[PATCH v4 2/4] watchdog: s3c2410_wdt: Fix max_timeout being calculated larger

[sw617.shin at samsung.com]

On Saturday, August 2, 2025 at 1:12 PM Sam Protsenko <semen.protsenko at linaro.org> wrote:

> How about something like this instead?
> 
> 8<--------------------------------------------------------------------->8
> static inline unsigned int s3c2410wdt_max_timeout(unsigned long freq) {
>     const u64 div_max = (S3C2410_WTCON_PRESCALE_MAX + 1) *
>                 S3C2410_WTCON_MAXDIV; /* 32768 */
>     const u64 n_max = S3C2410_WTCNT_MAXCNT * div_max;
>     u64 t_max = n_max / freq;
> 
>     if (t_max > UINT_MAX)
>         t_max = UINT_MAX;
> 
>     return (unsigned int)t_max;
> }
> 8<--------------------------------------------------------------------->8
> 
> This implementation's result:
>   - is never greater than real timeout, as it loses the decimal part after
> integer division in t_max
>   - much closer to the real timeout value, as it benefits from very big
> n_max in the numerator (this is the main trick here)
>   - prepared for using 32-bit max counter value in your next patch, as it
> uses u64 type for calculations
> 
> For example, at the clock frequency of 33 kHz:
>   - real timeout is: 65074.269 sec
>   - old function returns: 65535 sec
>   - your function returns: 32767 sec
>   - the suggested function returns: 65074 sec

Thank you for your feedback.
I'll make the code changes as follows in the next patch set:

static inline unsigned int s3c2410wdt_max_timeout(struct s3c2410_wdt *wdt)
 {
        const unsigned long freq = s3c2410wdt_get_freq(wdt);
+       const u64 div_max = (S3C2410_WTCON_PRESCALE_MAX + 1) *
+                       S3C2410_WTCON_MAXDIV;
+       const u64 n_max = S3C2410_WTCNT_MAXCNT * div_max;
+       u64 t_max = 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 (unsigned int)t_max;
 }