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

[boris.brezillon@free-electrons.com (Boris Brezillon)]

On Thu, 22 Sep 2016 09:07:46 +0200
Uwe Kleine-K?nig <u.kleine-koenig@pengutronix.de> wrote:

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

How about:

	div = DIV_ROUND_CLOSEST(port->uartclk, baud);
	cd = div / 16;
	fp = (div % 16) / 2;

	best_baud = port->uartclk / ((16 * cd) +  (8 * fp));

	/* Check if we can get a better approximation by rounding up. */
	if (div % 2) {
		int alt_baud, alt_fp, alt_cd;

		alt_fp = fp++;
		alt_cd = cd;
		if (alt_fp > 7) {
			alt_cd++;
			alt_fp = 0;
		}

		alt_baud = port->uartclk / ((16 * alt_cd) +  (8 *alt_fp));
		if (abs(best_baud - baud) > abs(alt_baud - baud)) {
			best_baud = alt_baud;
			fp = alt_fp;
			cd = alt_cd;
		}
	}

> 
> 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
>