ARCv3:DBG: add global variable to change UART baud in MDB

This patch adds the ability to change the UART baud using global
variable glb_uart_baud. This is for debugging/developing purposes.
Use this to set UART baud before starting a kernel right after load.

Author: xxkent

arch/arc/kernel/devtree.c

@@ -17,7 +17,9 @@
 
 static unsigned int __initdata arc_base_baud;
 
+//These two variables exist only for debugging perposes
 volatile unsigned int glb_clock_freq = 50000000;
+volatile unsigned int glb_uart_baud = 115200;
 
 unsigned int __init arc_early_base_baud(void)
 {

drivers/tty/serial/8250/8250_early.c

@@ -154,12 +154,16 @@ static void __init init_port(struct earlycon_device *device)
 	}
 }
 
+extern volatile unsigned int glb_uart_baud;
+
 int __init early_serial8250_setup(struct earlycon_device *device,
 					 const char *options)
 {
 	if (!(device->port.membase || device->port.iobase))
 		return -ENODEV;
 
+	device->baud = glb_uart_baud; // For changing UART baud in MDB runtime
+
 	if (!device->baud) {
 		struct uart_port *port = &device->port;
 		unsigned int ier;

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

@@ -2668,6 +2668,8 @@ static void serial8250_set_divisor(struct uart_port *port, unsigned int baud,
 		serial8250_do_set_divisor(port, baud, quot, quot_frac);
 }
 
+extern volatile unsigned int glb_uart_baud;
+
 static unsigned int serial8250_get_baud_rate(struct uart_port *port,
 					     struct ktermios *termios,
 					     struct ktermios *old)
@@ -2695,7 +2697,8 @@ static unsigned int serial8250_get_baud_rate(struct uart_port *port,
 	 * slower than nominal still match standard baud rates without
 	 * causing transmission errors.
 	 */
-	return uart_get_baud_rate(port, termios, old, min, max);
+	//return uart_get_baud_rate(port, termios, old, min, max);
+	return glb_uart_baud;  // For changing UART baud in MDB
 }
 
 /*