serial: core: Introduce callback for start_rx and do stop_rx in suspend only if this callback implementation is present.

In suspend sequence there is a need to perform stop_rx during suspend
sequence to prevent any asynchronous data over rx line. However this
can cause problem to drivers which dont do re-start_rx during set_termios.

Add new callback start_rx and perform stop_rx only when implementation of
start_rx is present. Also add call to start_rx in resume sequence so that
drivers who come across this problem can make use of this framework.

Fixes: c9d2325 ("serial: core: Do stop_rx in suspend path for console if console_suspend is disabled")
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Vijaya Krishna Nivarthi <quic_vnivarth@quicinc.com>
Link: https://lore.kernel.org/r/1654627965-1461-2-git-send-email-quic_vnivarth@quicinc.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Vijaya Krishna Nivarthi

drivers/tty/serial/serial_core.c

@@ -2214,11 +2214,12 @@ int uart_suspend_port(struct uart_driver *drv, struct uart_port *uport)
 	/*
 	 * Nothing to do if the console is not suspending
 	 * except stop_rx to prevent any asynchronous data
-	 * over RX line. Re-start_rx, when required, is
-	 * done by set_termios in resume sequence
+	 * over RX line. However ensure that we will be
+	 * able to Re-start_rx later.
 	 */
 	if (!console_suspend_enabled && uart_console(uport)) {
-		uport->ops->stop_rx(uport);
+		if (uport->ops->start_rx)
+			uport->ops->stop_rx(uport);
 		goto unlock;
 	}
 
@@ -2310,6 +2311,8 @@ int uart_resume_port(struct uart_driver *drv, struct uart_port *uport)
 		if (console_suspend_enabled)
 			uart_change_pm(state, UART_PM_STATE_ON);
 		uport->ops->set_termios(uport, &termios, NULL);
+		if (!console_suspend_enabled && uport->ops->start_rx)
+			uport->ops->start_rx(uport);
 		if (console_suspend_enabled)
 			console_start(uport->cons);
 	}

include/linux/serial_core.h

@@ -45,6 +45,7 @@ struct uart_ops {
 	void		(*unthrottle)(struct uart_port *);
 	void		(*send_xchar)(struct uart_port *, char ch);
 	void		(*stop_rx)(struct uart_port *);
+	void		(*start_rx)(struct uart_port *);
 	void		(*enable_ms)(struct uart_port *);
 	void		(*break_ctl)(struct uart_port *, int ctl);
 	int		(*startup)(struct uart_port *);