Date:	Sun, 10 Jan 2010 15:45:49 +0100
From:	Jean Delvare <khali@...ux-fr.org>
To:	Clemens Ladisch <clemens@...isch.de>
Cc:	linux-kernel@...r.kernel.org, lm-sensors@...sensors.org
Subject: Re: [PATCH v3] k10temp: temperature sensor for AMD Family 10h/11h 
 CPUs

Hi Clemens,

Sorry for the late answer...

On Fri, 27 Nov 2009 14:03:29 +0100, Clemens Ladisch wrote:
> Jean Delvare wrote:
> > On Wed, 25 Nov 2009 10:51:38 +0100, Clemens Ladisch wrote:
> > > temp1_input: -1000
> > > temp1_max: 40000
> > > temp1_relative: 0
> > > Should the values be labeled as "1 °C below normal" and "40 °C above
> > > normal", and how should the application know that 0 is to be labeled
> > > "normal"?  It might make more sense to display the temperature just as
> > > "41 °C below max", in which case the actual value of temp1_relative is
> > > not used at all.
> > 
> > Except that there may be no temp1_max, just a temperature value
> > relative to the "normal" operating point of the CPU. In that case we
> > can't fallback to the max limit.
> > 
> > Even your initial proposal doesn't work there yet: the hwmon interface
> > has no standard name for "normal operating temperature", so we can't put
> > that name in temp#_relative. [...]
> > If the base has a meaning (normal operating temperature, or critical
> > temperature, etc.) we have to let the user know somehow.
> 
> I chose that example because "normal" does not exist; and it's a bad
> example because "normal" actually has a meaning.

Indeed, we may want to add an standard name for this.

> Better take the AMD CPUs: The base of all relative values is zero (by
> definition), _not_ 70000, and the meaning of that base is just "70 °C
> below the temperature at which the processor wants 100% cooling".  This
> base value is meaningless for any monitoring purposes.

Correct. That being said, nothing prevents us from changing the base to
whatever we want it to be, pretty much by definition of relative
temperature reports. For example we could report 0 for temp1_max and
(register value - 70000) for temp1_input.

> If any point on the scale has a meaning, it should be reported with some
> temp#_whatever file.  However, the base itself does not necessarily have
> any meaning.
>
> As long as we have some corresponding _max or _crit limit that can be
> used for comparisons, we do not need a base value.  Only if there is
> no known predefined limit do we need a temp#_relative value.

I agree (again, unless we always change the base to be one of these
meaningful points... but I'm not sure if we want to do this.)

> > Or maybe create a new label (temp#_relative_label or similar) but I'm
> > not sure how we would integrate this into libsensors and applications.
> > In particular I am worried about translation issues if we make the
> > drivers too verbose.
> 
> All known CPUs with relative temperature scale also have known _max
> limits,

Not correct. For some variants of the Intel Core, we do not known the
max limit. I'm not even sure if the max limit value exists.

> and I don't think that a CPU with relative scale and both
> unknown _max and _crit will ever be designed.  In other words,
> temp#_relative* is not needed at the moment.  I think we should not
> try to define how the semantics of such an unknown scale can be
> described.

I agree that we shouldn't waste time designing an interface for
something that doesn't exist. All I'm saying is that, if we are adding
something to the interface today and it's not trivial, we should make
it reasonably flexible so that we don't have to do it all again next
year or so. Otherwise the interface could turn quite complex and
unappealing.

> > > > Additionally it wouldn't fit in libsensors as it exists today.
> > > 
> > > Then the best bet would probably be an entry like temp#_unit, with
> > > 0 = absolute °C (default); 1 = relative °C or °K; other values
> > > "unknown".  Even if some silly scale is introduced later, applications
> > > that read this entry then know that they must not display a unit like °C
> > > for unknown unit specifications.
> > 
> > This could work, yes. Note that current drivers and libsensors don't
> > have/know about this file yet, and they generally use an absolute °C
> > scale. So the absence of temp#_unit file would be interpreted exactly
> > as if the file was there and contained value 0.
> > 
> > (I'd rather name that file temp#_scale - but that's an implementation
> > detail.)
> 
> Like this?
> 
> --- a/Documentation/hwmon/sysfs-interface
> +++ b/Documentation/hwmon/sysfs-interface
> @@ -314,6 +314,19 @@ temp_reset_history
>  		Reset temp_lowest and temp_highest for all sensors
>  		WO
>  
> +temp[1-*]_scale	Temperature scale type.
> +		Integer
> +		RO
> +		0: millidegrees Celsius (default if no _scale entry)
> +		1: relative millidegrees Celsius; see below
> +		2: millivolts; see below
> +		other values: unknown
> +		When scale=1 (relative), the temperature value 0 does not
> +		correspond to zero degrees Celsius but to some unknown
> +		temperature. In this case, temperate values should not be
> +		interpreted or displayed as absolute values and make sense
> +		only when compared to other values of the same channel.
> +
>  Some chips measure temperature using external thermistors and an ADC, and
>  report the temperature measurement as a voltage. Converting this voltage
>  back to a temperature (or the other way around for limits) requires

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

> Hmm, which drivers use millivolt temperatures?

vt1211, vt8231, pc87360. These devices use thermistors for temperature
measurements, they measure the voltage at the pin (exactly as for a
voltage input) and leave it to user-space to turn the voltage value
back into a temperature value. The exact conversion formula depends on
the thermistor model and the value of the resistor used for the second
part of the bridge.

Note that virtually every chip monitoring voltages can be used to
monitor temperature instead, using thermistors. And this was seen once
already:
  http://article.gmane.org/gmane.linux.drivers.sensors/14564/
We do not have support for this at the moment though.

-- 
Jean Delvare
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