ar71xx: restore support for boot console with arbitrary baud rates

Commit 1bfbf2de6d ("ar71xx: serial: core: add support for boot console
with arbitrary baud rates") added support for arbitrary baud rates which
enabled 250000 baud rate for Yun. But the patch was not ported to kernel
4.9, and since then the kernel set its baud rate to 9600. This commit ports
the patch to kernel 4.14, thereby restoring the serial console of Yun.

Cc: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Sungbo Eo <mans0n@gorani.run>
(cherry picked from commit c90db26e051e4f0e7cd32333b3bd8c94a13d599a)

target/linux/ar71xx/patches-4.14/821-serial-core-add-support-for-boot-console-with-arbitr.patch

@@ -0,0 +1,54 @@
+From 4d3c17975c7814884a721fe693b3adf5c426d759 Mon Sep 17 00:00:00 2001
+From: Hauke Mehrtens <hauke@hauke-m.de>
+Date: Tue, 10 Nov 2015 22:18:39 +0100
+Subject: [RFC] serial: core: add support for boot console with arbitrary
+ baud rates
+
+The Arduino Yun uses a baud rate of 250000 by default. The serial is
+going over the Atmel ATmega and is used to connect to this chip.
+Without this patch Linux wants to switch the console to 9600 Baud.
+
+With this patch Linux will use the configured baud rate and not a
+default one specified in uart_register_driver().
+
+Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
+[rebased to 4.14, slightly reworded commit message]
+Signed-off-by: Sungbo Eo <mans0n@gorani.run>
+---
+ drivers/tty/serial/serial_core.c | 6 +++++-
+ include/linux/console.h          | 1 +
+ 2 files changed, 6 insertions(+), 1 deletions(-)
+
+--- a/drivers/tty/serial/serial_core.c
++++ b/drivers/tty/serial/serial_core.c
+@@ -232,6 +232,8 @@ static int uart_port_startup(struct tty_
+ 	if (retval == 0) {
+ 		if (uart_console(uport) && uport->cons->cflag) {
+ 			tty->termios.c_cflag = uport->cons->cflag;
++			tty->termios.c_ospeed = uport->cons->baud;
++			tty->termios.c_ispeed = uport->cons->baud;
+ 			uport->cons->cflag = 0;
+ 		}
+ 		/*
+@@ -2072,8 +2074,10 @@ uart_set_options(struct uart_port *port,
+ 	 * Allow the setting of the UART parameters with a NULL console
+ 	 * too:
+ 	 */
+-	if (co)
++	if (co) {
+ 		co->cflag = termios.c_cflag;
++		co->baud = baud;
++	}
+ 
+ 	return 0;
+ }
+--- a/include/linux/console.h
++++ b/include/linux/console.h
+@@ -145,6 +145,7 @@ struct console {
+ 	short	flags;
+ 	short	index;
+ 	int	cflag;
++	int	baud;
+ 	void	*data;
+ 	struct	 console *next;
+ };