watchdog: s3c2410_wdt: Fix max_timeout being calculated larger

Fix the issue of max_timeout being calculated larger than actual value.
The calculation result of freq / (S3C2410_WTCON_PRESCALE_MAX + 1) /
S3C2410_WTCON_MAXDIV is smaller than the actual value because the remainder
is discarded during the calculation process. This leads to a larger
calculated value for max_timeout compared to the actual settable value.
To resolve this issue, the order of calculations in the computation process
has been adjusted.

Reviewed-by: Sam Protsenko <semen.protsenko@linaro.org>
Signed-off-by: Sangwook Shin <sw617.shin@samsung.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>

Author: Sangwook Shin

drivers/watchdog/s3c2410_wdt.c

@@ -27,6 +27,7 @@
 #include <linux/mfd/syscon.h>
 #include <linux/regmap.h>
 #include <linux/delay.h>
+#include <linux/math64.h>
 
 #define S3C2410_WTCON		0x00
 #define S3C2410_WTDAT		0x04
@@ -410,9 +411,14 @@ static inline unsigned long s3c2410wdt_get_freq(struct s3c2410_wdt *wdt)
 static inline unsigned int s3c2410wdt_max_timeout(struct s3c2410_wdt *wdt)
 {
 	const unsigned long freq = s3c2410wdt_get_freq(wdt);
+	const u64 n_max = (u64)(S3C2410_WTCON_PRESCALE_MAX + 1) *
+			S3C2410_WTCON_MAXDIV * S3C2410_WTCNT_MAXCNT;
+	u64 t_max = div64_ul(n_max, freq);
 
-	return S3C2410_WTCNT_MAXCNT / (freq / (S3C2410_WTCON_PRESCALE_MAX + 1)
-				       / S3C2410_WTCON_MAXDIV);
+	if (t_max > UINT_MAX)
+		t_max = UINT_MAX;
+
+	return t_max;
 }
 
 static int s3c2410wdt_disable_wdt_reset(struct s3c2410_wdt *wdt, bool mask)