linux-kernel - Re: [PATCH] dell-smm-hwmon: Cache fan_type() calls and use fan

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Date:	Fri, 27 May 2016 15:21:10 +0200
From:	Pali Rohár <pali.rohar@...il.com>
To:	Thorsten Leemhuis <fedora@...mhuis.info>
Cc:	Jan C Peters <jcpeters89@...il.com>,
	David Santamaría Rogado <howl.nsp@...il.com>,
	Peter Saunderson <peteasa@...il.com>,
	Jean Delvare <jdelvare@...e.com>,
	Guenter Roeck <linux@...ck-us.net>,
	Tolga Cakir <cevelnet@...il.com>, linux-hwmon@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dell-smm-hwmon: Cache fan_type() calls and use fan_status() for fan detection

On Friday 27 May 2016 15:05:54 Thorsten Leemhuis wrote:
> Pali Rohár wrote on 27.05.2016 12:45:
> > […]
> > Looks like there are two different problems with dell-smm-hwmon
> > driver: 1) Fan speed going randomly up and down without system
> > freeze […]
> > So for problem 1) I need to know:
> > 
> > * Is it regression? […]
> 
> Yes, it is known to be a regression from f989e55452, as identified
> by Jan in https://bugzilla.kernel.org/show_bug.cgi?id=100121#c13
> 
> I just verified and reverted that change on top of 4.6; the
> problem with the fan speed indeed goes away.

Ok, thanks for testing!

> So I tried a few things
> and came to the conclusion: the problem shows up as soon as
> i8k_get_fan_type() (introduced in f989e55452) is called somewhere.

So, once kernel call i8k_get_fan_type() function, then fan speed going 
up/down? Even if it was called only at once? Can you confirm it? Caching 
patch cause that for each fan that function is called exactly one time.

If this is problem, we can probably create DMI list of machines which do 
not like i8k_get_fan_type() call and disable it for them.

To make sure that this is root of your problem, can you take some older 
kernel version (where is i8k working fine) and try to patch+call that 
i8k_get_fan_type() function? To check that something else cannot 
interference with it...

> Find below the minimal patch I could come up to that makes the fan
> act normal on the Studio 8000 I have here (it's just meant as a
> reference and not meant to be applied, as it leaves unused functions
> around).

Patch just make sure that i8k_get_fan_type() is never called.

-- 
Pali Rohár
pali.rohar@...il.com

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