PcAtChipsetPkg: Write RTC time in PcRtcInit() only when it is needed

In PcRtcInit(), it always read RTC time and then write it back. It could
potentially cause two issues:
1) There is time gap between read RTC and write RTC, writing RTC time on
every boot could cause RTC time drift after many reboot cycles

2) Writing RTC registers on every boot could cause some unnecessary delay,
slightly impact the boot performance.

The change is only writing RTC time when 1) the current RTC time is not
valid or 2) the RegisterB value is changed.

Signed-off-by: Chen Lin Z <lin.z.chen@intel.com>

PcAtChipsetPkg/PcatRealTimeClockRuntimeDxe/PcRtc.c

@@ -188,6 +188,67 @@ RtcWrite (
   }
 }
 
+/**
+  Sets the current local timezone & daylight information.
+
+  @param  TimeZone         Timezone info.
+  @param  Daylight         Daylight info.
+  @param  Global           For global use inside this module.
+
+  @retval EFI_SUCCESS      The operation completed successfully.
+  @retval EFI_DEVICE_ERROR The variable could not be set due due to hardware error.
+
+**/
+EFI_STATUS
+PcRtcSetTimeZone (
+  IN INT16                  TimeZone,
+  IN UINT8                  Daylight,
+  IN PC_RTC_MODULE_GLOBALS  *Global
+  )
+{
+  EFI_STATUS  Status;
+  UINT32      TimerVar;
+
+  ASSERT ((TimeZone == EFI_UNSPECIFIED_TIMEZONE) || ((TimeZone >= -1440) && (TimeZone <= 1440)));
+  ASSERT ((Daylight & (~(EFI_TIME_ADJUST_DAYLIGHT | EFI_TIME_IN_DAYLIGHT))) == 0);
+
+  //
+  // Write timezone and daylight to RTC variable
+  //
+  if ((TimeZone == EFI_UNSPECIFIED_TIMEZONE) && (Daylight == 0)) {
+    Status = EfiSetVariable (
+               mTimeZoneVariableName,
+               &gEfiCallerIdGuid,
+               0,
+               0,
+               NULL
+               );
+    if (Status == EFI_NOT_FOUND) {
+      Status = EFI_SUCCESS;
+    }
+  } else {
+    TimerVar = Daylight;
+    TimerVar = (UINT32)((TimerVar << 16) | (UINT16)(TimeZone));
+    Status   = EfiSetVariable (
+                 mTimeZoneVariableName,
+                 &gEfiCallerIdGuid,
+                 EFI_VARIABLE_BOOTSERVICE_ACCESS | EFI_VARIABLE_RUNTIME_ACCESS | EFI_VARIABLE_NON_VOLATILE,
+                 sizeof (TimerVar),
+                 &TimerVar
+                 );
+  }
+
+  //
+  // Set the variable that contains the TimeZone and Daylight fields
+  //
+  if (!EFI_ERROR (Status)) {
+    Global->SavedTimeZone = TimeZone;
+    Global->Daylight      = Daylight;
+  }
+
+  return Status;
+}
+
 /**
   Initialize RTC.
 
@@ -211,6 +272,9 @@ PcRtcInit (
   UINT32          TimerVar;
   BOOLEAN         Enabled;
   BOOLEAN         Pending;
+  BOOLEAN         NeedRtcUpdate;
+
+  NeedRtcUpdate = FALSE;
 
   //
   // Acquire RTC Lock to make access to RTC atomic
@@ -324,21 +388,32 @@ PcRtcInit (
     Time.Nanosecond = 0;
     Time.TimeZone   = EFI_UNSPECIFIED_TIMEZONE;
     Time.Daylight   = 0;
+    NeedRtcUpdate   = TRUE;
   }
 
   //
   // Set RTC configuration after get original time
   // The value of bit AIE should be reserved.
   //
-  RegisterB.Data = FixedPcdGet8 (PcdInitialValueRtcRegisterB) | (RegisterB.Data & BIT5);
-  RtcWrite (RTC_ADDRESS_REGISTER_B, RegisterB.Data);
+  if ((RegisterB.Data | BIT5) != (FixedPcdGet8 (PcdInitialValueRtcRegisterB) | BIT5)) {
+    RegisterB.Data = FixedPcdGet8 (PcdInitialValueRtcRegisterB) | (RegisterB.Data & BIT5);
+    RtcWrite (RTC_ADDRESS_REGISTER_B, RegisterB.Data);
+    NeedRtcUpdate = TRUE;
+  }
 
   //
   // Reset time value according to new RTC configuration
   //
-  Status = PcRtcSetTime (&Time, Global);
-  if (EFI_ERROR (Status)) {
-    return EFI_DEVICE_ERROR;
+  if (NeedRtcUpdate) {
+    Status = PcRtcSetTime (&Time, Global);
+    if (EFI_ERROR (Status)) {
+      return EFI_DEVICE_ERROR;
+    }
+  } else {
+    Status = PcRtcSetTimeZone (Time.TimeZone, Time.Daylight, Global);
+    if (EFI_ERROR (Status)) {
+      return EFI_DEVICE_ERROR;
+    }
   }
 
   //
@@ -563,7 +638,6 @@ PcRtcSetTime (
   EFI_TIME        RtcTime;
   RTC_REGISTER_A  RegisterA;
   RTC_REGISTER_B  RegisterB;
-  UINT32          TimerVar;
 
   if (Time == NULL) {
     return EFI_INVALID_PARAMETER;
@@ -598,31 +672,7 @@ PcRtcSetTime (
     return Status;
   }
 
-  //
-  // Write timezone and daylight to RTC variable
-  //
-  if ((Time->TimeZone == EFI_UNSPECIFIED_TIMEZONE) && (Time->Daylight == 0)) {
-    Status = EfiSetVariable (
-               mTimeZoneVariableName,
-               &gEfiCallerIdGuid,
-               0,
-               0,
-               NULL
-               );
-    if (Status == EFI_NOT_FOUND) {
-      Status = EFI_SUCCESS;
-    }
-  } else {
-    TimerVar = Time->Daylight;
-    TimerVar = (UINT32)((TimerVar << 16) | (UINT16)(Time->TimeZone));
-    Status   = EfiSetVariable (
-                 mTimeZoneVariableName,
-                 &gEfiCallerIdGuid,
-                 EFI_VARIABLE_BOOTSERVICE_ACCESS | EFI_VARIABLE_RUNTIME_ACCESS | EFI_VARIABLE_NON_VOLATILE,
-                 sizeof (TimerVar),
-                 &TimerVar
-                 );
-  }
+  Status = PcRtcSetTimeZone (Time->TimeZone, Time->Daylight, Global);
 
   if (EFI_ERROR (Status)) {
     if (!EfiAtRuntime ()) {
@@ -677,12 +727,6 @@ PcRtcSetTime (
     EfiReleaseLock (&Global->RtcLock);
   }
 
-  //
-  // Set the variable that contains the TimeZone and Daylight fields
-  //
-  Global->SavedTimeZone = Time->TimeZone;
-  Global->Daylight      = Time->Daylight;
-
   return EFI_SUCCESS;
 }