MdeModulePkg/SerialDxe: Inconsistent timeout processing in SerialRead

[Heyi Guo <heyi.guo@linaro.org>]

Hi Folks,

In SerialRead function in MdeModulePkg/Universal/SerialDxe/SerialIo.c, 
it seems the timeout processing in SerialRead is not consistent.

Since SerialPortPoll only checks the status of serial port and returns 
immediately, and SerialPortRead does not really implement a time out 
mechanism and will always wait for enough input, it will cause below 
results:

1. If there is no serial input at all, this interface will return 
timeout immediately without any waiting;

2. If there is A characters in serial port FIFO, and caller requires A+1 
characters, it will wait until a new input is coming and timeout will 
not really occur.

As SerialPortLib is a simple library implementation, I think it is 
better to improve SerialIoDxe driver instead of SerialPortLib.

Please let me know your comments about this.

Thanks and regards,

Gary (Heyi Guo)

EFI_STATUS
EFIAPI
SerialRead (
   IN EFI_SERIAL_IO_PROTOCOL *This,
   IN OUT UINTN              *BufferSize,
   OUT VOID                  *Buffer
   )
{
   UINTN Count;

   Count = 0;

   if (SerialPortPoll ()) {
     Count = SerialPortRead (Buffer, *BufferSize);
   }

   if (Count != *BufferSize) {
     *BufferSize = Count;
     return EFI_TIMEOUT;
   }

   return EFI_SUCCESS;
}