CPU idle in .NET framework

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15.2.4.3 CPU idle
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A DPowerController class derived object requires an implementation of CpuIdle(), which deals with CPU transition to idle state. The idle state is a transitional state, often the gateway to a power-saving retention mode. In Section 15.2.2.1, I will look at how the CPU can be moved to these retention states.
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Variant-speci c idle As I mentioned previously, the scheduling of the null thread is what signals the CPU idle condition.
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The null thread is the rst thread to start on a device at boot time, and it runs before the power manager has been registered with the kernel. Therefore, an alternative to the power manager s own CpuIdle() function must be provided, as a pure virtual method of the Asic class:
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class Asic { public: ... // power management virtual void Idle()=0; ... };
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This function is typically a dummy implementation, provided by the Asic class derived Variant class. Once the power manager has been registered, the kernel will call its CpuIdle() function instead.
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15.2.4.4 Enabling access to power controller from other kernel-side components
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It is common that other kernel-side components such as the variant, or peripheral drivers, need access to the power controller. This has no built-in accessible interfaces, other than to the power manager. When porting Symbian OS, the base porter may therefore wish to implement a derived power controller exported method to return a pointer to itself and an interface class, in this way:
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class TXXXPowerControllerInterface { public: ... // to allow Drivers access to power controller IMPORT_C static PowerController* PowerController(); inline static void RegisterPowerController( DXXXPowerController* aPowerController) {iPowerController=aPowerController;} public: ... static DXXXPowerController* iPowerController; }; EXPORT_C DXXXPowerController* TXXXPowerControllerInterface::PowerController() { return &iPowerController; }
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The power controller derived object s constructor should register the power controller with the interface, which is best done at construction time:
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DXXXPowerController::DXXXPowerController() { Register(); //register power ctrllr with power manager // register power controller with interface TXXXPowerController::RegisterPowerController(this); }
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15.2.4.5 Handling of wakeup events
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When the CPU and peripherals move to the standby state, their responsiveness and availability are greatly reduced. This is accepted by the user accepts who has chosen to switch the phone off and the framework uses that acceptance to save power. However, at the OS level, we need to enable a minimum capability to respond to user interactions, so that the framework can transition the phone back to a more available state when the user switches the phone back on. Also, some internal events, such as expiry of absolute timers, must be able to bring the phone back to a more available state. A DPowerController-derived object should implement the following pure virtual functions to handle wakeup events: EnableWakeupEvents() DisableWakeupEvents() AbsoluteTimerExpired().
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EnableWakeupEvents() Typically, the domain manager (or shutdown server) will start a transition to standby by requesting the kernel power framework to start monitoring wakeup events and notify it of their occurrence. As a result, the power manager calls the power controller s EnableWakeupEvents() to enable detection at platform-speci c level.
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Monitoring wakeup events The power controller may monitor some wakeup events directly. If that is the case, the implementation of EnableWakeupEvents() programs the hardware components involved in their detection, and hooks a handling function to service the event. This is commonly achieved with the use of an interrupt the ISR should schedule a DFC to notify the power manager of the event. More commonly, peripheral drivers monitor wakeup events. In this case, the implementation of EnableWakeupEvents() should store whether the event is enabled, like so:
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class DXXXPowerController : public DPowerController
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{ public: // from DPowerController ... void EnableWakeupEvents(); void AbsoluteTimerExpired(); void DisableWakeupEvents(); ... public: DXXXPowerController(); ... private: TInt iWakeupEventMask; ... }; void DXXXPowerController::EnableWakeupEvents() { ... // Set iWakeUpMask to a bit mask with one bit set for // each relevant wakeup event for the standby state if(iTargetState==EPwstandby) iWakeupEventMask=myMask; }
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There are two possible schemes: 1. Upon the occurrence of the event, the driver checks with the power controller to see if the event is enabled, and if it is, noti es the power manager by calling the power controller s WakeupEvent() method. (It checks by calling an API such as the next example IsWakeupEventEnable(...), and passing a bit mask containing the wakeup event that it is interested in.)
public: inline TBool IsWakeupEventEnabled(Tint aWakeupEvent) { (iWakeupEventMask & aWakeupEvent) return ETrue : return EFalse; }
The driver noti es the power controller whenever a wakeup event it monitors occurs, using an API such as the next example NotifyWakeupEvent(), and passing a bit mask containing the wakeup event that it monitors; the API checks to see if the wakeup event is enabled, and if it is, noti es the power manager by calling the WakeupEvent() method.
public: inline void NotifyWakeupEvent (Tint aWakeupEvent)