linux-kernel - Re: 2.6.22-rc1-mm1 [cannot change thermal trip points]

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Date:	Sat, 19 May 2007 14:56:49 -0500
From:	Thomas Renninger <trenn@...e.de>
To:	Len Brown <lenb@...nel.org>
Cc:	Pavel Machek <pavel@....cz>, Chuck Ebbert <cebbert@...hat.com>,
	len.brown@...el.com, Maciej Rutecki <maciej.rutecki@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
	torvalds@...ux-foundation.org
Subject: Re: 2.6.22-rc1-mm1 [cannot change thermal trip points]

On Thu, 2007-05-17 at 15:17 -0400, Len Brown wrote:
> On Thursday 17 May 2007 05:23, Pavel Machek wrote:
> 
> > >     ACPI: thermal trip points are read-only
> > 
> > What was the rationale? Can we get this one reverted? 
> > 
> > Some machines (HP omnibook xe3) have broken trip points -- too high --
> > so machine will overheat and trigger hw shutdown before starting
> > passive cooling.
> > 
> > That's really broken, and write to trip points is reasonable way to
> > 'fix' that. (I'd understand if you only ever let trip points to
> > decrease... but otoh root should be able to shoot himself....)
> 
> No, writing trip-points is neither a fix, nor it is reasonable.
> It is a workaround at best, and it is a dangerous and mis-leading hack.
Yes it is a workaround for critical ACPI bugs like that or similar:
https://bugs.launchpad.net/ubuntu/+source/linux-source-2.6.17/+bug/22336

It's also convenient to e.g. lower passive trip point to avoid fan
noise.

Some people are used to it, I already wanted to write a little userspace
prog to use them as it is really easy to fake cooling_mode (trip points
are modified by BIOS) and eliminate fan noise and other things by e.g.
reducing passsive or whatever trip point.

This is at least a major sysfs interface change, has this been discussed
somewhere before or declared deprecated?

It's there for a long time, why is this "a dangerous and mis-leading
hack." now?

I'd suggest to revert this and I can come with something like "only
allow lower values
than BIOS provides" patch if the current implementation is considered
dangerous.

      Thomas

> The OS has no capability to actually change the ACPI trip points
> that are used by the BIOS.  Changing the OS copy of them
> to make the user think that trip events will actually
> happen when the temperature crosses the OS copy is crazy.
> 
> If there are systems with broken thermals and the
> ACPI thermal control needs and over-ride to turn
> on the fan, then that is fine -- but using
> fake trip-points and giving the user the impression
> that they are real is not viable.


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