ArmPkg: Fix timer wrap-around

The timer counter register can wrap around and when this happens,
we'll get misbehavior for any MicroSecondDelay() calls. This adds
handling for that.

Signed-off-by: Carsten Haitzler <carsten.haitzler@foss.arm.com>

1 files changed, 10 insertions, 4 deletions

diff --git a/ArmPkg/Library/ArmArchTimerLib/ArmArchTimerLib.c b/ArmPkg/Library/ArmArchTimerLib/ArmArchTimerLib.c
index 76f94c9..cc1be01 100644
--- a/ArmPkg/Library/ArmArchTimerLib/ArmArchTimerLib.c
+++ b/ArmPkg/Library/ArmArchTimerLib/ArmArchTimerLib.c
@@ -62,6 +62,8 @@ MicroSecondDelay (
 {

   UINT64  TimerTicks64;

   UINT64  SystemCounterVal;

+  UINT64  PreviousSystemCounterVal;

+  UINT64  DeltaCounterVal;

 

   // Calculate counter ticks that represent requested delay:

   //  = MicroSeconds x TICKS_PER_MICRO_SEC

@@ -75,13 +77,17 @@ MicroSecondDelay (
                    );

 

   // Read System Counter value

-  SystemCounterVal = ArmGenericTimerGetSystemCount ();

-

-  TimerTicks64 += SystemCounterVal;

+  PreviousSystemCounterVal = ArmGenericTimerGetSystemCount ();

 

   // Wait until delay count expires.

-  while (SystemCounterVal < TimerTicks64) {

+  while (TimerTicks64 > 0) {

     SystemCounterVal = ArmGenericTimerGetSystemCount ();

+    // Get how much we advanced this tick. Wrap around still has delta correct

+    DeltaCounterVal = (SystemCounterVal - PreviousSystemCounterVal)

+                      & (MAX_UINT64 >> 8); // Account for a lesser (minimum) size

+    // Never wrap back around below zero by choosing the min and thus stop at 0

+    TimerTicks64            -= MIN (TimerTicks64, DeltaCounterVal);

+    PreviousSystemCounterVal = SystemCounterVal;

   }

 

   return MicroSeconds;