HpetTimer.c

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#include <PiDxe.h>
 
#include <Protocol/Cpu.h>
#include <Protocol/Timer.h>
 
#include <Library/IoLib.h>
#include <Library/PcdLib.h>
#include <Library/BaseLib.h>
#include <Library/DebugLib.h>
#include <Library/UefiBootServicesTableLib.h>
#include <Library/LocalApicLib.h>
#include <Library/IoApicLib.h>
 
#include <Register/LocalApic.h>
#include <Register/IoApic.h>
#include <Register/Hpet.h>
 
#define HPET_INVALID_TIMER_INDEX  0xff
 
 
EFI_STATUS
EFIAPI
TimerDriverRegisterHandler (
  IN EFI_TIMER_ARCH_PROTOCOL  *This,
  IN EFI_TIMER_NOTIFY         NotifyFunction
  );
 
EFI_STATUS
EFIAPI
TimerDriverSetTimerPeriod (
  IN EFI_TIMER_ARCH_PROTOCOL  *This,
  IN UINT64                   TimerPeriod
  );
 
EFI_STATUS
EFIAPI
TimerDriverGetTimerPeriod (
  IN EFI_TIMER_ARCH_PROTOCOL  *This,
  OUT UINT64                  *TimerPeriod
  );
 
EFI_STATUS
EFIAPI
TimerDriverGenerateSoftInterrupt (
  IN EFI_TIMER_ARCH_PROTOCOL  *This
  );
 
EFI_HANDLE  mTimerHandle = NULL;
 
EFI_TIMER_ARCH_PROTOCOL  mTimer = {
  TimerDriverRegisterHandler,
  TimerDriverSetTimerPeriod,
  TimerDriverGetTimerPeriod,
  TimerDriverGenerateSoftInterrupt
};
 
EFI_CPU_ARCH_PROTOCOL  *mCpu = NULL;
 
EFI_TIMER_NOTIFY  mTimerNotifyFunction = NULL;
 
UINT64  mTimerPeriod = 0;
 
UINT64  mTimerCount;
 
UINT64  mCounterMask;
 
volatile UINT64  mPreviousMainCounter;
 
volatile UINT64  mPreviousComparator;
 
UINTN  mTimerIndex;
 
UINT32  mTimerIrq;
 
HPET_GENERAL_CAPABILITIES_ID_REGISTER  mHpetGeneralCapabilities;
 
HPET_GENERAL_CONFIGURATION_REGISTER  mHpetGeneralConfiguration;
 
HPET_TIMER_CONFIGURATION_REGISTER  mTimerConfiguration;
 
volatile UINTN  mNumTicks;
 
UINT64
HpetRead (
  IN UINTN  Offset
  )
{
  return MmioRead64 (PcdGet32 (PcdHpetBaseAddress) + Offset);
}
 
UINT64
HpetWrite (
  IN UINTN   Offset,
  IN UINT64  Value
  )
{
  return MmioWrite64 (PcdGet32 (PcdHpetBaseAddress) + Offset, Value);
}
 
VOID
HpetEnable (
  IN BOOLEAN  Enable
  )
{
  mHpetGeneralConfiguration.Bits.MainCounterEnable = Enable ? 1 : 0;
  HpetWrite (HPET_GENERAL_CONFIGURATION_OFFSET, mHpetGeneralConfiguration.Uint64);
}
 
VOID
EFIAPI
TimerInterruptHandler (
  IN EFI_EXCEPTION_TYPE  InterruptType,
  IN EFI_SYSTEM_CONTEXT  SystemContext
  )
{
  UINT64  MainCounter;
  UINT64  Comparator;
  UINT64  TimerPeriod;
  UINT64  Delta;
 
  //
  // Count number of ticks
  //
  DEBUG_CODE (
    mNumTicks++;
    );
 
  //
  // Clear HPET timer interrupt status
  //
  HpetWrite (HPET_GENERAL_INTERRUPT_STATUS_OFFSET, LShiftU64 (1, mTimerIndex));
 
  //
  // Local APIC EOI
  //
  SendApicEoi ();
 
  //
  // Disable HPET timer when adjusting the COMPARATOR value to prevent a missed interrupt
  //
  HpetEnable (FALSE);
 
  //
  // Capture main counter value
  //
  MainCounter = HpetRead (HPET_MAIN_COUNTER_OFFSET);
 
