serial: 8250: Support rs485 devicetree properties

Retrieve rs485 devicetree properties on registration of 8250 ports in
case they are attached to an rs485 transceiver.

If the property "linux,rs485-enabled-at-boot-time" is present, invoke
the ->rs485_config() callback to immediately deassert RTS, thereby
ceasing control of the bus.
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Cc: Giulio Benetti <giulio.benetti@micronovasrl.com>
Cc: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/5908ea89b7f9da54872d6634b606d83db032297a.1582895077.git.lukas@wunner.deSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

@@ -1013,8 +1013,10 @@ int serial8250_register_8250_port(struct uart_8250_port *up)
 		if (uart->port.fifosize && !uart->tx_loadsz)
 			uart->tx_loadsz = uart->port.fifosize;
 
-		if (up->port.dev)
+		if (up->port.dev) {
 			uart->port.dev = up->port.dev;
+			uart_get_rs485_mode(uart->port.dev, &uart->port.rs485);
+		}
 
 		if (up->port.flags & UPF_FIXED_TYPE)
 			uart->port.type = up->port.type;

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

@@ -2989,6 +2989,9 @@ static void serial8250_config_port(struct uart_port *port, int flags)
 	if (flags & UART_CONFIG_TYPE)
 		autoconfig(up);
 
+	if (port->rs485.flags & SER_RS485_ENABLED)
+		port->rs485_config(port, &port->rs485);
+
 	/* if access method is AU, it is a 16550 with a quirk */
 	if (port->type == PORT_16550A && port->iotype == UPIO_AU)
 		up->bugs |= UART_BUG_NOMSR;