linux-kernel - Re: [PATCH] serial: sh-sci: Fix HSCIF RX sampling point calculation

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Date:   Thu, 4 Apr 2019 11:45:31 +0200
From:   Eugeniu Rosca <erosca@...adit-jv.com>
To:     Geert Uytterhoeven <geert+renesas@...der.be>
CC:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        Ulrich Hecht <uli+renesas@...nd.eu>,
        Dirk Behme <dirk.behme@...bosch.com>,
        <linux-serial@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        Simon Herkenhoff <sherkenhoff@...adit-jv.com>,
        Eugeniu Rosca <erosca@...adit-jv.com>
Subject: Re: [PATCH] serial: sh-sci: Fix HSCIF RX sampling point calculation

Hi Geert,

On Mon, Apr 01, 2019 at 01:25:10PM +0200, Geert Uytterhoeven wrote:
> There are several issues with the formula used for calculating the
> deviation from the intended rate:
>   1. While min_err and last_stop are signed, srr and baud are unsigned.
>      Hence the signed values are promoted to unsigned, which will lead
>      to a bogus value of deviation if min_err is negative,
>   2. Srr is the register field value, which is one less than the actual
>      sampling rate factor,
>   3. The divisions do not use rounding.
> 
> Fix this by casting unsigned variables to int, adding one to srr, and
> using a single DIV_ROUND_CLOSEST().
> 
> Fixes: 63ba1e00f178a448 ("serial: sh-sci: Support for HSCIF RX sampling point adjustment")
> Signed-off-by: Geert Uytterhoeven <geert+renesas@...der.be>
> ---
> Anyone with a good test setup for verifying this feature actually works
> as advertised?
> ---
>  drivers/tty/serial/sh-sci.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/serial/sh-sci.c b/drivers/tty/serial/sh-sci.c
> index 2bdaeba5d527a6ce..8dea59edde34c224 100644
> --- a/drivers/tty/serial/sh-sci.c
> +++ b/drivers/tty/serial/sh-sci.c
> @@ -2522,7 +2522,9 @@ static void sci_set_termios(struct uart_port *port, struct ktermios *termios,
>  			 * center of the last stop bit in sampling clocks.
>  			 */
>  			int last_stop = bits * 2 - 1;
> -			int deviation = min_err * srr * last_stop / 2 / baud;
> +			int deviation = DIV_ROUND_CLOSEST(min_err * last_stop *
> +							  (int)(srr + 1),
> +							  2 * (int)baud);
>  
>  			if (abs(deviation) >= 2) {
>  				/* At least two sampling clocks off at the

Just wanted to say thank you for your tremendous commitment and
continuity in supporting the R-Car platform (and beyond that), which
allowed us to gracefully overcome the problems encountered in the
project in the best possible time, specifically thanks to this patch.

Best regards,
Eugeniu.