serial: 8250: Deduplicate ->rs485_config() callback

Commit e490c9144c ("tty: Add software emulated RS485 support for 8250")
introduced support to use RTS as an rs485 Transmit Enable signal.
Drivers opt in to the feature by calling serial8250_em485_init() from
their ->rs485_config() callback.

So far there are two drivers doing that, 8250_omap.c and 8250_of.c.
Both use an identical callback.  We're about to add a third user of that
callback, therefore deduplicate it and move it to 8250_port.c.

Drivers now opt in to rs485 software emulation by assigning the generic
serial8250_rs485_config() callback introduced herein to their
.rs485_config struct member.  This change allows unexporting
serial8250_em485_init() and declaring it static.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Cc: Matwey V. Kornilov <matwey@sai.msu.ru>
Cc: Heiko Schocher <hs@denx.de>
Link: https://lore.kernel.org/r/fcef63642dc4eae41ae7842d23747b2bf5d40285.1582895077.git.lukas@wunner.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250.h

@@ -156,7 +156,7 @@ void serial8250_rpm_put(struct uart_8250_port *p);
 void serial8250_rpm_get_tx(struct uart_8250_port *p);
 void serial8250_rpm_put_tx(struct uart_8250_port *p);
 
-int serial8250_em485_init(struct uart_8250_port *p);
+int serial8250_em485_config(struct uart_port *port, struct serial_rs485 *rs485);
 void serial8250_em485_destroy(struct uart_8250_port *p);
 
 /* MCR <-> TIOCM conversion */

drivers/tty/serial/8250/8250_of.c

@@ -25,36 +25,6 @@ struct of_serial_info {
 	int line;
 };
 
-static int of_8250_rs485_config(struct uart_port *port,
-				  struct serial_rs485 *rs485)
-{
-	struct uart_8250_port *up = up_to_u8250p(port);
-
-	/* Clamp the delays to [0, 100ms] */
-	rs485->delay_rts_before_send = min(rs485->delay_rts_before_send, 100U);
-	rs485->delay_rts_after_send  = min(rs485->delay_rts_after_send, 100U);
-
-	port->rs485 = *rs485;
-
-	/*
-	 * Both serial8250_em485_init and serial8250_em485_destroy
-	 * are idempotent
-	 */
-	if (rs485->flags & SER_RS485_ENABLED) {
-		int ret = serial8250_em485_init(up);
-
-		if (ret) {
-			rs485->flags &= ~SER_RS485_ENABLED;
-			port->rs485.flags &= ~SER_RS485_ENABLED;
-		}
-		return ret;
-	}
-
-	serial8250_em485_destroy(up);
-
-	return 0;
-}
-
 /*
  * Fill a struct uart_port for a given device node
  */
@@ -184,7 +154,7 @@ static int of_platform_serial_setup(struct platform_device *ofdev,
 		port->flags |= UPF_SKIP_TEST;
 
 	port->dev = &ofdev->dev;
-	port->rs485_config = of_8250_rs485_config;
+	port->rs485_config = serial8250_em485_config;
 
 	switch (type) {
 	case PORT_RT2880:

drivers/tty/serial/8250/8250_omap.c

@@ -714,36 +714,6 @@ static void omap_8250_throttle(struct uart_port *port)
 	pm_runtime_put_autosuspend(port->dev);
 }
 
-static int omap_8250_rs485_config(struct uart_port *port,
-				  struct serial_rs485 *rs485)
-{
-	struct uart_8250_port *up = up_to_u8250p(port);
-
-	/* Clamp the delays to [0, 100ms] */
-	rs485->delay_rts_before_send = min(rs485->delay_rts_before_send, 100U);
-	rs485->delay_rts_after_send  = min(rs485->delay_rts_after_send, 100U);
-
-	port->rs485 = *rs485;
-
-	/*
-	 * Both serial8250_em485_init and serial8250_em485_destroy
-	 * are idempotent
-	 */
-	if (rs485->flags & SER_RS485_ENABLED) {
-		int ret = serial8250_em485_init(up);
-
-		if (ret) {
-			rs485->flags &= ~SER_RS485_ENABLED;
-			port->rs485.flags &= ~SER_RS485_ENABLED;
-		}
-		return ret;
-	}
-
-	serial8250_em485_destroy(up);
-
-	return 0;
-}
-
 static void omap_8250_unthrottle(struct uart_port *port)
 {
 	struct omap8250_priv *priv = port->private_data;
@@ -1187,7 +1157,7 @@ static int omap8250_probe(struct platform_device *pdev)
 	up.port.shutdown = omap_8250_shutdown;
 	up.port.throttle = omap_8250_throttle;
 	up.port.unthrottle = omap_8250_unthrottle;
-	up.port.rs485_config = omap_8250_rs485_config;
+	up.port.rs485_config = serial8250_em485_config;
 	up.port.has_sysrq = IS_ENABLED(CONFIG_SERIAL_8250_CONSOLE);
 
 	ret = of_alias_get_id(np, "serial");

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

@@ -615,7 +615,7 @@ EXPORT_SYMBOL_GPL(serial8250_rpm_put);
  *
  *	Return 0 - success, -errno - otherwise
  */
-int serial8250_em485_init(struct uart_8250_port *p)
+static int serial8250_em485_init(struct uart_8250_port *p)
 {
 	if (p->em485)
 		return 0;
@@ -636,7 +636,6 @@ int serial8250_em485_init(struct uart_8250_port *p)
 
 	return 0;
 }
-EXPORT_SYMBOL_GPL(serial8250_em485_init);
 
 /**
  *	serial8250_em485_destroy() - put uart_8250_port into normal state
@@ -664,6 +663,44 @@ void serial8250_em485_destroy(struct uart_8250_port *p)
 }
 EXPORT_SYMBOL_GPL(serial8250_em485_destroy);
 
+/**
+ * serial8250_em485_config() - generic ->rs485_config() callback
+ * @port: uart port
+ * @rs485: rs485 settings
+ *
+ * Generic callback usable by 8250 uart drivers to activate rs485 settings
+ * if the uart is incapable of driving RTS as a Transmit Enable signal in
+ * hardware, relying on software emulation instead.
+ */
+int serial8250_em485_config(struct uart_port *port, struct serial_rs485 *rs485)
+{
+	struct uart_8250_port *up = up_to_u8250p(port);
+
+	/* clamp the delays to [0, 100ms] */
+	rs485->delay_rts_before_send = min(rs485->delay_rts_before_send, 100U);
+	rs485->delay_rts_after_send  = min(rs485->delay_rts_after_send, 100U);
+
+	port->rs485 = *rs485;
+
+	/*
+	 * Both serial8250_em485_init() and serial8250_em485_destroy()
+	 * are idempotent.
+	 */
+	if (rs485->flags & SER_RS485_ENABLED) {
+		int ret = serial8250_em485_init(up);
+
+		if (ret) {
+			rs485->flags &= ~SER_RS485_ENABLED;
+			port->rs485.flags &= ~SER_RS485_ENABLED;
+		}
+		return ret;
+	}
+
+	serial8250_em485_destroy(up);
+	return 0;
+}
+EXPORT_SYMBOL_GPL(serial8250_em485_config);
+
 /*
  * These two wrappers ensure that enable_runtime_pm_tx() can be called more than
  * once and disable_runtime_pm_tx() will still disable RPM because the fifo is