[Fw_Os_Forum] Integration of cooling preferences for ACPI thermal zones

[Armin Wolf]

Hello,

i am currently integrating support for the _SCP ACPI control method into the ACPI thermal zone
driver inside the Linux kernel. During this i stumbled upon the following section inside the
ACPI specification:

	11.1.2.1 OSPM Change of Cooling Policy

	When OSPM changes the platform’s cooling policy from one cooling mode to the other, the following occurs:

	1. OSPM notifies the platform of the new cooling mode by running the Set Cooling Policy (_SCP) control method
	   in all thermal zones and invoking the OS-specific Set Cooling Policy interface to all participating devices in
	   each thermal zone.

	2. Thresholds are updated in the hardware and OSPM is notified of the change.

	3. OSPM re-evaluates the active and passive cooling temperature trip points for the zone and all devices in the zone
	   to obtain the new temperature thresholds.

This section seems to mandate that the system cooling policy is a global setting. In this case having different
_SCP values across two ACPI thermal zones would be considered invalid. In section 11.3.13 however the _SCP control
method is said to only affect the individual ACPI thermal zone.

I am now asking myself if treating the system cooling policy as a global setting is mandated by the ACPI specification
or should only be treated like a suggestion. The thing is that some firmware implementations might depend on the system
cooling policy being the same across all thermal zones inside a given system, so having some clarity here would be nice.

Thanks,
Armin Wolf