Advanced Configuration and Power Interface Specification

Processor Configuration and Control
When the OS running on a given processor detects there is no more work to schedule on that
processor, it needs to select an idle state. The state may affect more than just that processor. A
processor going idle could be the last one in the system, or in a processor container, and therefore
may select a power state what affects multiple processors. In order to select such a state, the OS
needs to choose a local power state for each affected level in the processor hierarchy.
Cluster 0's children
System
Level ID: 3
_LPI:
1: Power Down
Cluster0's local states
LevelID: 2
_LPI:
1: Clock Gate
2: Retention
3: Power Down
Cluster
0
Cluster1
LevelID: 2
_LPI:
1: Clock Gate
2: Retention
3: Power Down
Shallower states
Deeper states
Core0 Core1 Core2 Core3
Core0's local states
LevelID: 1
_LPI:
1: Clock Gate
2: Retention
3: Power Down
LevelID: 1
_LPI:
1: Clock Gate
2: Retention
3: Power Down
LevelID: 1
_LPI:
1: Clock Gate
2: Retention
3: Power Down
LevelID: 1
_LPI:
1: Clock Gate
2: Retention
3: Power Down
Figure 8-46 Power states for processor hierarchy
Consider a situation where Core 0 is the last active core depicted in the example system, Figure 8-
46. It may put the system into the lowest possible idle state. To do so, the OS chooses local state 3
(Power Down) for Core0, local state 3 (Power Down) for Cluster0, and local state 1 (Power Down)
for the system. However, most HW architectures only support a single power state request from the
OS to the platform. That is, it is not possible to make a separate local power state request per
hierarchy node to the platform. Therefore, the OS must combine the per level local power states into
a single Composite power state. The platform then acts on the Composite power state request.
A platform can only support a limited set of Composite power states, and not every combination of
Local Power states across levels is valid. The valid power states in our example system are depicted
in the following table:
Table 8-239 Valid Local State Combinations in Figure 2 example system
System Level Processor
Container
Cluster level Processor
Container
Processor
Running Running Clock Gated
Running Running Retention
Running Running Power Down
Running Clock Gated Clock Gated
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