[PATCH v2] tty: serial: Add RS422 flag to struct serial_rs485

[Crescent CY Hsieh]

Add "SER_RS422_ENABLED" flag within struct serial_rs485, so that serial
port can switching interface into RS422 if supported by using ioctl
command "TIOCSRS485".

In case of interfaces confliction, add checks within
uart_sanitize_serial_rs485() such that only one of RS422/RS485 is set.

Signed-off-by: Crescent CY Hsieh <crescentcy.hsieh@xxxxxxxx>

---
Changes in v2:
- Revise the logic that checks whether RS422/RS485 are enabled
  simultaneously.

v1: https://lore.kernel.org/all/20231030053632.5109-1-crescentcy.hsieh@xxxxxxxx/

---
 drivers/tty/serial/serial_core.c | 19 +++++++++++++++++--
 include/uapi/linux/serial.h      |  4 ++++
 2 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 831d03361..54a104c52 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -1305,7 +1305,7 @@ static int uart_get_icount(struct tty_struct *tty,
 
 #define SER_RS485_LEGACY_FLAGS	(SER_RS485_ENABLED | SER_RS485_RTS_ON_SEND | \
 				 SER_RS485_RTS_AFTER_SEND | SER_RS485_RX_DURING_TX | \
-				 SER_RS485_TERMINATE_BUS)
+				 SER_RS485_TERMINATE_BUS | SER_RS422_ENABLED)
 
 static int uart_check_rs485_flags(struct uart_port *port, struct serial_rs485 *rs485)
 {
@@ -1371,11 +1371,26 @@ static void uart_sanitize_serial_rs485(struct uart_port *port, struct serial_rs4
 {
 	u32 supported_flags = port->rs485_supported.flags;
 
-	if (!(rs485->flags & SER_RS485_ENABLED)) {
+	if (!(rs485->flags & (SER_RS485_ENABLED | SER_RS422_ENABLED))) {
 		memset(rs485, 0, sizeof(*rs485));
 		return;
 	}
 
+	/* Pick sane setting if the user enables both interfaces */
+	if (rs485->flags & SER_RS485_ENABLED && rs485->flags & SER_RS422_ENABLED) {
+		dev_warn_ratelimited(port->dev,
+			"%s (%d): Invalid serial interface setting, using RS485 instead\n",
+			port->name, port->line);
+		rs485->flags &= ~SER_RS422_ENABLED;
+	}
+
+	/* Clear other bits and return if RS422 is enabled */
+	if (rs485->flags & SER_RS422_ENABLED) {
+		memset(rs485, 0, sizeof(*rs485));
+		rs485->flags |= SER_RS422_ENABLED;
+		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) ==
diff --git a/include/uapi/linux/serial.h b/include/uapi/linux/serial.h
index 53bc1af67..427609fd5 100644
--- a/include/uapi/linux/serial.h
+++ b/include/uapi/linux/serial.h
@@ -137,6 +137,8 @@ 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_RS422_ENABLED		- RS422 enabled.
  */
 struct serial_rs485 {
 	__u32	flags;
@@ -149,6 +151,8 @@ struct serial_rs485 {
 #define SER_RS485_ADDR_RECV		(1 << 7)
 #define SER_RS485_ADDR_DEST		(1 << 8)
 
+#define SER_RS422_ENABLED		(1 << 9)
+
 	__u32	delay_rts_before_send;
 	__u32	delay_rts_after_send;
 
-- 
2.34.1