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@xxxxxxxxxxxxx
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