[PATCH] tty/serial: atmel: fix fractional baud rate computation

[Uwe Kleine-König]

On Wed, Sep 21, 2016 at 12:44:14PM +0200, Nicolas Ferre wrote:
> From: Alexey Starikovskiy <aystarik at gmail.com>
> 
> 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 at gmail.com>
> [nicolas.ferre at atmel.com: port to newer kernel and add commit log]
> Signed-off-by: Nicolas Ferre <nicolas.ferre at atmel.com>
> ---
>  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/  |