  //
  // Get the previous comparator counter
  //
  mPreviousComparator = HpetRead (HPET_TIMER_COMPARATOR_OFFSET + mTimerIndex * HPET_TIMER_STRIDE);
 
  //
  // Set HPET COMPARATOR to the value required for the next timer tick
  //
  Comparator = (mPreviousComparator + mTimerCount) & mCounterMask;
 
  if ((mPreviousMainCounter < MainCounter) && (mPreviousComparator > Comparator)) {
    //
    // When comparator overflows
    //
    HpetWrite (HPET_TIMER_COMPARATOR_OFFSET + mTimerIndex * HPET_TIMER_STRIDE, Comparator);
  } else if ((mPreviousMainCounter > MainCounter) && (mPreviousComparator < Comparator)) {
    //
    // When main counter overflows
    //
    HpetWrite (HPET_TIMER_COMPARATOR_OFFSET + mTimerIndex * HPET_TIMER_STRIDE, (MainCounter + mTimerCount) & mCounterMask);
  } else {
    //
    // When both main counter and comparator do not overflow or both do overflow
    //
    if (Comparator > MainCounter) {
      HpetWrite (HPET_TIMER_COMPARATOR_OFFSET + mTimerIndex * HPET_TIMER_STRIDE, Comparator);
    } else {
      HpetWrite (HPET_TIMER_COMPARATOR_OFFSET + mTimerIndex * HPET_TIMER_STRIDE, (MainCounter + mTimerCount) & mCounterMask);
    }
  }
 
  //
  // Enable the HPET counter once the new COMPARATOR value has been set.
  //
  HpetEnable (TRUE);
 
  //
  // Check to see if there is a registered notification function
  //
  if (mTimerNotifyFunction != NULL) {
    //
    // Compute time since last notification in 100 ns units (10 ^ -7)
    //
    if (MainCounter > mPreviousMainCounter) {
      //
      // Main counter does not overflow
      //
      Delta = MainCounter - mPreviousMainCounter;
    } else {
      //
      // Main counter overflows, first usb, then add
      //
      Delta = (mCounterMask - mPreviousMainCounter) + MainCounter;
    }
 
    TimerPeriod = DivU64x32 (
                    MultU64x32 (
                      Delta & mCounterMask,
                      mHpetGeneralCapabilities.Bits.CounterClockPeriod
                      ),
                    100000000
                    );
 
    //
    // Save main counter value before calling notification function
    // that may enable interrupts and allow interrupt nesting.
    //
    mPreviousMainCounter = MainCounter;
 
    //
    // Call registered notification function passing in the time since the last
    // interrupt in 100 ns units.
    //
    mTimerNotifyFunction (TimerPeriod);
    return;
  }
 
  //
  // Save main counter value
  //
  mPreviousMainCounter = MainCounter;
}
 
EFI_STATUS
EFIAPI
TimerDriverRegisterHandler (
  IN EFI_TIMER_ARCH_PROTOCOL  *This,
  IN EFI_TIMER_NOTIFY         NotifyFunction
  )
{
  //
  // Check for invalid parameters
  //
  if ((NotifyFunction == NULL) && (mTimerNotifyFunction == NULL)) {
    return EFI_INVALID_PARAMETER;
  }
 
  if ((NotifyFunction != NULL) && (mTimerNotifyFunction != NULL)) {
    return EFI_ALREADY_STARTED;
  }
 
  //
  // Cache the registered notification function
  //
  mTimerNotifyFunction = NotifyFunction;
 
  return EFI_SUCCESS;
}
 
EFI_STATUS
EFIAPI
TimerDriverSetTimerPeriod (
  IN EFI_TIMER_ARCH_PROTOCOL  *This,
  IN UINT64                   TimerPeriod
  )
{
  EFI_TPL                        Tpl;
  UINT64                         MainCounter;
  UINT64                         Delta;
  UINT64                         CurrentComparator;
  HPET_TIMER_MSI_ROUTE_REGISTER  HpetTimerMsiRoute;
 
  //
  // Disable interrupts
  //
  Tpl = gBS->RaiseTPL (TPL_HIGH_LEVEL);
 
