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Introduction to the Directed Power Management Framework

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Starting in Windows 10, version 1903, version 3 of the run-time power management framework (PoFx) provides an optional directed power model, Directed PoFx (DFx).

With DFx, the operating system directs device stacks to enter their appropriate low-power idle states when the system transitions to idle and there is no activator-brokered software activity, and thereby enables the system to enter low power more reliably.

The objective is to make systems more power-efficient and to reduce energy consumption for Windows devices across form factors.

DFx is currently supported for devices with D-state constraints only. DFx skips any device subtree with an F-state constraint.

DFx does not power down paging or debug devices.

Requirements for WDF (non-miniport) drivers

A WDF driver that is a power policy owner must implement an appropriate S0-Idle policy by specifying SystemManagedIdleTimeout or SystemManagedIdleTimeoutWithHint in the WDF_DEVICE_POWER_POLICY_IDLE_SETTINGS structure. This will allow the device to power down when it is idle. As an added resiliency measure, the driver can opt into DFx by adding the following registry key to the INF's AddReg directive section within the DDInstall.HW section:

HKR,"WDF","WdfDirectedPowerTransitionEnable",0x00010001,1

A WDF driver targeting version 31 and above will enable DFx by default. If this is undesired, the driver can opt out of DFx by setting the registry key to 0:

HKR,"WDF","WdfDirectedPowerTransitionEnable",0x00010001,0

A WDF driver targeting version 33 and above can alternatively opt out of DFx by setting the DirectedPoFxEnabled member of the WDF_POWER_FRAMEWORK_SETTINGS structure to WdfFalse.

Tip

To initialize its WDF_POWER_FRAMEWORK_SETTINGS structure, your driver should call WDF_POWER_FRAMEWORK_SETTINGS_INIT.

Because requesting system-managed idle timeout causes WDF to register with PoFx on the driver's behalf, the driver does not need to register with PoFx in this scenario.

If the driver specifies DriverManagedIdleTimeout, consider switching to system-managed idle timeout. If that is not feasible, use the guidelines in the WDM section below to opt into DFx.

If the WDF driver does not use runtime power management, add support for it and use system-managed idle timeout. To do so, provide an WDF_DEVICE_POWER_POLICY_IDLE_SETTINGS structure as input to WdfDeviceAssignS0IdleSettings.

Requirements for WDM (non-miniport) drivers

If your driver does not use the system-managed idle support provided by WDF (the driver is either a WDF driver using driver-managed idle, or a WDM driver), it can still get DFx support by registering itself with PoFx. In this scenario, the driver registers with PoFx by implementing:

Provide pointers to these callbacks in a PO_FX_DEVICE_V3 structure that is input to the PoFxRegisterDevice function.

To get DFx support, a driver must:

  • Provide the PO_FX_DIRECTED_POWER* callbacks when registering for PoFx
  • Call PoFxReportDevicePoweredOn from its PO_FX_DIRECTED_POWER_UP_CALLBACK callback function on return from idle. If this is a WDF driver, it can set a flag, and then in EvtDeviceD0Entry, check the flag and call PoFxReportDevicePoweredOn.
  • Call PoFxReportDevicePoweredOn on resume from Sx transition. If this is a WDF driver, it needs to preprocess IRP_MN_SET_POWER. The preprocessing callback should only proceed if Parameters.Power.Type == SystemPowerState. Because the device might stay in Dx state for the entirety of the sleep/resume cycle, the above approach of checking a flag in EvtDeviceD0Entry won't work. Instead, the EvtDeviceWdmIrpPreprocess event callback function should call IoSetCompletionRoutine to set an IoCompletion routine, and from the completion routine call PoFxReportDevicePoweredOn.

Example

The following example shows the self-registration option described above:

PO_FX_DEVICE_V3 MyPoFxDevice;
POHANDLE MyPoFxHandle;

RtlZeroMemory(&MyPoFxDevice, sizeof(PO_FX_DEVICE_V3));
MyPoFxDevice.Version = PO_FX_VERSION_V3;

// Initialize other PoFx callbacks and other fields like
// components and their idle states.

MyPoFxDevice.DirectedPowerUpCallback = <Driver's DFx power up callback>
MyPoFxDevice.DirectedPowerDownCallback = <Driver's DFx power down callback>

Status = PoFxRegisterDevice(
  <Driver's device object>,
  (PPO_FX_DEVICE)&MyPoFxDevice,
  &MyPoFxHandle);
  if (!NT_SUCCESS(Status)) {
  return Status;
}

If your driver specified PO_FX_VERSION_V1 previously, note that PO_FX_DEVICE_V3 structures uses PO_FX_COMPONENT_V2 for the component array structure.

Requirements for miniport drivers

For device classes that follow a port/miniport driver model, system-supplied port drivers typically handle power policy ownership. Most miniports are not expected to require any code changes to opt into DFx, as the corresponding port driver is expected to handle DFx support.

Guidance for third-party miniports of KS.sys

Starting with Windows 10, version 2004 (also known as 20H1 or build 19041), KS.sys opts-out of DFx and related HLK requirements by default. Third-party miniports of KS.sys can opt-in to DFx and related HLK by registering itself with PoFx and adding KsDFxSupportEnable registry key to the INF.

A driver can register itself with PoFx by using the implementation mentioned in this section. Additionally, the following line needs to be added in the AddReg directive section.

HKR, , KSDFxSupportEnable, 0x00010001, 1

The AddReg section can be invoked by either the device's [DDInstall.HW] section or the driver's [service-install-section]. Adding it in the [DDInstall.HW] section changes only that particular device. This is useful if the same driver is used for different VID/PID combinations, but DFx needs to be enabled only for a specific device.

Adding the AddReg section in the [service-install-section] opts-in DFx for all devices using that driver.

Testing

Microsoft provides three tests for DFx: a single-device test in the Windows Driver Kit intended for testing user-specified devices, a device-level HLK test, and a system-level HLK test intended for testing all devices on a system.

The single-device test is available as part of the PwrTest tool that ships with the WDK. To access it, run the tool with the /directedfx switch. For more information, see PwrTest DirectedFx Scenario.

For information about HLK tests, please see the following pages:

Testing DFx after an S4 transition is recommended in order to catch any cases where a driver may not be correctly calling PoFxReportDevicePoweredOn after resume from S4.

DFx and S-state transitions

  • The target D-state for DFx transitions should match that for Runtime D3 (RTD3), which may be different than the target D-state for S3/S4 transitions. Consider a scenario in which a device enters D2 for RTD3, but enters D3 for S3/S4. In this case, the target D-state for DFx should be D2.
  • Similarly, the arm-for-wake behavior for DFx should match that for RTD3, which may differ from that used in S3/S4 transitions. For example, a device may enter D2/wake-armed for RTD3, but enter D3/no-wake-armed for S3/S4. In this scenario, DFx transitions should also enter D2/wake-armed.

DFx and Runtime D3 (RTD3)

  • With RTD3, a device typically enters a lower power D-state when it goes idle. If new work arrives, the device immediately wakes to D0. With DFx, the device should continue to remain in its target D-state (and pend new work on its queues) until PoFx directs it to power back up.

See Also