linux-kernel - Re: Dell Latitude E6440 & i8k

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Date:	Sat, 17 May 2014 15:18:54 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Jean Delvare <jdelvare@...e.de>
CC:	Pali Rohár <pali.rohar@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Dell Latitude E6440 & i8k

On 05/17/2014 02:09 PM, Jean Delvare wrote:
> On Sat, 17 May 2014 08:25:38 -0700, Guenter Roeck wrote:
>> On 05/16/2014 12:11 PM, Jean Delvare wrote:
>>> Load the i8k driver with fan_mult=1.
>>
>> Would it make sense to change the default multiplier to 1 ?
>> Lots of people have problems with it, and trying to figure out
>> affected machines one by one would be an all but impossible task.
>
> That would cause a regression on many (presumably older) machines. I
> doubt this is acceptable. One option would be to use the ACPI year to
> change the default, if indeed all new machines need fan_mult=1. I don't
> know if this is the case.
>
> One this I had in mind was auto-detecting the scaling factor. AFAIK
> only 30 and 1 are possible values, so any value above ~300 would imply
> scaling factor of 1 (30 would make it > 9000 RPM which is unrealistic.)
> But I don't know if we can actually do that, as such a simple heuristic
> could easily fail is the fan is stopped (30 * 0 == 1 * 0) or if the
> returned raw speed is temporarily unreliable for whatever reason.
>
Sounds like an idea. We could make the cutoff higher, such as 500 or
even 1000. I am not much concerned about 0 rpm - the code could simply check
the returned rpm and adjust the scaling factor to 1 if the reading is too
high. Since 30*0 is still 0, there is no problem if the fan is stopped.

> I have to admit that working on a reverse engineered driver for
> hardware I don't even have isn't quite at the top of my to-do list.
>

;-). We have several of those systems, so there is at least some interest
on my side. If I have time, I'll play around with it. The driver does need
major cleanup, though, so that may take a while.

Guenter

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