[PATCH 4.19 56/96] serial: sh-sci: Fix HSCIF RX sampling point calculation

Greg Kroah-Hartman
2019-04-24T17:31:27Z

From: Geert Uytterhoeven <geert+renesas@glider.be>

commit ace965696da2611af759f0284e26342b7b6cec89 upstream.

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>

---
drivers/tty/serial/sh-sci.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

--- a/drivers/tty/serial/sh-sci.c
+++ b/drivers/tty/serial/sh-sci.c
@@ -2497,7 +2497,9 @@ 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