[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>

2 files changed, 10 insertions, 2 deletions

diff --git a/drivers/serial/atmel_serial.c b/drivers/serial/atmel_serial.c
index 935f48fa501d..3320bcd92c0a 100644
--- a/drivers/serial/atmel_serial.c
+++ b/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) {
diff --git a/drivers/serial/atmel_serial.h b/drivers/serial/atmel_serial.h
index 11b44360e108..e0141776517c 100644
--- a/drivers/serial/atmel_serial.h
+++ b/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)