linux-kernel - Re: [PATCH v3] k10temp: temperature sensor for AMD Family 10h/11h CPUs

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Date:	Fri, 15 Jan 2010 10:57:58 +0100
From:	Clemens Ladisch <clemens@...isch.de>
To:	Jean Delvare <khali@...ux-fr.org>
CC:	linux-kernel@...r.kernel.org, lm-sensors@...sensors.org
Subject: Re: [PATCH v3] k10temp: temperature sensor for AMD Family 10h/11h
 CPUs

Jean Delvare wrote:
> On Fri, 27 Nov 2009 14:03:29 +0100, Clemens Ladisch wrote:
> > +temp[1-*]_scale	Temperature scale type.
> > +		0: millidegrees Celsius (default if no _scale entry)
> > +		1: relative millidegrees Celsius; see below
> > +		2: millivolts; see below
> > +		other values: unknown
> 
> Maybe, yes. I am a little worried that older versions of libsensors
> will ignore this attribute. The good thing about this is that users
> will still get some value until they upgrade. The bad thing is that
> they will not know that the value isn't absolute. They are likely to
> get frightened by unexpected values and/or to complain to us about them.
> 
> I am wondering if a totally separate channel type wouldn't be
> preferable. The pros and cons would be inverted of course: older
> versions of libsensors would have zero support for that, and all
> applications would have to be updated to support it, but at least the
> meaning of the value would be totally clear. This would come at the
> price of some code duplication both in libsensors and applications
> though.
> 
> I guess it basically depends whether we want to consider a thermal
> margin as a "temperature measurement except that it's relative" or as
> something completely separate.

It's different from the millidegree/millivolt issue; millivolts can be
transparently converted by libsensors, while relative values must be
handled/displayed differently by the application.  So I think at least
in the libsensors API, relative values should be different.

(In any case, we should add temp#_scale at least for millivolts.)

> Honestly, I've been thinking about this
> for some time now and I simply don't know what we'd rather do :(

The sysfs interface is a somewhat internal interface; I think the main
question is whether old userspace (old libsensors or old apps using a
new libsensors) should be able to see relative values without knowing
that they are relative.

Coretemp and k10temp already exist and show relative values.  If we
introduced a new channel for relative values now, we would still have
problems with those drivers, so it might already be too late to avoid
problems for old userspace.


Best regards,
Clemens
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