ArmPkg: Flush the FIFO when initializing the PL011 Serial Lib (#227)

Adds logic to the PL011 initialization that will flush the FIFO. Without
this change it's possible for garbage or stale data to be read from the
UART which can causes unexpected behavior or hangs during boot.

- [x] Impacts functionality?
- [ ] Impacts security?
- [ ] Breaking change?
- [ ] Includes tests?
- [ ] Includes documentation?

Tested on physical and virtual platform

N/A

Author: cfernald

ArmPlatformPkg/Library/PL011SerialPortLib/PL011SerialPortLib.c

@@ -34,22 +34,34 @@ SerialPortInitialize (
   EFI_PARITY_TYPE     Parity;
   UINT8               DataBits;
   EFI_STOP_BITS_TYPE  StopBits;
+  EFI_STATUS          Status;
+  UINT8               Scratch;
 
   BaudRate         = FixedPcdGet64 (PcdUartDefaultBaudRate);
   ReceiveFifoDepth = 0;         // Use default FIFO depth
   Parity           = (EFI_PARITY_TYPE)FixedPcdGet8 (PcdUartDefaultParity);
   DataBits         = FixedPcdGet8 (PcdUartDefaultDataBits);
   StopBits         = (EFI_STOP_BITS_TYPE)FixedPcdGet8 (PcdUartDefaultStopBits);
 
-  return PL011UartInitializePort (
-           (UINTN)PcdGet64 (PcdSerialRegisterBase),
-           PL011UartClockGetFreq (),
-           &BaudRate,
-           &ReceiveFifoDepth,
-           &Parity,
-           &DataBits,
-           &StopBits
-           );
+  // MU_CHANGE START: Flush FIFO when initializing the UART.
+  Status =  PL011UartInitializePort (
+              (UINTN)PcdGet64 (PcdSerialRegisterBase),
+              PL011UartClockGetFreq (),
+              &BaudRate,
+              &ReceiveFifoDepth,
+              &Parity,
+              &DataBits,
+              &StopBits
+              );
+
+  // Flush the FIFO.
+  if (!EFI_ERROR (Status)) {
+    while (SerialPortRead (&Scratch, sizeof (Scratch)) != 0) {
+    }
+  }
+
+  return Status;
+  // MU_CHANGE END
 }
 
 /**