  //
  // Disable HPET timer when adjusting the timer period
  //
  HpetEnable (FALSE);
 
  if (TimerPeriod == 0) {
    if (mTimerPeriod != 0) {
      //
      // Check if there is possibly a pending interrupt
      //
      MainCounter = HpetRead (HPET_MAIN_COUNTER_OFFSET);
      if (MainCounter < mPreviousMainCounter) {
        Delta = (mCounterMask - mPreviousMainCounter) + MainCounter;
      } else {
        Delta = MainCounter - mPreviousMainCounter;
      }
 
      if ((Delta & mCounterMask) >= mTimerCount) {
        //
        // Interrupt still happens after disable HPET, wait to be processed
        // Wait until interrupt is processed and comparator is increased
        //
        CurrentComparator = HpetRead (HPET_TIMER_COMPARATOR_OFFSET + mTimerIndex * HPET_TIMER_STRIDE);
        while (CurrentComparator == mPreviousComparator) {
          CurrentComparator = HpetRead (HPET_TIMER_COMPARATOR_OFFSET + mTimerIndex * HPET_TIMER_STRIDE);
          CpuPause ();
        }
      }
    }
 
    //
    // If TimerPeriod is 0, then mask HPET Timer interrupts
    //
 
    if ((mTimerConfiguration.Bits.MsiInterruptCapability != 0) && FeaturePcdGet (PcdHpetMsiEnable)) {
      //
      // Disable HPET MSI interrupt generation
      //
      mTimerConfiguration.Bits.MsiInterruptEnable = 0;
    } else {
      //
      // Disable I/O APIC Interrupt
      //
      IoApicEnableInterrupt (mTimerIrq, FALSE);
    }
 
    //
    // Disable HPET timer interrupt
    //
    mTimerConfiguration.Bits.InterruptEnable = 0;
    HpetWrite (HPET_TIMER_CONFIGURATION_OFFSET + mTimerIndex * HPET_TIMER_STRIDE, mTimerConfiguration.Uint64);
  } else {
    //
    // Convert TimerPeriod to femtoseconds and divide by the number if femtoseconds
    // per tick of the HPET counter to determine the number of HPET counter ticks
    // in TimerPeriod 100 ns units.
    //
    mTimerCount = DivU64x32 (
                    MultU64x32 (TimerPeriod, 100000000),
                    mHpetGeneralCapabilities.Bits.CounterClockPeriod
                    );
 
    //
    // Program the HPET Comparator with the number of ticks till the next interrupt
    //
    MainCounter = HpetRead (HPET_MAIN_COUNTER_OFFSET);
    if (MainCounter > mPreviousMainCounter) {
      Delta = MainCounter - mPreviousMainCounter;
    } else {
      Delta = (mCounterMask - mPreviousMainCounter) + MainCounter;
    }
 
    if ((Delta & mCounterMask) >= mTimerCount) {
      HpetWrite (HPET_TIMER_COMPARATOR_OFFSET + mTimerIndex * HPET_TIMER_STRIDE, (MainCounter + 1) & mCounterMask);
    } else {
      HpetWrite (HPET_TIMER_COMPARATOR_OFFSET + mTimerIndex * HPET_TIMER_STRIDE, (mPreviousMainCounter + mTimerCount) & mCounterMask);
    }
 
    //
    // Enable HPET Timer interrupt generation
    //
    if ((mTimerConfiguration.Bits.MsiInterruptCapability != 0) && FeaturePcdGet (PcdHpetMsiEnable)) {
      //
      // Program MSI Address and MSI Data values in the selected HPET Timer
      // Program HPET register with APIC ID of current BSP in case BSP has been switched
      //
      HpetTimerMsiRoute.Bits.Address = GetApicMsiAddress ();
      HpetTimerMsiRoute.Bits.Value   = (UINT32)GetApicMsiValue (PcdGet8 (PcdHpetLocalApicVector), LOCAL_APIC_DELIVERY_MODE_LOWEST_PRIORITY, FALSE, FALSE);
      HpetWrite (HPET_TIMER_MSI_ROUTE_OFFSET + mTimerIndex * HPET_TIMER_STRIDE, HpetTimerMsiRoute.Uint64);
      //
      // Enable HPET MSI Interrupt
      //
      mTimerConfiguration.Bits.MsiInterruptEnable = 1;
    } else {
      //
      // Enable timer interrupt through I/O APIC
      // Program IOAPIC register with APIC ID of current BSP in case BSP has been switched
      //
      IoApicConfigureInterrupt (mTimerIrq, PcdGet8 (PcdHpetLocalApicVector), IO_APIC_DELIVERY_MODE_LOWEST_PRIORITY, TRUE, FALSE);
      IoApicEnableInterrupt (mTimerIrq, TRUE);
    }
 
