On Thu, 22 Sep 2016 09:39:04 +0200
Boris Brezillon <boris.brezillon@xxxxxxxxxxxxxxxxxx> wrote:
> On Thu, 22 Sep 2016 09:07:46 +0200
> Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx> wrote:
>
> > 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
>
> 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)) {
After a lengthy discussion that happened on IRC (#armlinux), Uwe
proved me wrong. This should actually be
/*
* Calculate the Error in the time domain:
* Error = (RealBaudPeriod - ExpectedBaudPeriod) /
* ExpectedBaudPeriod;
*
* which after conversion to the frequency domain gives:
* Error = 1 - (ExpectedBaudRate/RealBaudRate);
*
* and since we want to compare 2 errors and avoid
* approximation, we have:
*
* if (RealBaudRate2 * (RealBaudRate1 - ExpectedBaudRate) <
* RealBaudRate1 * (RealBaudRate2 - ExpectedBaudRate))
* ...
*
*/
if (alt_baud * abs(best_baud - baud) >
best_baud * abs(alt_baud - baud))
Thanks for your patience ;-).
> 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
> >
>
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