[PATCH] tty: improve encode_baud_rate logic

Mostly so people can see the work in progress.  This enhances the encode
function which isn't currently used in the base tree but is when using some of
the testing tty patches.

This resolves a problem with some hardware where applications got confusing
information from the tty ioctls.  Correct but confusing.

In some situations asking for, say, 9600 baud actually gets you 9595 baud or
similar near-miss values.  With the old code this meant that a request for
B9600 got a return of BOTHER, 9595 which programs interpreted as a failure.

The new code now works on the following basis

- If you ask for specific rate via BOTHER, you get a precise return

- If you ask for a standard Bfoo rate and the result is close you get a Bfoo
  return

- If you ask for a standard Bfoo rate and get something way off you get a
  BOTHER/rate return

This seems to fix up the cases I've found where this broke compatibility.
Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/char/tty_ioctl.c

@@ -225,7 +225,7 @@ EXPORT_SYMBOL(tty_termios_input_baud_rate);
 
 /**
  *	tty_termios_encode_baud_rate
- *	@termios: termios structure
+ *	@termios: ktermios structure holding user requested state
  *	@ispeed: input speed
  *	@ospeed: output speed
  *
@@ -233,7 +233,10 @@ EXPORT_SYMBOL(tty_termios_input_baud_rate);
  *	used as a library helper for drivers os that they can report back
  *	the actual speed selected when it differs from the speed requested
  *
- *	For now input and output speed must agree.
+ *	For maximal back compatibility with legacy SYS5/POSIX *nix behaviour
+ *	we need to carefully set the bits when the user does not get the
+ *	desired speed. We allow small margins and preserve as much of possible
+ *	of the input intent to keep compatiblity.
  *
  *	Locking: Caller should hold termios lock. This is already held
  *	when calling this function from the driver termios handler.
@@ -242,32 +245,44 @@ EXPORT_SYMBOL(tty_termios_input_baud_rate);
 void tty_termios_encode_baud_rate(struct ktermios *termios, speed_t ibaud, speed_t obaud)
 {
 	int i = 0;
-	int ifound = 0, ofound = 0;
+	int ifound = -1, ofound = -1;
+	int iclose = ibaud/50, oclose = obaud/50;
+	int ibinput = 0;
 
 	termios->c_ispeed = ibaud;
 	termios->c_ospeed = obaud;
 
+	/* If the user asked for a precise weird speed give a precise weird
+	   answer. If they asked for a Bfoo speed they many have problems
+	   digesting non-exact replies so fuzz a bit */
+
+	if ((termios->c_cflag & CBAUD) == BOTHER)
+		oclose = 0;
+	if (((termios->c_cflag >> IBSHIFT) & CBAUD) == BOTHER)
+		iclose = 0;
+	if ((termios->c_cflag >> IBSHIFT) & CBAUD)
+		ibinput = 1;	/* An input speed was specified */
+
 	termios->c_cflag &= ~CBAUD;
-	/* Identical speed means no input encoding (ie B0 << IBSHIFT)*/
-	if (termios->c_ispeed == termios->c_ospeed)
-		ifound = 1;
 
 	do {
-		if (obaud == baud_table[i]) {
+		if (obaud - oclose >= baud_table[i] && obaud + oclose <= baud_table[i]) {
 			termios->c_cflag |= baud_bits[i];
-			ofound = 1;
-			/* So that if ibaud == obaud we don't set it */
-			continue;
+			ofound = i;
 		}
-		if (ibaud == baud_table[i]) {
-			termios->c_cflag |= (baud_bits[i] << IBSHIFT);
-			ifound = 1;
+		if (ibaud - iclose >= baud_table[i] && ibaud + iclose <= baud_table[i]) {
+			/* For the case input == output don't set IBAUD bits if the user didn't do so */
+			if (ofound != i || ibinput)
+				termios->c_cflag |= (baud_bits[i] << IBSHIFT);
+			ifound = i;
 		}
 	}
 	while(++i < n_baud_table);
-	if (!ofound)
+	if (ofound == -1)
 		termios->c_cflag |= BOTHER;
-	if (!ifound)
+	/* Set exact input bits only if the input and output differ or the
+	   user already did */
+	if (ifound == -1 && (ibaud != obaud  || ibinput))
 		termios->c_cflag |= (BOTHER << IBSHIFT);
 }