On Thu, 22 Sep 2016 11:43:08 +0200
Boris Brezillon <boris.brezillon@xxxxxxxxxxxxxxxxxx> wrote:
> 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?
Okay, it seems I was wrong here. It appears that 117702.17 is better
than 112797.92, and Alexey's patch is calculating the best cd and fp
values for all cases.
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