tty: Remove baudrate dead code & make ktermios params const

With the architectures currently in-tree, either:
  1) CBAUDEX is zero
  2) The earlier BOTHER if check covers cbaud < 1 case
  3) All CBAUD bits are covered by the baud_table

Thus, the check for cbaud being out-of-range for CBAUDEX case cannot
ever be true.

The ktermios parameters can now be made const.
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20220816115739.10928-2-ilpo.jarvinen@linux.intel.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/tty.h

@@ -73,7 +73,7 @@ void tty_buffer_set_lock_subclass(struct tty_port *port);
 bool tty_buffer_restart_work(struct tty_port *port);
 bool tty_buffer_cancel_work(struct tty_port *port);
 void tty_buffer_flush_work(struct tty_port *port);
-speed_t tty_termios_input_baud_rate(struct ktermios *termios);
+speed_t tty_termios_input_baud_rate(const struct ktermios *termios);
 void tty_ldisc_hangup(struct tty_struct *tty, bool reset);
 int tty_ldisc_reinit(struct tty_struct *tty, int disc);
 long tty_ioctl(struct file *file, unsigned int cmd, unsigned long arg);

drivers/tty/tty_baudrate.c

@@ -49,13 +49,13 @@ static int n_baud_table = ARRAY_SIZE(baud_table);
  *
  *	Convert termios baud rate data into a speed. This should be called
  *	with the termios lock held if this termios is a terminal termios
- *	structure. May change the termios data. Device drivers can call this
- *	function but should use ->c_[io]speed directly as they are updated.
+ *	structure. Device drivers can call this function but should use
+ *	->c_[io]speed directly as they are updated.
  *
  *	Locking: none
  */
 
-speed_t tty_termios_baud_rate(struct ktermios *termios)
+speed_t tty_termios_baud_rate(const struct ktermios *termios)
 {
 	unsigned int cbaud;
 
@@ -67,11 +67,7 @@ speed_t tty_termios_baud_rate(struct ktermios *termios)
 
 	if (cbaud & CBAUDEX) {
 		cbaud &= ~CBAUDEX;
-
-		if (cbaud < 1 || cbaud + 15 > n_baud_table)
-			termios->c_cflag &= ~CBAUDEX;
-		else
-			cbaud += 15;
+		cbaud += 15;
 	}
 	return cbaud >= n_baud_table ? 0 : baud_table[cbaud];
 }
@@ -83,13 +79,13 @@ EXPORT_SYMBOL(tty_termios_baud_rate);
  *
  *	Convert termios baud rate data into a speed. This should be called
  *	with the termios lock held if this termios is a terminal termios
- *	structure. May change the termios data. Device drivers can call this
- *	function but should use ->c_[io]speed directly as they are updated.
+ *	structure. Device drivers can call this function but should use
+ *	->c_[io]speed directly as they are updated.
  *
  *	Locking: none
  */
 
-speed_t tty_termios_input_baud_rate(struct ktermios *termios)
+speed_t tty_termios_input_baud_rate(const struct ktermios *termios)
 {
 	unsigned int cbaud = (termios->c_cflag >> IBSHIFT) & CBAUD;
 
@@ -102,11 +98,7 @@ speed_t tty_termios_input_baud_rate(struct ktermios *termios)
 
 	if (cbaud & CBAUDEX) {
 		cbaud &= ~CBAUDEX;
-
-		if (cbaud < 1 || cbaud + 15 > n_baud_table)
-			termios->c_cflag &= ~(CBAUDEX << IBSHIFT);
-		else
-			cbaud += 15;
+		cbaud += 15;
 	}
 	return cbaud >= n_baud_table ? 0 : baud_table[cbaud];
 }

include/linux/tty.h

@@ -434,7 +434,7 @@ int tty_hung_up_p(struct file *filp);
 void do_SAK(struct tty_struct *tty);
 void __do_SAK(struct tty_struct *tty);
 void no_tty(void);
-speed_t tty_termios_baud_rate(struct ktermios *termios);
+speed_t tty_termios_baud_rate(const struct ktermios *termios);
 void tty_termios_encode_baud_rate(struct ktermios *termios, speed_t ibaud,
 		speed_t obaud);
 void tty_encode_baud_rate(struct tty_struct *tty, speed_t ibaud,