Hi Ulrich,
On Wed, Apr 4, 2018 at 5:48 PM, Ulrich Hecht
<ulrich.hecht+renesas@xxxxxxxxx> wrote:
> HSCIF has facilities that allow moving the RX sampling point by between
> -8 and 7 sampling cycles (one sampling cycles equals 1/15 of a bit
> by default) to improve the error margin in case of slightly mismatched
> bit rates between sender and receiver.
>
> This patch tries to determine if shifting the sampling point can improve
> the error margin and will enable it if so.
>
> Signed-off-by: Ulrich Hecht <ulrich.hecht+renesas@xxxxxxxxx>
> ---
> This revision dumps the sysfs interface and works out the optimal shift
> on its own. It also moves setting of the HSSRR register back to its
> original location.
Thanks for the update!
Reviewed-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>
One suggestion for improvement below.
> --- a/drivers/tty/serial/sh-sci.c
> +++ b/drivers/tty/serial/sh-sci.c
> @@ -2406,8 +2427,27 @@ static void sci_set_termios(struct uart_port *port, struct ktermios *termios,
> serial_port_out(port, SCSCR, scr_val | s->hscif_tot);
> serial_port_out(port, SCSMR, smr_val);
> serial_port_out(port, SCBRR, brr);
> - if (sci_getreg(port, HSSRR)->size)
> - serial_port_out(port, HSSRR, srr | HSCIF_SRE);
> + if (sci_getreg(port, HSSRR)->size) {
> + unsigned int hssrr = srr | HSCIF_SRE;
> + /* Calculate deviation from intended rate at the
> + * center of the last stop bit in sampling clocks.
> + */
> + int last_stop = bits * 2 - 1;
> + int deviation = min_err * srr * last_stop / 2 / baud;
DIV_ROUND_CLOSEST()?
> +
> + if (abs(deviation) >= 2) {
> + /* At least two sampling clocks off at the
> + * last stop bit; we can increase the error
> + * margin by shifting the sampling point.
> + */
> + int shift = min(-8, max(7, deviation / 2));
> +
> + hssrr |= (shift << HSCIF_SRHP_SHIFT) &
> + HSCIF_SRHP_MASK;
> + hssrr |= HSCIF_SRDE;
> + }
> + serial_port_out(port, HSSRR, hssrr);
> + }
>
> /* Wait one bit interval */
> udelay((1000000 + (baud - 1)) / baud);
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
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