Advanced Configuration and Power Interface Specification

Advanced Configuration and Power Interface Specification
The registers are optional, and if the feature is not present the platform must use a NULL register of
the following form:
ResourceTemplate() {Register {(SystemMemory, 0, 0, 0, 0)}}
The Usage Count register counts how many times the local state has been used. Whether it counts
entries or exits is implementation defined.
The residency register counts how long the hierarchy node has been in the given LPI state, at a rate
given by LPI’s residency counter frequency field. A frequency of 0 indicates that the counter runs at
an architectural-specific frequency.
8.4.4.3.7 Wake from LPI States
With _LPI, the platform can describe deep S0-idle states which may turn off fundamental resources
like bus clocks, interrupt controllers, etc. so special care must be taken to ensure that the platform
can be woken from these states. This section describes handling for device initiated wakes. There
are other wake sources such as timers, which are described elsewhere.
For device wakes, the requirement is that OSPM must not enter any LPI state that would prevent a
device enabled for wake from waking the system. This means not entering any LPI state for which
any Power Resource listed in _RDI (see the _RDI section Section 8.4.4.4) is required to be ON. Note
that on a platform coordinated system, the OSPM may choose to enter an _LPI state even if there are
resources listed in its companion RDI that are still on. However, if the OSPM has already enabled a
device for wake, and ensured the power resources needed for wake are on, the platform will demote
the LPI state to one where said resources remain on.
The wake device uses the standard _PRx and _PRW methods to describe power resources it requires
to be ON based on its D-state and wake enabled status. This further implies that any device enabled
for wake which depends on a resource which may be turned off as part of an LPI state must describe
that dependency via _PRx/_PRW => _RDI => _LPI.
This is illustrated in the following example:
456 April, 2015 Version 6.0