Wireless Intel SpeedStep Power Manager

White PaperWireless Intel SpeedStep ® Power ManagerIt is important that all drivers maintain their own device powerstate management. The drivers must at least be able to monitortheir own activity and self-manage their own state, such asturning off the peripheral device when it makes sense andinforming the Power Manager software of the transition. ThePower Manager software tracks the state of each driver, butdoes not mandate the device's state management.9.3.1 DEVICE POWER STATESSince many operating systems currently have some level ofdevice power management, the Device Driver Interface Layertranslates an OS-specific device state into an appropriatePower Manager software-specific state. If an operating systemdoes not supply power management device states, the devicedrivers use the power states created by the Power Managersoftware. When a registered driver transitions its device's powerstate, the driver must inform the Power Manager software ofthis transition. Power Manager software device states areshown in Table 3.STATEOnOffLow PowerDESCRIPTIONThe device is fully powered up and fullyfunctional.The device is off.The device is on, but is in a reduced powerstate. Depending on the device there could beseveral of these low-power states.Table 3. Device States9.3.2 DEVICE STATE SENSITIVITYAs part of the driver’s registration, the driver must report to thePower Manager software what the device’s sensitivity is to allsupported processor modes. If a driver reports that it is sensitiveto a given state, the Power Manager software notifies the driverof an impending state transition before the Power Managersoftware makes the state transition.9.3.3 DEVICE FREQUENCY SENSITIVITYAs part of a driver’s registration, the driver must inform thePower Manager software as to whether or not the driver requiresnotification when the Policy Manager intends to scale thedevice’s frequency and/or voltage. For example, Table 1 showsthat when the Policy Manager intends to scale the Intel PXA27xprocessor’s Core Run Frequency from 208 MHz to 104 MHz,other Intel PXA27x processor frequencies (e.g., System Bus,Memory Controller, LCD Controller) must be scaled as well.10.0 Power Manager Software-AwareApplication Interface (Optional)10.1 OverviewApplications can be easily enhanced to provide their performance/processing requirements (cycles, deadlines, etc.), to the PowerManager software, thus making frequency change and voltagechange scheduling simpler and more effective. For example,consider a mobile platform that uses the Intel PXA27x processorto control a small-sized display. For an MPEG4 video applicationto decode and display 30 fps, the Intel PXA27x processor canrun at its maximum possible frequency and voltage so thatsignificant idle time exists during the execution bursts that areneeded to service the application, as shown in Figure 2. TheIdle Profiler alone would either not detect this idle time (if it isvery small) or would incur a latency in detecting the idle time.Optionally, if the application is slightly enhanced to convey theidle time information to the Power Manager, the Policy Managecan make good decisions to reduce power dissipationduring the idle time.10.2 Application Programming InterfaceThe Applications Programming Interface (API) is shown inFigure 11 below.App[0]App[1]App[n-1]Power Manager Application Interface LayerPower Manager Policy ManagerFigure 11. Applications Programming Interface14