ACPI Device Performance States

This page captures some of the new proposals to ACPI to support device performance states.

The changes have been prototyped with Linux kernel DVFS framework and have not yet been discussed on public forums yet.

Device Performance States

Device performance states support is implemented through two optional objects

Device Performance Control Object ( _DPC )
Device Performance States Object ( _DPS )

If the platform and device support multiple performance states then the above two objects should be implemented for the device.

Device Performance Control Object ( _DPC )

_DPC provides the binding to place the device in a specific performance state along with various metadata.

Arguments:

None

Returns:

A package containing performance control information for the device.

Package
{
    NumEntries,           // Integer
    Revision,             // Integer
    NominalPerformance,   // Integer or Buffer (Resource Descriptor)
    NominalFrequency,     // Integer or Buffer (Resource Descriptor)
    DesiredPerformance,   // Buffer (Resource Descriptor)
    DeliveredPerformance, // Buffer (Resource Descriptor)
    SustainedPerformance, // Buffer (Resource Descriptor)
    PerformanceDomain,    // Integer or Buffer (Resource Descriptor)
})

Element	Object Type	Description

Element	Object Type	Description
Num Entries	Integer	Number of entries in the _DPC package
Revision	Integer	Revision number
Nominal Performance	Integer or Register Buffer	Performance level value that corresponds to the highest sustained performance level without considering any contraints.
Nominal Frequency	Integer or Register Buffer	Base frequency (KHz) corresponding to the nominal performance level.
Desired Performance	Register Buffer	Register to write the desired performance level.
Delivered Performance	Register Buffer	Register to read to get the current delivered performance level.
Sustained Performance	Register Buffer	This is an optional object providing OSPM the current maximum sustained performance level taking into account all known external constraints (power budgeting, thermal constraints, power source, etc.). A notify event will generated by the platform if the sustained performance changes.
Performance Domain	Integer or Register Buffer	This is an optional object providing the performance domain identifier. OSPM can use this to coordinate performance states transition for devices in same performance domain.

All _DPC registers can be in PCC, System Memory, System IO, or Functional Fixed Hardware address spaces.

Device Performance States Object ( _DPS )

_DPS defines all of the supported device performance states for the device.

Arguments:

None

Returns:

A variable-length Package containing a list of performance states sub-packages as described below

Package
{
        P0,
        P1,
        ...
        Pn
})

Each performance state sub-Package contains the elements described below:

Performance Level

Performance level provided on an abstract integer scale.

The implementation can decide the scale it represents, it can be actual frequencies or in percentage of the maximum performance domain etc. However, the scales must be linear.

Power

Power dissipation in Milliwatts

Latency

Performance state transition latency in microseconds

Example

Device(DDR1)
  {
      Name (_HID, "LIN0002")  // _HID: Hardware ID
      Name (_UID, One)  // _UID: Unique ID
      Method (_STA)
      {
          Return(0xF)
      }

      // Device performance states
      Name (_DPS, Package()
      {
              /* performance level(abstract), power cost(mW), transition latency(us) */ 
              Package(){25, 1800, 0},
              Package(){100, 1800, 0},
              Package(){750, 1800, 0}
      })

      Name (_DPC, Package ()
      {
          // Num Entries
          9,
          // Revision
          0,
          // Nominal Performance (abstract scale) - Highest sustained performance level
          750,
          // Nominal Frequency (KHz)
          750000,
          // Desired Performance Register
          ResourceTemplate(){Register(FFixedHW, 0, 0, 0xFFFFFFFFC200000E)},
          // Delivered Performance Register
          ResourceTemplate(){Register(FFixedHW, 0, 0, 0xFFFFFFFFC200000F)},
          // Sustained Performance Level Register (Optional) - Current max. perf considering all constraints
          ResourceTemplate(){Register(FFixedHW, 0, 0, 0xFFFFFFFFC2000011)},
          // Performance Domain Index (Optional)
          1,
      })

  }

Linux Kernel Prototype

The changes are currently in the following merge request

https://gitlab.com/LinaroLtd/clientpc/linux/-/merge_requests/10

It contains following changes

Prototype implementation of device performance state framework
An example implementation of I.MX DDR DevFreq driver with ACPI device performance states
Bug fixes to Linux OPP framework

Note: The prototype uses a specific FFH address space for SMC calls and is not part of ARM FFH spec yet.