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[Xen-devel] Passive and Active ACPI Thermal control under Xen?


With summer in full swing these days, I've been trying to somehow cool
down my laptop, which brought me into tinkering with various ACPI stuff,
and raised a few questions on how Xen and Dom0 cooperate to enforce
thermal policy.

I'm using Xen 4.1 booted *without* cpufreq=dom0-kernel and I can see
xenpm returning all the various info about my processors' C and P
states. I can even set the policy, e.g.:

xenpm set-scaling-gov powersave

The only problem being it has absolutely zero impact on the system

According to the ACPI spec, there are two primary methods for thermal
control of the platform: 1) active control and 2) passive control. The
former works by OSPM enabling various fans at different speeds,
depending on the measured temperatures from various sensors, while the
latter works by throttling down the CPU frequencies and perhaps by
controlling transitions to the deeper C states.

My understanding is that the cpufreq mechanism in Xen is essentially an
implementation of the passive thermal control? If this is so, does Xen
honor the ACPI-defined _TC1 and _TC2 constants (that one usually can set
in the BIOS)? I could imagine that this should be the correct behavior
when one selects the 'powersave' governor, shouldn't it?

Next question relates to the active thermal control -- who is in charge
of spinning the fans? My understanding is that on a baremetal Linux,
this is the job of the thermal.ko module? (which also apparently should
take care about the passive control, although not when booted under Xen)

Unfortunately I cannot get my cooling fan to spin :( I tried setting the
temp thresholds in BIOS for when the fan should be spinning at low and
high speed (_AC0, _AC1), and even pass them directly to the thermal.ko
module (as 'act' param), but my fan remain silent even though I can see
the temperature going above the thresholds.

The only moment I can hear my fan spinning at full speed is during early
boot stage (I assume this is when the _AC0, _AC1 constants I set in BIOS
are actually honored by the SMI handler), but then, after booting into
Xen and Dom0 the fan is being turned off.

Xen 4.1.0, Dom0 kernel: 2.6.38 (xenlinux), platform: Sony Vaio Z12


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