On Thu, 22 Sep 2016 09:07:46 +0200
Uwe Kleine-König <u.kleine-koe...@pengutronix.de> wrote:
> On Wed, Sep 21, 2016 at 12:44:14PM +0200, Nicolas Ferre wrote:
> > From: Alexey Starikovskiy <aysta...@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 <aysta...@gmail.com>
> > [nicolas.fe...@atmel.com: port to newer kernel and add commit log]
> > Signed-off-by: Nicolas Ferre <nicolas.fe...@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
>
