serial: sh-sci: Fix HSCIF RX sampling point calculation

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@glider.be>
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Cc: stable <stable@vger.kernel.org>
Reviewed-by: Ulrich Hecht <uli+renesas@fpond.eu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/serial/sh-sci.c b/drivers/tty/serial/sh-sci.c
index 2d1c626..55ef6e5 100644 (file)
--- a/drivers/tty/serial/sh-sci.c
+++ b/drivers/tty/serial/sh-sci.c
@@ -2512,7 +2512,9 @@ done:
                         * 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