Re: removing an envsys temperature limit

["Constantine A. Murenin" <cnst%NetBSD.org@localhost>]

On 16 February 2016 at 04:47, Edgar Fuß <ef%math.uni-bonn.de@localhost> wrote:
>> Is this a continuation of your prior thread:
>>     http://mail-index.netbsd.org/tech-userlevel/2016/01/30/msg009639.html ?
> No. That was about a 54 degC CritMax for CPU Temp.

So, this is about the 45degC CritMin part?  I see.

Full envsys output from the prior thread for reference:
    http://mail-index.netbsd.org/tech-userlevel/2016/02/01/msg009646.html

>
>> If so, what you're effectively asking is for
>> http://bxr.su/n/sys/dev/i2c/dbcool.c#dbcool_get_temp_limits to stop
>> reading the limits from the chip, which I don't think is supported.
> It doesn't report any limit for sensor0 (l_temp), so there must be a way of
> having "no limit".
>
>> Did you find out what was causing the limit to be low in the first place?
> The low CritMax the other thread was about? No. It must be the BIOS, but
> there's nothing I can sensibly do about that.

I am curious on your resolution -- when you modify these limits
through envsys, the values are supposed to get written back into the
chip (http://bxr.su/n/sys/dev/i2c/dbcool.c#dbcool_set_temp_limits).

Do they persist between the reboots and cold boots?  Or does CritMax
change back to 54degC at certain times?

>> Why do you want to remove the limit, instead of setting it to an actual
>> reasonable value?
> I did set the CritMax limit to 70 degC in envsys.conf.
>
>
> This thread is about a CritMin CPU Temp limit of 45 degC.
> I don't think there's a sensible minimal CPU temperature. Maybe AMD specifies
> a minimum temperature of, say, -20 degC for the Opteron 246, but, what shall
> I do if I fall below that? Start some idle jobs to heat the CPU?

It depends on the environment.  What if someone's trying to use liquid
nitrogen to cool down and steal your memory modules together with
their content (like passwords and private keys)?  Or your datacentre
is in the North, and a wall or window broke down, and a snow storm is
coming? :-)

>
> Of course, I can set the limit to some ridiculously low value, but that's
> still displayed in envstat output.

No, depending on your chip, you might be limited to 0degC on the low
end, which is not that ridiculously low.

http://bxr.su/n/sys/dev/i2c/dbcool.c#dbcool_set_temp_limits

2056        if (sc->sc_temp_offset) {
2057            limit += sc->sc_temp_offset;
2058            if (limit < 0)
2059                limit = 0;
2060            else if (limit > 255)
2061                limit = 255;

But some chips supported by dbcool let you go down to -127degC (at
least as far as the register settings go!), which is still not /that/
ridiculously low when you consider the absolute zero of -273,15degC.

>
> One could argue that a CPU temp reading below 45 degC must be a sensor failure,
> so it's OK to shut down.

Hardly; but it certainly depends on the CPU, system load and the
ambient temperature.

C.