    //
    // Enable HPET Interrupt Generation
    //
    mTimerConfiguration.Bits.InterruptEnable = 1;
    HpetWrite (HPET_TIMER_CONFIGURATION_OFFSET + mTimerIndex * HPET_TIMER_STRIDE, mTimerConfiguration.Uint64);
  }
 
  //
  // Save the new timer period
  //
  mTimerPeriod = TimerPeriod;
 
  //
  // Enable the HPET counter once new timer period has been established
  // The HPET counter should run even if the HPET Timer interrupts are
  // disabled.  This is used to account for time passed while the interrupt
  // is disabled.
  //
  HpetEnable (TRUE);
 
  //
  // Restore interrupts
  //
  gBS->RestoreTPL (Tpl);
 
  return EFI_SUCCESS;
}
 
EFI_STATUS
EFIAPI
TimerDriverGetTimerPeriod (
  IN EFI_TIMER_ARCH_PROTOCOL  *This,
  OUT UINT64                  *TimerPeriod
  )
{
  if (TimerPeriod == NULL) {
    return EFI_INVALID_PARAMETER;
  }
 
  *TimerPeriod = mTimerPeriod;
 
  return EFI_SUCCESS;
}
 
EFI_STATUS
EFIAPI
TimerDriverGenerateSoftInterrupt (
  IN EFI_TIMER_ARCH_PROTOCOL  *This
  )
{
  UINT64   MainCounter;
  EFI_TPL  Tpl;
  UINT64   TimerPeriod;
  UINT64   Delta;
 
  //
  // Disable interrupts
  //
  Tpl = gBS->RaiseTPL (TPL_HIGH_LEVEL);
 
  //
  // Capture main counter value
  //
  MainCounter = HpetRead (HPET_MAIN_COUNTER_OFFSET);
 
  //
  // Check to see if there is a registered notification function
  //
  if (mTimerNotifyFunction != NULL) {
    //
    // Compute time since last interrupt in 100 ns units (10 ^ -7)
    //
    if (MainCounter > mPreviousMainCounter) {
      //
      // Main counter does not overflow
      //
      Delta = MainCounter - mPreviousMainCounter;
    } else {
      //
      // Main counter overflows, first usb, then add
      //
      Delta = (mCounterMask - mPreviousMainCounter) + MainCounter;
    }
 
    TimerPeriod = DivU64x32 (
                    MultU64x32 (
                      Delta & mCounterMask,
                      mHpetGeneralCapabilities.Bits.CounterClockPeriod
                      ),
                    100000000
                    );
 
    //
    // Call registered notification function passing in the time since the last
    // interrupt in 100 ns units.
    //
    mTimerNotifyFunction (TimerPeriod);
  }
 
  //
  // Save main counter value
  //
  mPreviousMainCounter = MainCounter;
 
  //
  // Restore interrupts
  //
  gBS->RestoreTPL (Tpl);
 
  return EFI_SUCCESS;
}
 
EFI_STATUS
EFIAPI
TimerDriverInitialize (
  IN EFI_HANDLE        ImageHandle,
  IN EFI_SYSTEM_TABLE  *SystemTable
  )
{
  EFI_STATUS                     Status;
  UINTN                          TimerIndex;
  UINTN                          MsiTimerIndex;
  HPET_TIMER_MSI_ROUTE_REGISTER  HpetTimerMsiRoute;
 
  DEBUG ((DEBUG_INFO, "Init HPET Timer Driver\n"));
 
  //
  // Make sure the Timer Architectural Protocol is not already installed in the system
  //
  ASSERT_PROTOCOL_ALREADY_INSTALLED (NULL, &gEfiTimerArchProtocolGuid);
 
  //
  // Find the CPU architectural protocol.
  //
  Status = gBS->LocateProtocol (&gEfiCpuArchProtocolGuid, NULL, (VOID **)&mCpu);
  ASSERT_EFI_ERROR (Status);
 
  //
  // Retrieve HPET Capabilities and Configuration Information
  //
  mHpetGeneralCapabilities.Uint64  = HpetRead (HPET_GENERAL_CAPABILITIES_ID_OFFSET);
  mHpetGeneralConfiguration.Uint64 = HpetRead (HPET_GENERAL_CONFIGURATION_OFFSET);
 
