linux-kernel - Re: [PATCH] ACPI thermal: do not always return THERMAL_TREND

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Message-ID: <20130427183800.GA8929@amd.pavel.ucw.cz>
Date:	Sat, 27 Apr 2013 20:38:00 +0200
From:	Pavel Machek <pavel@....cz>
To:	Zhang Rui <rui.zhang@...el.com>
Cc:	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
	linux-acpi@...r.kernel.org, rjw@...k.pl, lenb@...nel.org,
	eduardo.valentin@...com
Subject: Re: [PATCH] ACPI thermal: do not always return
 THERMAL_TREND_RAISING for active trip points

Hi!

> > Is that even right algoritm?
> > 
> sorry the changelog is not clear enough here.
> I should say,
> "keep the fan on when the temperature is raising"
> "turn off the fan when the temperature is dropping"
> 
> > Assume I'm running at very cold room, lets say -10C. Assume idle CPU
> > will hover around 30C with no fan, or hover around 10C with fan
> > running, trip point being 50C.
> > 
> > You _could_ leave fan off until 50C, having silent, passively cooled
> > system.
> > 
> > What it will do instead is annoyingly pulse fan at 10C.
> > 
> First, fan will never spins on if the temperature never hits 50C.
> 
> let's see how ACPI 3.0 fan (just have on and off state) is supposed to
> work after overheat.
> 1. temperature goes above 50C
> 2. Fan starts to spin.
> 3. a) if temperature goes higher (if possible), fan keeps on.
>    b) if temperature drops, but still above 50C. the fan keeps on.
>    c) if temperature drops below 50C, the last temperature is above 50C,
> thus the fan is turned off.
>    d) if the temperature goes higher again, the fan will not spin on
> until the temperature is higher than the trip point (50C) again.

So the fan is spinning iff the temperature is over threshold, right?

> is this behavior what you want?

That's better, yes. Even better would be if it had some hysteresis:
turn the fan on at threshold temperature, but turn it off only if it
drops below temperature-5C, or if it is below threshold for 1 minute.

Thanks,
      	    		       	     	      		      Pavel

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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