[Patch 1/1] MdeModulePkg/PciSioSerialDxe: Flush Tx before config change

["Michael D Kinney" <michael.d.kinney@intel.com>]

https://bugzilla.tianocore.org/show_bug.cgi?id=3114

Add logic to flush all UART transmit buffers if there is a
config change from Reset(), SetAttributes() or SetControl().
Use a timeout in the flush operation, so the system can
continue to boot if the transmit buffers can not be
flushed for any reason.

This change prevents lost characters on serial debug logs
and serial consoles when a config change is made.  It also
prevents a UART from getting into a bad state or reporting
error status due to characters being transmitted at the same
time registers are updated with new communications settings.

Cc: Hao A Wu <hao.a.wu@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
---
 .../Bus/Pci/PciSioSerialDxe/SerialIo.c        | 91 ++++++++++++++++++-
 1 file changed, 90 insertions(+), 1 deletion(-)

diff --git a/MdeModulePkg/Bus/Pci/PciSioSerialDxe/SerialIo.c b/MdeModulePkg/Bus/Pci/PciSioSerialDxe/SerialIo.c
index 8377ffa13c7a..56c5faf5db1f 100644
--- a/MdeModulePkg/Bus/Pci/PciSioSerialDxe/SerialIo.c
+++ b/MdeModulePkg/Bus/Pci/PciSioSerialDxe/SerialIo.c
@@ -1,7 +1,7 @@
 /** @file
   SerialIo implementation for PCI or SIO UARTs.
 
-Copyright (c) 2006 - 2018, Intel Corporation. All rights reserved.<BR>
+Copyright (c) 2006 - 2020, Intel Corporation. All rights reserved.<BR>
 SPDX-License-Identifier: BSD-2-Clause-Patent
 
 **/
@@ -436,6 +436,68 @@ SerialReceiveTransmit (
   return EFI_SUCCESS;
 }
 
+/**
+  Flush the serial hardware transmit FIFO, holding register, and shift register.
+
+  @param SerialDevice  The device to flush.
+
+  @retval  EFI_SUCCESS  The transmit FIFO is completely flushed.
+  @retval  EFI_TIMEOUT  A timeout occured waiting for the transmit FIFO to flush.
+**/
+EFI_STATUS
+SerialFlushTransmitFifo (
+  SERIAL_DEV  *SerialDevice
+  )
+{
+  SERIAL_PORT_LSR  Lsr;
+  UINTN            Timeout;
+  UINTN            Elapsed;
+
+  //
+  // If this is the first time the UART is being configured, then the current
+  // UART settings are not known, so compute a timeout to wait for the Tx FIFO
+  // assuming the worst case current settings.
+  //
+  // Timeout = (Max Bits per Char) * (Max Pending Chars) / (Slowest Baud Rate)
+  //   Max Bits per Char = Start bit + 8 data bits + parity + 2 stop bits = 12
+  //   Max Pending Chars = Largest Tx FIFO + hold + shift = 64 + 1 + 1 = 66
+  //   Slowest Reasonable Baud Rate = 300 baud
+  // Timeout = 12 * 66 / 300 = 2.64 seconds = 2,640,000 uS
+  //
+  Timeout = 2640000;
+
+  //
+  // Use the largest of the computed timeout, the default timeout, and the
+  // currently set timeout.
+  //
+  Timeout = MAX (Timeout, SERIAL_PORT_DEFAULT_TIMEOUT);
+  Timeout = MAX (Timeout, SerialDevice->SerialMode.Timeout);
+
+  //
+  // Wait for the shortest time possible for the serial port to be ready making
+  // sure the transmit FIFO, holding register, and shift register are all
+  // empty.  The actual wait time is expected to be very small because the
+  // number characters currently in the FIFO should be small when a
+  // configuration change is requested.
+  //
+  // NOTE: Do not use any DEBUG() or REPORT_STATUS_CODE() or any other calls
+  // in the rest of this function that may send additional characters to this
+  // UART device invalidating the flush operation.
+  //
+  Elapsed = 0;
+  Lsr.Data = READ_LSR (SerialDevice);
+  while (Lsr.Bits.Temt == 0 || Lsr.Bits.Thre == 0) {
+    if (Elapsed >= Timeout) {
+      return EFI_TIMEOUT;
+    }
+    gBS->Stall (TIMEOUT_STALL_INTERVAL);
+    Elapsed += TIMEOUT_STALL_INTERVAL;
+    Lsr.Data = READ_LSR (SerialDevice);
+  }
+
+  return EFI_SUCCESS;
+}
+
 //
 // Interface Functions
 //
@@ -476,6 +538,15 @@ SerialReset (
 
   Tpl = gBS->RaiseTPL (TPL_NOTIFY);
 
+  //
+  // Wait for all data to be transmitted before changing the UART configuration.
+  //
+  // NOTE: Do not use any DEBUG() or REPORT_STATUS_CODE() or any other calls
+  // that may send additional characters to this UART device until the UART
+  // configuration change is complete.
+  //
+  SerialFlushTransmitFifo (SerialDevice);
+
   //
   // Make sure DLAB is 0.
   //
@@ -654,6 +725,15 @@ SerialSetAttributes (
 
   Tpl = gBS->RaiseTPL (TPL_NOTIFY);
 
+  //
+  // Wait for all data to be transmitted before changing the UART configuration.
+  //
+  // NOTE: Do not use any DEBUG() or REPORT_STATUS_CODE() or any other calls
+  // that may send additional characters to this UART device until the UART
+  // configuration change is complete.
+  //
+  SerialFlushTransmitFifo (SerialDevice);
+
   //
   // Put serial port on Divisor Latch Mode
   //
@@ -826,6 +906,15 @@ SerialSetControl (
 
   Tpl = gBS->RaiseTPL (TPL_NOTIFY);
 
+  //
+  // Wait for all data to be transmitted before changing the UART configuration.
+  //
+  // NOTE: Do not use any DEBUG() or REPORT_STATUS_CODE() or any other calls
+  // that may send additional characters to this UART device until the UART
+  // configuration change is complete.
+  //
+  SerialFlushTransmitFifo (SerialDevice);
+
   Mcr.Data = READ_MCR (SerialDevice);
   Mcr.Bits.DtrC = 0;
   Mcr.Bits.Rts = 0;
-- 
2.29.2.windows.2