[PATCH] MdeModulePkg SerialDxe: Handle Timeout change more robustly

[Star Zeng <star.zeng@intel.com>]

https://lists.01.org/pipermail/edk2-devel/2017-October/016479.html
reported "Xen Console input very slow in recent UEFI" that appears
after 4cf3f37c87ba1f9d58072444bd735e40e4779e70 "MdeModulePkg
SerialDxe: Process timeout consistently in SerialRead".

Julien did more debugging and find out the following is happening in
TerminalConInTimerHandler (MdeModulePkg/Universal/Console/TerminalDxe)
when a character is received:
1) GetControl will return EFI_SERIAL_INPUT_BUFFER_EMPTY unset
  => Entering in the loop to fetch character from the serial
2) GetOneKeyFromSerial()
  => Return directly with the character read
3) Looping as the fifo is not full and no error
4) GetOneKeyFromSerial() -> SerialRead()
  => No more character so SerialPortPoll() will return FALSE and loop
     until timeout
  => Return EFI_TIMEOUT
5) Exiting the loop from TerminalConInTimerHandler
6) Characters are printed

After some investigation, I found it is related to the Timeout value.

The Timeout is 1000000 (1s) by default to follow UEFI spec.
And the Terminal driver will recalculate and set the Timeout value
based on the properties of UART in TerminalDriverBindingStart()/
TerminalConInTimerHandler().

  SerialInTimeOut = 0;
  if (Mode->BaudRate != 0) {
    //
    // According to BAUD rate to calculate the timeout value.
    //
    SerialInTimeOut = (1 + Mode->DataBits + Mode->StopBits) *
                      2 * 1000000 / (UINTN) Mode->BaudRate;
  }

For example, based on the PCD values of PcdUartDefaultBaudRate,
PcdUartDefaultDataBits and PcdUartDefaultStopBits, SerialInTimeOut =
(1 + 8  + 1) * 2 * 1000000 / (UINTN) 115200 = 173 (us).

When SerialDxe is used,
TerminalDriverBindingStart()/TerminalConInTimerHandler() ->
  SerialIo->SetAttributes() ->
    SerialSetAttributes() ->
      SerialPortSetAttributes()

Some implementations of SerialPortSetAttributes() could handle the
input parameters and return RETURN_SUCCESS, for example
BaseSerialPortLib16550, then Timeout value will be changed to 173 (us),
no "slow down" will be observed.
But some implementations of SerialPortSetAttributes() just return
RETURN_UNSUPPORTED, for example XenConsoleSerialPortLib, then Timeout
value will be not changed and kept 1000000 (1s), "slow down" will be
observed.

SerialPortLib instance can be enhanced to
1. Handle the input parameters and return status accordingly instead of
just returning RETURN_UNSUPPORTED in SerialPortSetAttributes().
2. Just return RETURN_SUCCESS instead of RETURN_UNSUPPORTED in
SerialPortSetAttributes() if the instance does not care the input
parameters at all.

And SerialDxe can also be enhanced like this patch to be more robust
to handle Timeout change.

Cc: Julien Grall <julien.grall@linaro.org>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
---
 MdeModulePkg/Universal/SerialDxe/SerialIo.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/MdeModulePkg/Universal/SerialDxe/SerialIo.c b/MdeModulePkg/Universal/SerialDxe/SerialIo.c
index ebcd92726314..060ea56c2b1a 100644
--- a/MdeModulePkg/Universal/SerialDxe/SerialIo.c
+++ b/MdeModulePkg/Universal/SerialDxe/SerialIo.c
@@ -285,7 +285,21 @@ SerialSetAttributes (
 
   Status = SerialPortSetAttributes (&BaudRate, &ReceiveFifoDepth, &Timeout, &Parity, &DataBits, &StopBits);
   if (EFI_ERROR (Status)) {
-    return Status;
+    //
+    // If it is just to set Timeout value and unsupported is returned,
+    // do not return error.
+    //
+    if ((Status == EFI_UNSUPPORTED) &&
+        (This->Mode->Timeout          != Timeout) &&
+        (This->Mode->ReceiveFifoDepth == ReceiveFifoDepth) &&
+        (This->Mode->BaudRate         == BaudRate) &&
+        (This->Mode->DataBits         == (UINT32) DataBits) &&
+        (This->Mode->Parity           == (UINT32) Parity) &&
+        (This->Mode->StopBits         == (UINT32) StopBits)) {
+      Status = EFI_SUCCESS;
+    } else {
+      return Status;
+    }
   }
 
   //
-- 
2.7.0.windows.1