Advanced Configuration and Power Interface Specification

Advanced Configuration and Power Interface Specification
Note: FFH is permitted and applicable to both full and HW-reduced ACPI implementations.
4.1.1 Hardware-Reduced Events
HW-reduced ACPI platforms require alternatives to some of the features supported in the ACPI HW
Specification, where none already exists. There are two areas that require such alternatives: The
ACPI Platform Event Model, and System and Device Wakeup.
4.1.1.1 GPIO-Signaled Events
General Purpose Input/Output (GPIO) hardware can be used for signaling platform events. GPIO
HW is a generalization of the GPE model, and is a shared hardware resource used for many
applications. ACPI support for GPIO is described in section 3.11.3, "Connection Resources".
ACPI Events are signaled by GPIO interrupt connections, which describe the connection to a GPIO
controller and pin, and which are mapped to the ACPI Event Handling mechanism via the ACPI
Event Information namespace object (_AEI). OSPM treats GPIO Interrupt Connections listed in
_AEI exactly as it does SCI interrupts: it executes the Event Method associated with the specific
event. The name of the method to run is determined by the pin information contained in the GPIO
Interrupt Connection resource.
GPIO-signaled events can also be wake events, just as GPE events can on traditional ACPI
platforms. Designating which events are wake events is done through attributes of the GPIO
Interrupt Connection resource used.
4.1.1.2 Interrupt-based Wake Events
Wake events on HW-reduced ACPI platforms are always caused by an interrupt reaching the
processor. Therefore, there are two requirements for waking the system from a sleep or low-power
idle state, or a device from a low-power state. First, the interrupt line must be Wake-Capable. Wakecapable
interrupts are designed to be able to be delivered to the processor from low-power states.
This implies that it must also cause the processor and any required platform hardware to power-up so
that an Interrupt Service Routine can run. Secondly, an OS driver must enable the interrupt before
entering a low-power state, or before OSPM puts the system into a sleep or low-power idle state.
Wake-capable interrupts are designated as such in their Extended Interrupt or GPIO Interrupt
Connection resource descriptor.
4.2 Fixed Hardware Programming Model
Because of the changes needed for migrating legacy hardware to the fixed category, ACPI limits the
features specified by fixed hardware. Fixed hardware features are defined by the following criteria:
• Performance sensitive features
• Features that drivers require during wake
• Features that enable catastrophic OS software failure recovery
ACPI defines register-based interfaces to fixed hardware. CPU clock control and the power
management timer are defined as fixed hardware to reduce the performance impact of accessing this
hardware, which will result in more quickly reducing a thermal condition or extending battery life. If
this logic were allowed to reside in PCI configuration space, for example, several layers of drivers
58 April, 2015 Version 6.0