serial: 8250: Create serial_lsr_in()

LSR register readers need to be careful in order to not lose bits that
are not preserved across reads. Create a helper that takes care of
storing the non-preserved bits into lsr_save_flags.
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20220608095431.18376-3-ilpo.jarvinen@linux.intel.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250.h

@@ -123,6 +123,26 @@ static inline void serial_out(struct uart_8250_port *up, int offset, int value)
 	up->port.serial_out(&up->port, offset, value);
 }
 
+/**
+ *	serial_lsr_in - Read LSR register and preserve flags across reads
+ *	@up:	uart 8250 port
+ *
+ *	Read LSR register and handle saving non-preserved flags across reads.
+ *	The flags that are not preserved across reads are stored into
+ *	up->lsr_saved_flags.
+ *
+ *	Returns LSR value or'ed with the preserved flags (if any).
+ */
+static inline unsigned int serial_lsr_in(struct uart_8250_port *up)
+{
+	unsigned int lsr = up->lsr_saved_flags;
+
+	lsr |= serial_in(up, UART_LSR);
+	up->lsr_saved_flags = lsr & LSR_SAVE_FLAGS;
+
+	return lsr;
+}
+
 /*
  * For the 16C950
  */

drivers/tty/serial/8250/8250_core.c

@@ -276,8 +276,7 @@ static void serial8250_backup_timeout(struct timer_list *t)
 	 * the "Diva" UART used on the management processor on many HP
 	 * ia64 and parisc boxes.
 	 */
-	lsr = serial_in(up, UART_LSR);
-	up->lsr_saved_flags |= lsr & LSR_SAVE_FLAGS;
+	lsr = serial_lsr_in(up);
 	if ((iir & UART_IIR_NO_INT) && (up->ier & UART_IER_THRI) &&
 	    (!uart_circ_empty(&up->port.state->xmit) || up->port.x_char) &&
 	    (lsr & UART_LSR_THRE)) {

drivers/tty/serial/8250/8250_port.c

@@ -1508,11 +1508,9 @@ static inline void __stop_tx(struct uart_8250_port *p)
 	struct uart_8250_em485 *em485 = p->em485;
 
 	if (em485) {
-		unsigned char lsr = serial_in(p, UART_LSR);
+		unsigned char lsr = serial_lsr_in(p);
 		u64 stop_delay = 0;
 
-		p->lsr_saved_flags |= lsr & LSR_SAVE_FLAGS;
-
 		if (!(lsr & UART_LSR_THRE))
 			return;
 		/*
@@ -1569,10 +1567,8 @@ static inline void __start_tx(struct uart_port *port)
 
 	if (serial8250_set_THRI(up)) {
 		if (up->bugs & UART_BUG_TXEN) {
-			unsigned char lsr;
+			unsigned char lsr = serial_lsr_in(up);
 
-			lsr = serial_in(up, UART_LSR);
-			up->lsr_saved_flags |= lsr & LSR_SAVE_FLAGS;
 			if (lsr & UART_LSR_THRE)
 				serial8250_tx_chars(up);
 		}
@@ -2002,8 +1998,7 @@ static unsigned int serial8250_tx_empty(struct uart_port *port)
 	serial8250_rpm_get(up);
 
 	spin_lock_irqsave(&port->lock, flags);
-	lsr = serial_port_in(port, UART_LSR);
-	up->lsr_saved_flags |= lsr & LSR_SAVE_FLAGS;
+	lsr = serial_lsr_in(up);
 	spin_unlock_irqrestore(&port->lock, flags);
 
 	serial8250_rpm_put(up);
@@ -2079,9 +2074,7 @@ static void wait_for_lsr(struct uart_8250_port *up, int bits)
 
 	/* Wait up to 10ms for the character(s) to be sent. */
 	for (;;) {
-		status = serial_in(up, UART_LSR);
-
-		up->lsr_saved_flags |= status & LSR_SAVE_FLAGS;
+		status = serial_lsr_in(up);
 
 		if ((status & bits) == bits)
 			break;