watchdog: renesas_wdt: don't set divider while watchdog is running

The datasheet says we must stop the timer before changing the clock
divider. This can happen when the restart handler is called while the
watchdog is running.
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>

drivers/watchdog/renesas_wdt.c

@@ -74,12 +74,17 @@ static int rwdt_init_timeout(struct watchdog_device *wdev)
 static int rwdt_start(struct watchdog_device *wdev)
 {
 	struct rwdt_priv *priv = watchdog_get_drvdata(wdev);
+	u8 val;
 
 	pm_runtime_get_sync(wdev->parent);
 
-	rwdt_write(priv, 0, RWTCSRB);
-	rwdt_write(priv, priv->cks, RWTCSRA);
+	/* Stop the timer before we modify any register */
+	val = readb_relaxed(priv->base + RWTCSRA) & ~RWTCSRA_TME;
+	rwdt_write(priv, val, RWTCSRA);
+
 	rwdt_init_timeout(wdev);
+	rwdt_write(priv, priv->cks, RWTCSRA);
+	rwdt_write(priv, 0, RWTCSRB);
 
 	while (readb_relaxed(priv->base + RWTCSRA) & RWTCSRA_WRFLG)
 		cpu_relax();