linux-kernel - Re: [PATCH] acerhdf: Fix fan activation with new thermal governor

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Date:	Mon, 25 Feb 2013 11:36:04 +0100
From:	Peter Feuerer <peter@...e.net>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Zhang Rui <rui.zhang@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Durgadoss R <durgadoss.r@...el.com>,
	Borislav Petkov <bp@...e.de>, Andreas Mohr <andi@...as.de>,
	Alexander Lam <azl@...rew.cmu.edu>
Subject: Re: [PATCH] acerhdf: Fix fan activation with new thermal governor

Hi,

sorry for not answering to earlier mails, but sitting in front of the 
computer and writing English mails is still very exhausting for me.


Borislav Petkov writes:

> On Mon, Feb 25, 2013 at 11:06:40AM +0800, Zhang Rui wrote:
>> when the temperature goes up to 50C, will the fan be turned on or not?
> 
> Right, let's first clarify this:
> 
> so we want the fan to go on when it reaches and/or exceeds the 'fanon'
> temperature and to go off after it goes under the 'fanoff' temperature.
> It is supposed to remain on in the [fanoff,fanon] interval.
> 
> Now, my observation from yesterday was that, if we don't do the
> flattening of the abovementioned interval in acerhdf_set_cur_state() but
> we simply turn on the fan if we receive state=1 and turn it off when
> state=0, the fan gets turned on only for short bursts when the governor
> decides it is time to throttle, i.e. the temperature in the zone is
> rising and turns it off when the temperature starts to fall again. And
> it would turn on the fan at lower temperatures than fanon.
> 
> But this is annoying. The main reason why this acerhdf thing was added
> in the first place was to do a better fan control on those machines
> because the fans are noisy. Basically, enable the fan only when really
> needed - and leave it off when not because it is really annoying.
> 
> When enabled, however, leave it on longer until a safe, 'fanoff'
> temperature is reached.
> 
> Am I making more sense now? Peter?

This sounds good, couldn't have stated it more clear. Just one thing to add 
here: This is standard idea of a "schmitt trigger", we implemented 
in acerhdf :)
http://en.wikipedia.org/wiki/Schmitt_trigger

-- 
kind regards,
--peter;
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