diff --git a/MdePkg/Include/Library/TimerLib.h b/MdePkg/Include/Library/TimerLib.h
index 8523bd0dfb..8f31bbb5f3 100644
--- a/MdePkg/Include/Library/TimerLib.h
+++ b/MdePkg/Include/Library/TimerLib.h
@@ -93,8 +93,8 @@ GetPerformanceCounter (
 UINT64
 EFIAPI
 GetPerformanceCounterProperties (
-  IN      UINT64                    *StartValue,  OPTIONAL
-  IN      UINT64                    *EndValue     OPTIONAL
+  OUT      UINT64                    *StartValue,  OPTIONAL
+  OUT      UINT64                    *EndValue     OPTIONAL
   );
 
 #endif
diff --git a/MdePkg/Library/BaseTimerLibLocalApic/Ebc/EbcTimerLib.c b/MdePkg/Library/BaseTimerLibLocalApic/Ebc/EbcTimerLib.c
index 97cf3e28af..416a44f4ea 100644
--- a/MdePkg/Library/BaseTimerLibLocalApic/Ebc/EbcTimerLib.c
+++ b/MdePkg/Library/BaseTimerLibLocalApic/Ebc/EbcTimerLib.c
@@ -111,8 +111,8 @@ GetPerformanceCounter (
 UINT64
 EFIAPI
 GetPerformanceCounterProperties (
-  IN      UINT64                    *StartValue,
-  IN      UINT64                    *EndValue
+  OUT      UINT64                    *StartValue,  OPTIONAL
+  OUT      UINT64                    *EndValue     OPTIONAL
   )
 {
   //
diff --git a/MdePkg/Library/BaseTimerLibLocalApic/Ipf/IpfTimerLib.c b/MdePkg/Library/BaseTimerLibLocalApic/Ipf/IpfTimerLib.c
index 800b5258e3..12803d9fa4 100644
--- a/MdePkg/Library/BaseTimerLibLocalApic/Ipf/IpfTimerLib.c
+++ b/MdePkg/Library/BaseTimerLibLocalApic/Ipf/IpfTimerLib.c
@@ -188,8 +188,8 @@ GetPerformanceCounter (
 UINT64
 EFIAPI
 GetPerformanceCounterProperties (
-  IN      UINT64                    *StartValue,
-  IN      UINT64                    *EndValue
+  OUT      UINT64                    *StartValue,  OPTIONAL
+  OUT      UINT64                    *EndValue     OPTIONAL
   )
 {
   PAL_PROC_RETURN                   PalRet;
diff --git a/MdePkg/Library/BaseTimerLibLocalApic/x86TimerLib.c b/MdePkg/Library/BaseTimerLibLocalApic/x86TimerLib.c
index dd240d88d0..09b614119d 100644
--- a/MdePkg/Library/BaseTimerLibLocalApic/x86TimerLib.c
+++ b/MdePkg/Library/BaseTimerLibLocalApic/x86TimerLib.c
@@ -239,8 +239,8 @@ GetPerformanceCounter (
 UINT64
 EFIAPI
 GetPerformanceCounterProperties (
-  IN      UINT64                    *StartValue,
-  IN      UINT64                    *EndValue
+  OUT      UINT64                    *StartValue,  OPTIONAL
+  OUT      UINT64                    *EndValue     OPTIONAL
   )
 {
   UINTN                             ApicBase;