fr.org>
Cc: Jan Dittmer <j.dittmer@portrix.net>, azarah@gentoo.org, greg@kroah.com,
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Subject: Re: lm sensors sysfs file structure
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* Jean Delvare <khali@linux-fr.org> [2003-03-27 14:31:40 +0100]:
> > Ah, and I don't want to do small corrections like 1 or 2 percent up or
> > or down, but things like this for lm80:
> >      compute in0 (24/14.7 + 1) * @ ,       @ / (24/14.7 + 1)
> >      compute in2 (22.1/30 + 1) * @ ,       @ / (22.1/30 + 1)
> >      compute in3 (2.8/1.9) * @,            @ * 1.9/2.8
> >      compute in4 (160/30.1 + 1) * @,       @ / (160/30.1 + 1)
> > 
> > These differ a lot. And as stated in the sensors.conf seem to be from 
> > the datasheet of the lm80 and not mainboard specific.
> 
> They may be mainboard specific. The formulae rely on *suggested*
> resistor values, which the integrators may or may not follow. There
> definitely is a need for userspace conversion. Libsensors needs to be
> tunable by the user.
> 
> I don't know about the kernel conversion Jan is talking about (never
> coded a sensor driver myself), so I can't talk about this half, but I'm
> pretty sure you shouldn't even think of moving all conversions into
> kernel space.
> 
> Mark, Phil, maybe you could explain the reasons better than I would?

I'm not the Mark to which Jean refers, but anyway...  he is correct.
The sensor chip cannot read temperatures directly, only voltage.  The
conversion from degress C to V is dependent on the mainboard for the
reasons Jean mentions.

But also, the chip driver cannot read voltage directly, only bits in a
register.  *That* conversion is *not* mainboard dependent; it is
sensor chip specific.

Even when the subject in question is voltage (e.g. power supply +12V),
there is mainboard dependent nonsense between the "real value" and what
is presented at the pin of the sensor chip.

So there is a legitimate need for two conversions.  IMHO, the 
mainboard dependent one *must* be done in userspace (as Jean says).
But the conversion of raw register values to volts should happen in
the kernel; fixed point maths are sufficient for that.

(Yes I'm simplifying; external sensors can present data to sensors
chips in other electrical formats, e.g. PWM.  Same argument applies.)

Regards,

-- 
Mark M. Hoffman
mhoffman@lightlink.com