  //
  // If Revision is not valid, then ASSERT() and unload the driver because the HPET
  // device is not present.
  //
  ASSERT (mHpetGeneralCapabilities.Uint64 != 0);
  ASSERT (mHpetGeneralCapabilities.Uint64 != 0xFFFFFFFFFFFFFFFFULL);
  if ((mHpetGeneralCapabilities.Uint64 == 0) || (mHpetGeneralCapabilities.Uint64 == 0xFFFFFFFFFFFFFFFFULL)) {
    DEBUG ((DEBUG_ERROR, "HPET device is not present.  Unload HPET driver.\n"));
    return EFI_DEVICE_ERROR;
  }
 
  //
  // Force the HPET timer to be disabled while setting everything up
  //
  HpetEnable (FALSE);
 
  //
  // Dump HPET Configuration Information
  //
  DEBUG_CODE_BEGIN ();
  DEBUG ((DEBUG_INFO, "HPET Base Address = 0x%08x\n", PcdGet32 (PcdHpetBaseAddress)));
  DEBUG ((DEBUG_INFO, "  HPET_GENERAL_CAPABILITIES_ID  = 0x%016lx\n", mHpetGeneralCapabilities));
  DEBUG ((DEBUG_INFO, "  HPET_GENERAL_CONFIGURATION    = 0x%016lx\n", mHpetGeneralConfiguration.Uint64));
  DEBUG ((DEBUG_INFO, "  HPET_GENERAL_INTERRUPT_STATUS = 0x%016lx\n", HpetRead (HPET_GENERAL_INTERRUPT_STATUS_OFFSET)));
  DEBUG ((DEBUG_INFO, "  HPET_MAIN_COUNTER             = 0x%016lx\n", HpetRead (HPET_MAIN_COUNTER_OFFSET)));
  DEBUG ((DEBUG_INFO, "  HPET Main Counter Period      = %d (fs)\n", mHpetGeneralCapabilities.Bits.CounterClockPeriod));
  for (TimerIndex = 0; TimerIndex <= mHpetGeneralCapabilities.Bits.NumberOfTimers; TimerIndex++) {
    DEBUG ((DEBUG_INFO, "  HPET_TIMER%d_CONFIGURATION     = 0x%016lx\n", TimerIndex, HpetRead (HPET_TIMER_CONFIGURATION_OFFSET + TimerIndex * HPET_TIMER_STRIDE)));
    DEBUG ((DEBUG_INFO, "  HPET_TIMER%d_COMPARATOR        = 0x%016lx\n", TimerIndex, HpetRead (HPET_TIMER_COMPARATOR_OFFSET    + TimerIndex * HPET_TIMER_STRIDE)));
    DEBUG ((DEBUG_INFO, "  HPET_TIMER%d_MSI_ROUTE         = 0x%016lx\n", TimerIndex, HpetRead (HPET_TIMER_MSI_ROUTE_OFFSET     + TimerIndex * HPET_TIMER_STRIDE)));
  }
 
  DEBUG_CODE_END ();
 
  //
  // Capture the current HPET main counter value.
  //
  mPreviousMainCounter = HpetRead (HPET_MAIN_COUNTER_OFFSET);
 
  //
  // Determine the interrupt mode to use for the HPET Timer.
  // Look for MSI first, then unused PIC mode interrupt, then I/O APIC mode interrupt
  //
  MsiTimerIndex = HPET_INVALID_TIMER_INDEX;
  mTimerIndex   = HPET_INVALID_TIMER_INDEX;
  for (TimerIndex = 0; TimerIndex <= mHpetGeneralCapabilities.Bits.NumberOfTimers; TimerIndex++) {
    //
    // Read the HPET Timer Capabilities and Configuration register
    //
    mTimerConfiguration.Uint64 = HpetRead (HPET_TIMER_CONFIGURATION_OFFSET + TimerIndex * HPET_TIMER_STRIDE);
 
    //
    // Check to see if this HPET Timer supports MSI
    //
    if (mTimerConfiguration.Bits.MsiInterruptCapability != 0) {
      //
      // Save the index of the first HPET Timer that supports MSI interrupts
      //
      if (MsiTimerIndex == HPET_INVALID_TIMER_INDEX) {
        MsiTimerIndex = TimerIndex;
      }
    }
 
