On Thu, Jul 24, 2025 at 3:13 AM Sangwook Shin <sw617.shin@xxxxxxxxxxx> wrote:
>
> 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.
> A ceiling operation is applied in the calculation process to resolve this.
>
> Reviewed-by: Alim Akhtar <alim.akhtar@xxxxxxxxxxx>
> Signed-off-by: Sangwook Shin <sw617.shin@xxxxxxxxxxx>
> ---
> drivers/watchdog/s3c2410_wdt.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/watchdog/s3c2410_wdt.c b/drivers/watchdog/s3c2410_wdt.c
> index 95f7207e390a..31f7e1ec779e 100644
> --- a/drivers/watchdog/s3c2410_wdt.c
> +++ b/drivers/watchdog/s3c2410_wdt.c
> @@ -411,8 +411,8 @@ static inline unsigned int s3c2410wdt_max_timeout(struct s3c2410_wdt *wdt)
> {
> const unsigned long freq = s3c2410wdt_get_freq(wdt);
>
> - return S3C2410_WTCNT_MAXCNT / (freq / (S3C2410_WTCON_PRESCALE_MAX + 1)
> - / S3C2410_WTCON_MAXDIV);
> + return S3C2410_WTCNT_MAXCNT / DIV_ROUND_UP(freq,
> + (S3C2410_WTCON_PRESCALE_MAX + 1) * S3C2410_WTCON_MAXDIV);
> }
>
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
I've prepared the test program you can run on your machine to play
with all implementations: [1].
Thanks!
[1] https://gist.github.com/joe-skb7/c2b2290bb0b0572a4018d81fc4ba1f3e
> static int s3c2410wdt_disable_wdt_reset(struct s3c2410_wdt *wdt, bool mask)
> --
> 2.25.1
>
