On Wed, Sep 21, 2016 at 12:44:14PM +0200, Nicolas Ferre wrote:
> From: Alexey Starikovskiy <aystarik@xxxxxxxxx>
>
> 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@xxxxxxxxx>
> [nicolas.ferre@xxxxxxxxx: port to newer kernel and add commit log]
> Signed-off-by: Nicolas Ferre <nicolas.ferre@xxxxxxxxx>
> ---
> drivers/tty/serial/atmel_serial.c | 10 ++++++----
> include/linux/atmel_serial.h | 1 +
> 2 files changed, 7 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/tty/serial/atmel_serial.c b/drivers/tty/serial/atmel_serial.c
> index 5f550d9feed9..fd8aa1f4ba78 100644
> --- 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;
given baud = 115200 and uartclk = 5414300 this results in:
div = DIV_ROUND_CLOSEST(5414300, 115200 * 2) = 23
cd = 2
fp = 7
which yields a rate of 5414300 / 46 = 117702.17. With cd = 3 and fp = 0
however the resulting rate is 5414300 / 48 = 112797.92.
Which one is better?
> } else {
> cd = uart_get_divisor(port, baud);
> }
> diff --git a/include/linux/atmel_serial.h b/include/linux/atmel_serial.h
> index f8e452aa48d7..bd2560502f3c 100644
> --- 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
Is there another user of this header? If not, this can be folded into
the driver.
Best regards
Uwe
--
Pengutronix e.K. | Uwe Kleine-König |
Industrial Linux Solutions | http://www.pengutronix.de/ |
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