void OEMIdle(DWORD idleParam)

{

     UINT32 baseMSec, idleMSec, idleSysTicks;

     INT32 usedCounts, idleCounts;

     ULARGE_INTEGER idle;

     // Get current system timer counter

     baseMSec = CurMSec;

     // Compute the remaining idle time

     // 计算距离下次系统调度的时间，如果需要马上开始调度，则立即返回

     idleMSec = dwReschedTime - baseMSec;

     // Idle time has expired - we need to return

     if ((INT32)idleMSec <= 0) return;

     // Limit the maximum idle time to what is supported. 

     // Counter size is the limiting parameter.  When kernel

     // profiler or interrupt latency timing is active it is set

     // to one system tick.

     // 是不是大于当前OAL中配置的Timer所支持的最大时间，如果大于它，则取最大值

     // 其实这个最大值就是g_oalTimer.maxPeriodMSec，在OALTimerInit中配置，计算方法可以参照

     // Timer的Spec，该值越大越好

     if (idleMSec > g_oalTimer.maxPeriodMSec) {

         idleMSec = g_oalTimer.maxPeriodMSec;

     }

     // We can wait only full systick

     idleSysTicks = idleMSec/g_oalTimer.msecPerSysTick;

     // This is idle time in hi-res ticks

     idleCounts = idleSysTicks * g_oalTimer.countsPerSysTick;

     // Find how many hi-res ticks was already used

     // 计算当前用了多少个Counter，即过了多少个Timer Base Clock的时钟周期

     usedCounts = OALTimerCountsSinceSysTick();

     if (usedCounts == g_oalTimer.countsPerSysTick)

     {

         return;

     }

     // Prolong beat period to idle time -- don't do it idle time isn't

     // longer than one system tick. Even if OALTimerExtendSysTick function

     // should accept this value it can cause problems if kernel profiler

     // or interrupt latency timing is active.

     if (idleSysTicks > 1) {

         // Extend timer period

          // 根据计算出来的值重新配置Timer Counter的初始值，也即配置系统中断唤醒的时间。

          // 如果计算出来的时间等于一个Tick的时间长，则不用进行配置

         OALTimerUpdate(idleCounts-usedCounts, g_oalTimer.countsMargin);

         // Update value for timer interrupt which wakeup from idle

          // 更新系统Tick的时间长，以MS为单位，可以看到此处更新的是actualMSecPerSysTick的值

          // 而msecPerSysTick的值并不变化

          // 随之夜更新actualCountsPerSysTick的值

         g_oalTimer.actualMSecPerSysTick = idleMSec;

         g_oalTimer.actualCountsPerSysTick = idleCounts;

     }

     g_idleMSec = idleMSec;

     // Move SoC/CPU to idle mode

     OALCPUIdle();

     // Return system tick period back to original. Don't call when idle

     // time was one system tick. See comment above.

     if (idleSysTicks > 1) {

         // If there wasn't timer interrupt we have to update CurMSec&curCounts

         if (CurMSec == baseMSec) {

              // Return system tick period back to original

              // 这个需要特别注意，否则系统的CPU Loading会计算错误

              idleCounts = OALTimerUpdate(g_oalTimer.countsPerSysTick, g_oalTimer.countsMargin)+usedCounts;

              g_idleMSec = g_oalTimer.msecPerSysTick;

              //            RETAILMSG(1,(TEXT("(%x)"), idleSysTicks));

              // Restore original values

              g_oalTimer.actualMSecPerSysTick = g_oalTimer.msecPerSysTick;

              g_oalTimer.actualCountsPerSysTick = g_oalTimer.countsPerSysTick;

              // Fix system tick counters & idle counter

              // 这个地方一定要注意，否则系统的调度会出现问题

              CurMSec += idleCounts/g_oalTimer.countsPerMSec;

              g_oalTimer.curCounts += idleCounts;

              idleCounts += OALTimerCountsSinceSysTick();

              usedCounts = 0;

         }

     } else {

         if (CurMSec == baseMSec) {

              // Update actual idle counts, if there wasn't timer interrupt

              idleCounts = OALTimerCountsSinceSysTick();

              usedCounts = 0;

         }

     }

     // Get real idle value. If result is negative we didn't idle at all.

     idleCounts -= usedCounts;

     if (idleCounts < 0) idleCounts = 0;

     //   RETAILMSG(1,(TEXT("(%x, %x)"), idleCounts, (CurMSec - baseMSec)*g_oalTimer.countsPerSysTick));

     // Update idle counters

     // idle.QuadPart的值其实就是GetIdleTime的时间

     idle.LowPart = curridlelow;

     idle.HighPart = curridlehigh;

     idle.QuadPart += idleCounts;

     curridlelow  = idle.LowPart;

     curridlehigh = idle.HighPart;

}