    //
    // Check to see if this HPET Timer supports I/O APIC interrupts
    //
    if (mTimerConfiguration.Bits.InterruptRouteCapability != 0) {
      //
      // Save the index of the first HPET Timer that supports I/O APIC interrupts
      //
      if (mTimerIndex == HPET_INVALID_TIMER_INDEX) {
        mTimerIndex = TimerIndex;
        mTimerIrq   = (UINT32)LowBitSet32 (mTimerConfiguration.Bits.InterruptRouteCapability);
      }
    }
  }
 
  if (FeaturePcdGet (PcdHpetMsiEnable) && (MsiTimerIndex != HPET_INVALID_TIMER_INDEX)) {
    //
    // Use MSI interrupt if supported
    //
    mTimerIndex = MsiTimerIndex;
 
    //
    // Program MSI Address and MSI Data values in the selected HPET Timer
    //
    HpetTimerMsiRoute.Bits.Address = GetApicMsiAddress ();
    HpetTimerMsiRoute.Bits.Value   = (UINT32)GetApicMsiValue (PcdGet8 (PcdHpetLocalApicVector), LOCAL_APIC_DELIVERY_MODE_LOWEST_PRIORITY, FALSE, FALSE);
    HpetWrite (HPET_TIMER_MSI_ROUTE_OFFSET + mTimerIndex * HPET_TIMER_STRIDE, HpetTimerMsiRoute.Uint64);
 
    //
    // Read the HPET Timer Capabilities and Configuration register and initialize for MSI mode
    //   Clear LevelTriggeredInterrupt to use edge triggered interrupts when in MSI mode
    //
    mTimerConfiguration.Uint64                       = HpetRead (HPET_TIMER_CONFIGURATION_OFFSET + mTimerIndex * HPET_TIMER_STRIDE);
    mTimerConfiguration.Bits.LevelTriggeredInterrupt = 0;
  } else {
    //
    // If no HPET timers support MSI or I/O APIC modes, then ASSERT() and unload the driver.
    //
    ASSERT (mTimerIndex != HPET_INVALID_TIMER_INDEX);
    if (mTimerIndex == HPET_INVALID_TIMER_INDEX) {
      DEBUG ((DEBUG_ERROR, "No HPET timers support MSI or I/O APIC mode.  Unload HPET driver.\n"));
      return EFI_DEVICE_ERROR;
    }
 
    //
    // Initialize I/O APIC entry for HPET Timer Interrupt
    //   Fixed Delivery Mode, Level Triggered, Asserted Low
    //
    IoApicConfigureInterrupt (mTimerIrq, PcdGet8 (PcdHpetLocalApicVector), IO_APIC_DELIVERY_MODE_LOWEST_PRIORITY, TRUE, FALSE);
 
    //
    // Read the HPET Timer Capabilities and Configuration register and initialize for I/O APIC mode
    //   Clear MsiInterruptCapability to force rest of driver to use I/O APIC mode
    //   Set LevelTriggeredInterrupt to use level triggered interrupts when in I/O APIC mode
    //   Set InterruptRoute field based in mTimerIrq
    //
    mTimerConfiguration.Uint64                       = HpetRead (HPET_TIMER_CONFIGURATION_OFFSET + mTimerIndex * HPET_TIMER_STRIDE);
    mTimerConfiguration.Bits.LevelTriggeredInterrupt = 1;
    mTimerConfiguration.Bits.InterruptRoute          = mTimerIrq;
  }
 
  //
  // Configure the selected HPET Timer with settings common to both MSI mode and I/O APIC mode
  //   Clear InterruptEnable to keep interrupts disabled until full init is complete
  //   Clear PeriodicInterruptEnable to use one-shot mode
  //   Configure as a 32-bit counter
  //
  mTimerConfiguration.Bits.InterruptEnable         = 0;
  mTimerConfiguration.Bits.PeriodicInterruptEnable = 0;
  mTimerConfiguration.Bits.CounterSizeEnable       = 1;
  HpetWrite (HPET_TIMER_CONFIGURATION_OFFSET + mTimerIndex * HPET_TIMER_STRIDE, mTimerConfiguration.Uint64);
 
