serial: core: Preserve termios c_cflag for console resume

commit ae84db96 upstream.

When a tty is opened for the serial console, the termios c_cflag
settings are inherited from the console line settings.
However, if the tty is subsequently closed, the termios settings
are lost. This results in a garbled console if the console is later
suspended and resumed.

Preserve the termios c_cflag for the serial console when the tty
is shutdown; this reflects the most recent line settings.

Fixes: Bugzilla #69751, 'serial console does not wake from S3'
Reported-by: Valerio Vanni <valerio.vanni@inwind.it>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>

drivers/tty/serial/serial_core.c

@@ -235,6 +235,9 @@ static void uart_shutdown(struct tty_struct *tty, struct uart_state *state)
 		/*
 		 * Turn off DTR and RTS early.
 		 */
+		if (uart_console(uport) && tty)
+			uport->cons->cflag = tty->termios.c_cflag;
+
 		if (!tty || (tty->termios.c_cflag & HUPCL))
 			uart_clear_mctrl(uport, TIOCM_DTR | TIOCM_RTS);