From mboxrd@z Thu Jan 1 00:00:00 1970 From: Matthew Garrett Subject: Re: [lm-sensors] Fintek f71882fg ACPI conflict Date: Thu, 28 Jun 2012 02:45:49 +0100 Message-ID: <20120628014548.GA15994@srcf.ucam.org> References: <4FEA4C10.2060904@googlemail.com> <20120627171505.GB12712@roeck-us.net> <4FEB90A8.8050901@googlemail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from cavan.codon.org.uk ([93.93.128.6]:44736 "EHLO cavan.codon.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750806Ab2F1BqD (ORCPT ); Wed, 27 Jun 2012 21:46:03 -0400 Content-Disposition: inline In-Reply-To: <4FEB90A8.8050901@googlemail.com> Sender: linux-acpi-owner@vger.kernel.org List-Id: linux-acpi@vger.kernel.org To: Michael Zintakis Cc: Guenter Roeck , lm-sensors@lm-sensors.org, Len Brown , linux-acpi@vger.kernel.org On Thu, Jun 28, 2012 at 12:00:56AM +0100, Michael Zintakis wrote: > This is precisely what I stumbled upon when reading the > documentation about the kernel acpi parameters today and tried it > straight away (though, in all honesty, I didn't have a clue that it > is actually a bad idea until I read the article above - thanks for > posting!). Could any of the more knowledgeable on here explain why > is it such a bad idea please? Because there's no handshaking between the firmware and the OS driver, and accesses to hardware sensors are often indexed. Imagine this scenario: ACPI lmsensors ---- --------- select temperature register select fan speed register read value read value ACPi will read the fan speed register instead of the temperature register, and the value may be far too high and cause a critical shutdown of the system. That's the *good* outcome. The bad outcome involves these register accesses racing in a way that disables fan control or thermal trip points and risks causing hardware damage. It's not safe for two different codepaths to access the same hardware without having any kind of locking, so if your system firmware declares that ACPI is using the temperature device the hardware sensors framework will refuse to. -- Matthew Garrett | mjg59@srcf.ucam.org