  //
  // Read the HPET Timer Capabilities and Configuration register back again.
  // CounterSizeEnable will be read back as a 0 if it is a 32-bit only timer
  //
  mTimerConfiguration.Uint64 = HpetRead (HPET_TIMER_CONFIGURATION_OFFSET + mTimerIndex * HPET_TIMER_STRIDE);
  if ((mTimerConfiguration.Bits.CounterSizeEnable == 1) && (sizeof (UINTN) == sizeof (UINT64))) {
    DEBUG ((DEBUG_INFO, "Choose 64-bit HPET timer.\n"));
    //
    // 64-bit BIOS can use 64-bit HPET timer
    //
    mCounterMask = 0xffffffffffffffffULL;
    //
    // Set timer back to 64-bit
    //
    mTimerConfiguration.Bits.CounterSizeEnable = 0;
    HpetWrite (HPET_TIMER_CONFIGURATION_OFFSET + mTimerIndex * HPET_TIMER_STRIDE, mTimerConfiguration.Uint64);
  } else {
    DEBUG ((DEBUG_INFO, "Choose 32-bit HPET timer.\n"));
    mCounterMask = 0x00000000ffffffffULL;
  }
 
  //
  // Install interrupt handler for selected HPET Timer
  //
  Status = mCpu->RegisterInterruptHandler (mCpu, PcdGet8 (PcdHpetLocalApicVector), TimerInterruptHandler);
  ASSERT_EFI_ERROR (Status);
  if (EFI_ERROR (Status)) {
    DEBUG ((DEBUG_ERROR, "Unable to register HPET interrupt with CPU Arch Protocol.  Unload HPET driver.\n"));
    return EFI_DEVICE_ERROR;
  }
 
  //
  // Force the HPET Timer to be enabled at its default period
  //
  Status = TimerDriverSetTimerPeriod (&mTimer, PcdGet64 (PcdHpetDefaultTimerPeriod));
  ASSERT_EFI_ERROR (Status);
  if (EFI_ERROR (Status)) {
    DEBUG ((DEBUG_ERROR, "Unable to set HPET default timer rate.  Unload HPET driver.\n"));
    return EFI_DEVICE_ERROR;
  }
 
  //
  // Show state of enabled HPET timer
  //
  DEBUG_CODE_BEGIN ();
  if ((mTimerConfiguration.Bits.MsiInterruptCapability != 0) && FeaturePcdGet (PcdHpetMsiEnable)) {
    DEBUG ((DEBUG_INFO, "HPET Interrupt Mode MSI\n"));
  } else {
    DEBUG ((DEBUG_INFO, "HPET Interrupt Mode I/O APIC\n"));
    DEBUG ((DEBUG_INFO, "HPET I/O APIC IRQ         = 0x%02x\n", mTimerIrq));
  }
 
  DEBUG ((DEBUG_INFO, "HPET Interrupt Vector     = 0x%02x\n", PcdGet8 (PcdHpetLocalApicVector)));
  DEBUG ((DEBUG_INFO, "HPET Counter Mask         = 0x%016lx\n", mCounterMask));
  DEBUG ((DEBUG_INFO, "HPET Timer Period         = %d\n", mTimerPeriod));
  DEBUG ((DEBUG_INFO, "HPET Timer Count          = 0x%016lx\n", mTimerCount));
  DEBUG ((DEBUG_INFO, "HPET_TIMER%d_CONFIGURATION = 0x%016lx\n", mTimerIndex, HpetRead (HPET_TIMER_CONFIGURATION_OFFSET + mTimerIndex * HPET_TIMER_STRIDE)));
  DEBUG ((DEBUG_INFO, "HPET_TIMER%d_COMPARATOR    = 0x%016lx\n", mTimerIndex, HpetRead (HPET_TIMER_COMPARATOR_OFFSET    + mTimerIndex * HPET_TIMER_STRIDE)));
  DEBUG ((DEBUG_INFO, "HPET_TIMER%d_MSI_ROUTE     = 0x%016lx\n", mTimerIndex, HpetRead (HPET_TIMER_MSI_ROUTE_OFFSET     + mTimerIndex * HPET_TIMER_STRIDE)));
 
  //
  // Wait for a few timer interrupts to fire before continuing
  //
  while (mNumTicks < 10) {
  }
 
  DEBUG_CODE_END ();
 
  //
  // Install the Timer Architectural Protocol onto a new handle
  //
  Status = gBS->InstallMultipleProtocolInterfaces (
                  &mTimerHandle,
                  &gEfiTimerArchProtocolGuid,
                  &mTimer,
                  NULL
                  );
  ASSERT_EFI_ERROR (Status);
 
  return Status;
}