Advanced Configuration and Power Interface Specification

Advanced Configuration and Power Interface Specification
8.4.7.1.2.5 Time Window Register
Optional
Register Location: PCC or Functional Fixed Hardware
Attribute:
Read/Write
Size:
8-32 bits
Units:
milliseconds
When Autonomous Selection is not enabled, OSPM may write a value to the Time Window
Register to indicate a time window over which the platform must provide the desired performance
level (subject to the Performance Reduction Tolerance). OSPM sets the time window when electing
a new desired performance The time window represents the minimum time duration for OSPM’s
evaluation of the platform’s delivered performance (see Section 8.4.7.1.3.1 “Performance Counters”
for details on how OSPM computes delivered performance). If OSPM evaluates delivered
performance over an interval smaller than the specified time window, it has no expectations of the
performance delivered by the platform. For any evaluation interval equal to or greater than the time
window, the platform must deliver the OSPM desired performance within the specified tolerance
bound.
If OSPM specifies a time window of zero or if the platform does not support the time window
register, the platform must deliver performance within the bounds of Performance Reduction
Tolerance irrespective of the duration of the evaluation interval.
When Autonomous Selection is enabled, values written to the Time Window Register are ignored.
Reads of the Time Window register indicate minimum length of time (in ms) between successive
reads of the platform's performance counters. If the Time Window register is not supported then
there is no minimum time requirement between successive reads of the platform's performance
counters.
8.4.7.1.3 Performance Feedback
The platform provides performance feedback via set of performance counters, and a performance
limited indicator.
8.4.7.1.3.1 Performance Counters
To determine the actual performance level delivered over time, OSPM may read a set of
performance counters from the Reference Performance Counter Register and the Delivered
Performance Counter Register.
OSPM calculates the delivered performance over a given time period by taking a beginning and
ending snapshot of both the reference and delivered performance counters, and calculating:
delivered performance = reference performance
X
∆delivered performance counter
∆reference performance counter
The delivered performance should always fall in the range [Lowest Performance, Highest
Performance], inclusive. OSPM may use the delivered performance counters as a feedback
mechanism to refine the desired performance state it selects.
When Autonomous Selection is not enabled, there are constraints that govern how and when the
performance delivered by the platform may deviate from the OSPM Desired Performance.
486 April, 2015 Version 6.0