Bug#740098: Patch to fix setting USB-serial adapters to 38400 bps

[Jan Niehusmann]

tag 740098 +patch
thanks

The following patch should fix the reported issue.

On serial ports not supporting the TIOCGSERIAL/TIOCSSERIAL mechanism,
this still means that the custom baud rates do not work, but at least
38400 works, again.

Perhaps an even better fix would be switching to the newer termios2 API,
as described here:
https://mail.python.org/pipermail/python-list/2012-October/633871.html

That API allows setting custom baud rates on the USB serial adapters I
have tested, where TIOCGSERIAL/TIOCSSERIAL was not supported.

But unfortunately, termios2 is not available through glibc, so one would
need to call linux-specific ioctls directly, which wouldn't be portable
at all, and I don't know how it interacts with the BSD / Hurd ports.

Therefore this is the minimal fix, for now.

--- a/src/SerialImp.c
+++ b/src/SerialImp.c
@@ -872,10 +872,14 @@
 	   out and select baud rates 38400 then 28800 then 38400, you will get
 	   a final baud rate of 28800 because you did not update the divisor.
 
+	   However, if the serial port doesn't support TIOCGSERIAL or custom speeds,
+	   don't fail: In this case, just use B38400 and assume that there is no way
+	   to set a custom speed on this port.
+
 	   See the next ifdef below for the divisor.
 	*/
 #if defined(TIOCGSERIAL)
-	if ( cspeed == B38400 )
+	if ( cspeed == B38400 && ioctl( fd, TIOCGSERIAL, &sstruct ) == 0 )
 		cspeed = 38400;
 #endif /* TIOCGSERIAL */
 	if(     cfsetispeed( &ttyset, cspeed ) < 0 ||
@@ -905,10 +909,9 @@
 			return(1);
 		}
 		sstruct.custom_divisor = ( sstruct.baud_base/cspeed );
-		cspeed = B38400;
 #endif /* TIOCGSERIAL */
-		if(     cfsetispeed( &ttyset, cspeed ) < 0 ||
-			cfsetospeed( &ttyset, cspeed ) < 0 )
+		if(     cfsetispeed( &ttyset, B38400 ) < 0 ||
+			cfsetospeed( &ttyset, B38400 ) < 0 )
 		{
 			/* OK, we tried everything */
 			report( "nativeSetSerialPortParams: Cannot Set Speed\n" );
@@ -916,8 +919,11 @@
 		}
 #if defined(TIOCSSERIAL)
 		/*  It is assumed Win32 does this for us */
+		/*  Ignore errors if the target speed is 38400: In this case,
+		 *  just assume TIOCSSERIAL is not supported and custom speeds are
+		 *  not available. */
 		if (	sstruct.baud_base < 1 ||
-		ioctl( fd, TIOCSSERIAL, &sstruct ) < 0 )
+		ioctl( fd, TIOCSSERIAL, &sstruct ) < 0 && cspeed != 38400 )
 		{
 			return( 1 );
 		}