Advanced Configuration and Power Interface Specification

Processor Configuration and Control
8.4.7.1.2.3 Desired Performance Register
Optional (depending on Autonomous Selection support)
Register Location: PCC or Functional Fixed Hardware
Attribute:
Read/Write
Size:
8-32 bits
When Autonomous Selection is disabled, the Desired Performance Register is required and
conveys the performance level OSPM is requesting from the platform. Desired performance may be
set to any performance value in the range [Minimum Performance, Maximum Performance],
inclusive. Desired performance may take one of two meanings, depending on whether the desired
performance is above or below the guaranteed performance level.
• Below the guaranteed performance level, desired performance expresses the average
performance level the platform must provide subject to the Performance Reduction Tolerance.
• Above the guaranteed performance level, the platform must provide the guaranteed performance
level. The platform should attempt to provide up to the desired performance level, if current
operating conditions allow for it, but it is not required to do so
When Autonomous Selection is enabled, it is not necessary for OSPM to assess processor workload
performance demand and convey a corresponding performance delivery request to the platform via
the Desired Register. If the Desired Performance Register exists, OSPM may provide an explicit
performance requirement hint to the platform by writing a non-zero value. In this case, the delivered
performance is not bounded by the Performance Reduction Tolerance Register, however, OSPM can
influence the delivered performance by writing appropriate values to the Energy Performance
Preference Register. Writing a zero value to the Desired Performance Register or the non-existence
of the Desired Performance Register causes the platform to autonomously select a performance level
appropriate to the current workload.
Note: The Desired Performance Register is optional only when OPSM indicates support for CPPC2 in
the platform-wide _OSC capabilities and the Autonomous Selection Enable field is encoded as an
Integer with a value of 1.
8.4.7.1.2.4 Performance Reduction Tolerance Register
Optional
Register Location: PCC or Functional Fixed Hardware
Attribute:
Read/Write
Size:
8-32 bits
The Performance Reduction Tolerance Register is used by OSPM to convey the deviation below
the Desired Performance that is tolerable. It is expressed by OSPM as an absolute value on the
performance scale. Performance Tolerance must be less than or equal to the Desired Performance. If
the platform supports the Time Window Register, the Performance Reduction Tolerance conveys the
minimal performance value that may be delivered on average over the Time Window. If this register
is not implemented, the platform must assume Performance Reduction Tolerance = Desired
Performance.
When Autonomous Selection is enabled, values written to the Performance Reduction Tolerance
Register are ignored.
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