tty/serial: atmel: fix fractional baud rate computation

The problem with previous code was it rounded values in wrong
place and produced wrong baud rate in some cases.

Signed-off-by: Alexey Starikovskiy <aystarik@gmail.com>
[nicolas.ferre@atmel.com: port to newer kernel and add commit log]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/serial/atmel_serial.c b/drivers/tty/serial/atmel_serial.c
index 5f550d9..fd8aa1f 100644 (file)
--- a/drivers/tty/serial/atmel_serial.c
+++ b/drivers/tty/serial/atmel_serial.c
@@ -2170,13 +2170,15 @@ static void atmel_set_termios(struct uart_port *port, struct ktermios *termios,
         * accurately. This feature is enabled only when using normal mode.
         * baudrate = selected clock / (8 * (2 - OVER) * (CD + FP / 8))
         * Currently, OVER is always set to 0 so we get
-        * baudrate = selected clock (16 * (CD + FP / 8))
+        * baudrate = selected clock / (16 * (CD + FP / 8))
+        * then
+        * 8 CD + FP = selected clock / (2 * baudrate)
         */
        if (atmel_port->has_frac_baudrate &&
            (mode & ATMEL_US_USMODE) == ATMEL_US_USMODE_NORMAL) {
-               div = DIV_ROUND_CLOSEST(port->uartclk, baud);
-               cd = div / 16;
-               fp = DIV_ROUND_CLOSEST(div % 16, 2);
+               div = DIV_ROUND_CLOSEST(port->uartclk, baud * 2);
+               cd = div >> 3;
+               fp = div & ATMEL_US_FP_MASK;
        } else {
                cd = uart_get_divisor(port, baud);
        }

diff --git a/include/linux/atmel_serial.h b/include/linux/atmel_serial.h
index f8e452a..bd25605 100644 (file)
--- a/include/linux/atmel_serial.h
+++ b/include/linux/atmel_serial.h
@@ -119,6 +119,7 @@
 #define ATMEL_US_BRGR          0x20    /* Baud Rate Generator Register */
 #define        ATMEL_US_CD             GENMASK(15, 0)  /* Clock Divider */
 #define ATMEL_US_FP_OFFSET     16      /* Fractional Part */
+#define ATMEL_US_FP_MASK       0x7
 
 #define ATMEL_US_RTOR          0x24    /* Receiver Time-out Register for USART */
 #define ATMEL_UA_RTOR          0x28    /* Receiver Time-out Register for UART */