PoFxSetComponentLatency function (wdm.h)

Article
02/25/2022

The PoFxSetComponentLatency routine specifies the maximum latency that can be tolerated in the transition from the idle condition to the active condition in the specified component.

Syntax

void PoFxSetComponentLatency(
  [in] POHANDLE  Handle,
  [in] ULONG     Component,
  [in] ULONGLONG Latency
);

Parameters

[in] Handle

A handle that represents the registration of the device with the power management framework (PoFx). The device driver previously received this handle from the PoFxRegisterDevice routine.

[in] Component

The index that identifies the component. This parameter is an index into the Components array in the PO_FX_DEVICE structure that the device driver used to register the device with PoFx. If the Components array contains N elements, component indexes range from 0 to N–1.

[in] Latency

The maximum latency, in 100-nanosecond units, that can be tolerated in the specified component. For more information, see Remarks.

Return value

None

Remarks

The calling driver supplies a maximum latency that clients of the device can tolerate in the specified component. This latency is the time required to complete a transition from the idle condition to the active condition. PoFx uses this latency as a hint to improve performance.

When the component switches from the active condition to the idle condition, PoFx selects an appropriate low-power Fx state for the component to enter. After this transition completes, the time required to switch back to the active condition depends on the selected Fx state. PoFx tries to select an Fx state that does not violate the maximum latency requirement.

For example, if a driver supports power states F0, F1, and F2 in the specified component, the F2 state represents a lower power setting than the F1 state. Typically, the time T2 required to wake (to the fully on state, F0) from F2 is longer than the time T1 required to wake from F1. If the maximum latency is greater than T1 but less than T2, PoFx can safely switch the component to the F1 state and still meet the maximum latency requirement. However, PoFx avoids switching the component to the F2 state because doing so violates this requirement.

The device driver can call PoFxSetComponentLatency each time a change in conditions requires a change in the maximum latency. After each call, the new maximum latency remains in effect until the driver calls PoFxSetComponentLatency again to update it.

If a component is in the idle condition when PoFxSetComponentLatency is called, PoFx might change the component’s current Fx state to accommodate the new maximum latency specified by the caller.

Requirements

Requirement	Value
Minimum supported client	Available starting with Windows 8.
Target Platform	Universal
Header	wdm.h
Library	Ntoskrnl.lib
DLL	Ntoskrnl.exe
IRQL	<= DISPATCH_LEVEL