ACPI Device Performance States

ACPI defines performance states for processors and devices but only provides standard bindings for processor performance states via P-States and CPPC bindings. For devices, performance states are defined by the buses like PCIe, USB, etc., and no standard bindings are defined by ACPI.

In this section, we are exploring generic ACPI-related bindings to define performance states for platform devices.

ACPI Definition for Device Performance States

From section 2.3.1 Device Performance States:

Device performance states (Px states) are power consumption and capability states within the active (D0) device power state. Performance states allow OSPM to make tradeoffs between performance and energy conservation. Device performance states have the greatest impact when the implementation is such that the states invoke different device efficiency levels as opposed to a linear scaling of performance and energy consumption. Since performance state transitions occur in the active device states, care must be taken to ensure that performance state transitions do not adversely impact the system. Device performance states, when necessary, are defined on a per device class basis (See Appendix A: Device Class Specifications for more information).

ACPI Bindings Required

_DPT

ACPI object to describe polling time in milliseconds.

This will be the frequency at which the OSPM will poll the device and change the performance states.

_DPS

ACPI object to get supported device performance states (Px)

The format of each Px state is defined as follows


Device Frequency	Device core frequency in MHz
Power	Power dissipation in Milliwatts
Latency	Performance state transition latency in nanoseconds

        Name (_DPS, Package()
        {
                P0,
                P1,
                ...
                Pn
        })

Device Frequency and Power are only indication to OSPM for the performance levels and OSPM should not rely on the values.

_DPC

OSPM can use this method to choose a performance level for the device.

Method(_DPC, 1, Serialized)
{
    ...
}

_DPU

OSPM can use this method to get the device current utilization in percentage.

Method(_DUT, 0, Serialized)
{
   ....
}

_DPA

OSPM can use this method to check the available device performance levels.

Method(_DPA, 0, Serialized)
{
   Return <available starting Px index>
}

_DPA will return the first available Px index that OSPM can use for dynamic performance level setting.

Example with I.MX8MP DDR Controller

Device utilization is derived from PMU counters. TF-A implements functions that can be invoked from AML via FFH and SMCC to collect the device utilization and configure the DDR frequency.

ASL Code

 Device(DDR1)
 {
        Name (_HID, "NXP099")  // _HID: Hardware ID
        Name (_UID, One)      // _UID: Unique ID
        Method (_STA)
        {
            Return(0xF)
        }

        Method (_CRS, 0x0, NotSerialized)
        {
            Name ( RBUF, ResourceTemplate () {
              ClockInput(100000, 1, Hz, Variable, “\\_SB.CLK”,)
            })
            Return(RBUF)
        }
  
        // Polling time in miliseconds
        Name(_DPT, 12)

        // Get Device Performance States
        Name (_DPS, Package()
        {
                Package(){200, 1800, 100}, // Performance State zero (P0)
                Package(){400, 1800, 100}, // Performance State zero (P1)
                Package(){800, 1800, 100}, // Performance State zero (P2)
        })

        // Configure Device Performance States
        // does it have to be asynchronous ???
        Method(_DPC, 1, Serialized)
        {
            If (Arg0 == 0) {
                local0 = 200
            } Elseif (Arg0 == 1){
                local0 = 400
            } Else {
                local0 = 800
            }
            // SMC call
            OperationRegion(AFFH, FFixedHW, 1, 24)
            Field (AFFH, BufferAcc, NoLock, Preserve) { SMCC, 192 }
            Name(BUFF, Buffer(24){})
            CreateQWordField(BUFF, 0x00, BFX0)
            CreateQWordField(BUFF, 0x08, BFX1) // X1
            CreateQWordField(BUFF, 0x10, BFX2) // X2

            BFX0 = 0xc2000004 // IMX_SIP_DDR_DVFS FID
            BFX1 = local0 // target frequency
            BFX2 = 0x1010101 // online CPUs - assumes all of them are online
            BUFF = (SMCC = BUFF)
            
            Return (BFX0)
        }

        // Device utilization in percentage
        Method(_DPU, 0, Serialized)
        {
            // SMC call
            OperationRegion(AFFH, FFixedHW, 1, 8)
            Field (AFFH, BufferAcc, NoLock, Preserve) { SMCC, 64 }
            Name(BUFF, Buffer(8){})
            CreateQWordField(BUFF, 0x00, BFX0)

            BFX0 = 0xc200000D //  FID
            BUFF = (SMCC = BUFF)

            Return (BFX0)
        }

        // Device Performance States Availability
        Method(_DPA, 0, Serialized)
        {
            Return (0) // all Pn states available
        }

    }