linux-kernel - Re: [PATCH v4 2/4] watchdog: s3c2410_wdt: Fix max

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Message-ID: <CAPLW+4nRh9DEnkhunG68xvGdaNJswC8fN4v4uBA1Aaao_5pxfw@mail.gmail.com>
Date: Fri, 1 Aug 2025 23:11:44 -0500
From: Sam Protsenko <semen.protsenko@...aro.org>
To: Sangwook Shin <sw617.shin@...sung.com>
Cc: krzk@...nel.org, alim.akhtar@...sung.com, wim@...ux-watchdog.org, 
	linux@...ck-us.net, khwan.seo@...sung.com, dongil01.park@...sung.com, 
	linux-arm-kernel@...ts.infradead.org, linux-samsung-soc@...r.kernel.org, 
	linux-watchdog@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/4] watchdog: s3c2410_wdt: Fix max_timeout being
 calculated larger

On Thu, Jul 24, 2025 at 3:13 AM Sangwook Shin <sw617.shin@...sung.com> 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@...sung.com>
> Signed-off-by: Sangwook Shin <sw617.shin@...sung.com>
> ---
>  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
>