tty: Add software emulated RS485 support for 8250

Implementation of software emulation of RS485 direction handling is based
on omap_serial driver.
Before and after transmission RTS is set to the appropriate value.

Note that before calling serial8250_em485_init() the caller has to
ensure that UART will interrupt when shift register empty. Otherwise,
emultaion cannot be used.

Both serial8250_em485_init() and serial8250_em485_destroy() are
idempotent functions.

Signed-off-by: Matwey V. Kornilov <matwey@sai.msu.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250.h

@@ -117,6 +117,8 @@ static inline void serial_dl_write(struct uart_8250_port *up, int value)
 struct uart_8250_port *serial8250_get_port(int line);
 void serial8250_rpm_get(struct uart_8250_port *p);
 void serial8250_rpm_put(struct uart_8250_port *p);
+int serial8250_em485_init(struct uart_8250_port *p);
+void serial8250_em485_destroy(struct uart_8250_port *p);
 
 #if defined(__alpha__) && !defined(CONFIG_PCI)
 /*