tty: serial: Add RS422 flag to struct serial_rs485

Add "SER_RS485_MODE_RS422" flag to struct serial_rs485, so that serial
port can switch interface into RS422 if supported by using ioctl command
"TIOCSRS485".

By treating RS422 as a mode of RS485, which means while enabling RS422
there are two flags need to be set (SER_RS485_ENABLED and
SER_RS485_MODE_RS422), it would make things much easier. For example
some places that checks for "SER_RS485_ENABLED" won't need to be rewritten.
Signed-off-by: Crescent CY Hsieh <crescentcy.hsieh@moxa.com>
Link: https://lore.kernel.org/r/20231201071554.258607-3-crescentcy.hsieh@moxa.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/serial_core.c

@@ -1370,6 +1370,12 @@ static void uart_sanitize_serial_rs485(struct uart_port *port, struct serial_rs4
 		return;
 	}
 
+	/* Clear other RS485 flags but SER_RS485_TERMINATE_BUS and return if enabling RS422 */
+	if (rs485->flags & SER_RS485_MODE_RS422) {
+		rs485->flags &= (SER_RS485_ENABLED | SER_RS485_MODE_RS422 | SER_RS485_TERMINATE_BUS);
+		return;
+	}
+
 	/* Pick sane settings if the user hasn't */
 	if ((supported_flags & (SER_RS485_RTS_ON_SEND|SER_RS485_RTS_AFTER_SEND)) &&
 	    !(rs485->flags & SER_RS485_RTS_ON_SEND) ==

include/uapi/linux/serial.h

@@ -138,6 +138,7 @@ struct serial_icounter_struct {
  * * %SER_RS485_ADDRB		- Enable RS485 addressing mode.
  * * %SER_RS485_ADDR_RECV - Receive address filter (enables @addr_recv). Requires %SER_RS485_ADDRB.
  * * %SER_RS485_ADDR_DEST - Destination address (enables @addr_dest). Requires %SER_RS485_ADDRB.
+ * * %SER_RS485_MODE_RS422	- Enable RS422. Requires %SER_RS485_ENABLED.
  */
 struct serial_rs485 {
 	__u32	flags;
@@ -149,6 +150,7 @@ struct serial_rs485 {
 #define SER_RS485_ADDRB			_BITUL(5)
 #define SER_RS485_ADDR_RECV		_BITUL(6)
 #define SER_RS485_ADDR_DEST		_BITUL(7)
+#define SER_RS485_MODE_RS422		_BITUL(8)
 
 	__u32	delay_rts_before_send;
 	__u32	delay_rts_after_send;