Thermal Management

ACPI supports operating system directed thermal management. Prior to ACPI there was no unified interface for thermal management (e.g. APM did not support it). On some systems the platform specific BIOS code handles thermal management, but this is invisible to the operating system and few OS developers noticed it. Thermal management is becoming more important important as system efficiency increases and the system gets hotter when working.

The ACPI thermal management subsystem provides a way to get the current temperature, and it provides hints to control thermal policy. The thermal policy information includes information on how to get the system cooler and at what point the cooling method should be invoked. There are two ways to cool the system down: active cooling, which activates cooling devices such as fans, and passive cooling, in which the CPU operation slows down to decrease heat generation.

Thermal management is controlled in the ThermalZone section of the AML namespace. Note that in ACPI, all temperatures are in tenths of degrees kelvin. Important sections of the thermal management part of the ACPI namespace include:

_TMP:

gets the current temperature.

_ACx:

the temperature at which the system should switch to active cooling mode ``x.''

_ALx:

a list of objects that should be active when the system is in cooling mode ``x.''

_CRT:

the temperature at which we should halt the entire system.

_PSV:

the temperature at which the system should switch to passive cooling mode.

_PSL:

a list of objects (typically the CPU) that should be slowed in passive cooling mode.

_SCP:

a method that allows the operating system to set the cooling policy.

_TCx:

a parameter for passive cooling mode ``x.''

Note that in the event of a thermal emergency (e.g. a bug in ACPI software), ACPI allows the hardware to take over thermal management in order to protect the hardware from damage.