[ARM] 4357/1: AT91: Support slower serial baud-rates

Allow slower serial baud-rates by switching the UART clock from MCK to
MCK/8.

Based on patches by Mike Wolfram and Russell King.
Signed-off-by: Andrew Victor <andrew@sanpeople.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

drivers/serial/atmel_serial.c

@@ -484,11 +484,16 @@ static void atmel_set_termios(struct uart_port *port, struct ktermios * termios,
 	unsigned long flags;
 	unsigned int mode, imr, quot, baud;
 
+	/* Get current mode register */
+	mode = UART_GET_MR(port) & ~(ATMEL_US_USCLKS | ATMEL_US_CHRL | ATMEL_US_NBSTOP | ATMEL_US_PAR);
+
 	baud = uart_get_baud_rate(port, termios, old, 0, port->uartclk/16);
 	quot = uart_get_divisor(port, baud);
 
-	/* Get current mode register */
-	mode = UART_GET_MR(port) & ~(ATMEL_US_CHRL | ATMEL_US_NBSTOP | ATMEL_US_PAR);
+	if (quot > 65535) {		/* BRGR is 16-bit, so switch to slower clock */
+		quot /= 8;
+		mode |= ATMEL_US_USCLKS_MCK_DIV8;
+	}
 
 	/* byte size */
 	switch (termios->c_cflag & CSIZE) {

drivers/serial/atmel_serial.h

@@ -46,6 +46,9 @@
 #define			ATMEL_US_USMODE_ISO7816_T1	6
 #define			ATMEL_US_USMODE_IRDA		8
 #define		ATMEL_US_USCLKS		(3   <<  4)		/* Clock Selection */
+#define			ATMEL_US_USCLKS_MCK		(0 <<  4)
+#define			ATMEL_US_USCLKS_MCK_DIV8	(1 <<  4)
+#define			ATMEL_US_USCLKS_SCK		(3 <<  4)
 #define		ATMEL_US_CHRL		(3   <<  6)		/* Character Length */
 #define			ATMEL_US_CHRL_5			(0 <<  6)
 #define			ATMEL_US_CHRL_6			(1 <<  6)