[PATCH v5] tty: serial: Add RS422 flag to struct serial_rs485

[Crescent CY Hsieh]

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@xxxxxxxx>

---
Changes from v4 to v5:
- Revise commit message.
- Delete RS422 checks within uart_set_rs485_termination().

Changes from v3 to v4:
- Include 'linux/const.h' header in '/include/uapi/linux/serial.h'
- Replace BIT() with _BITUL() which defined in
  '/include/uapi/linux/const.h'

Changes from v2 to v3:
- Remove "SER_RS422_ENABLED" flag from legacy flags.
- Revise "SER_RS422_ENABLED" into "SER_RS485_MODE_RS422".
- Remove the code which checks the conflicts between SER_RS485_ENABLED
  and SER_RS422_ENABLED.
- Add return check in uart_set_rs485_termination().

Changes from v1 to v2:
- Revise the logic that checks whether RS422/RS485 are enabled
  simultaneously.

v4: https://lore.kernel.org/all/20231113094136.52003-1-crescentcy.hsieh@xxxxxxxx/
v3: https://lore.kernel.org/all/20231108060719.11775-1-crescentcy.hsieh@xxxxxxxx/
v2: https://lore.kernel.org/all/20231101064404.45711-1-crescentcy.hsieh@xxxxxxxx/
v1: https://lore.kernel.org/all/20231030053632.5109-1-crescentcy.hsieh@xxxxxxxx/

---
 drivers/tty/serial/serial_core.c |  7 +++++++
 include/uapi/linux/serial.h      | 19 +++++++++++--------
 2 files changed, 18 insertions(+), 8 deletions(-)

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 831d03361..db1ebed7f 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -1376,6 +1376,13 @@ static void uart_sanitize_serial_rs485(struct uart_port *port, struct serial_rs4
 		return;
 	}
 
+	/* Clear other RS485 flags and return if enabling RS422 */
+	if (rs485->flags & SER_RS485_MODE_RS422) {
+		memset(rs485, 0, sizeof(*rs485));
+		rs485->flags |= (SER_RS485_ENABLED | SER_RS485_MODE_RS422);
+		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..9086367db 100644
--- a/include/uapi/linux/serial.h
+++ b/include/uapi/linux/serial.h
@@ -11,6 +11,7 @@
 #ifndef _UAPI_LINUX_SERIAL_H
 #define _UAPI_LINUX_SERIAL_H
 
+#include <linux/const.h>
 #include <linux/types.h>
 
 #include <linux/tty_flags.h>
@@ -137,17 +138,19 @@ 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;
-#define SER_RS485_ENABLED		(1 << 0)
-#define SER_RS485_RTS_ON_SEND		(1 << 1)
-#define SER_RS485_RTS_AFTER_SEND	(1 << 2)
-#define SER_RS485_RX_DURING_TX		(1 << 4)
-#define SER_RS485_TERMINATE_BUS		(1 << 5)
-#define SER_RS485_ADDRB			(1 << 6)
-#define SER_RS485_ADDR_RECV		(1 << 7)
-#define SER_RS485_ADDR_DEST		(1 << 8)
+#define SER_RS485_ENABLED		_BITUL(0)
+#define SER_RS485_RTS_ON_SEND		_BITUL(1)
+#define SER_RS485_RTS_AFTER_SEND	_BITUL(2)
+#define SER_RS485_RX_DURING_TX		_BITUL(3)
+#define SER_RS485_TERMINATE_BUS		_BITUL(4)
+#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;
-- 
2.34.1