Re: [PATCH 1/2] watchdog: realtek-otto: prevent PHASE2 underflows

[Guenter Roeck]

On Fri, May 15, 2026 at 11:23:50PM +0200, Sander Vanheule wrote:
> For small pretimeout values, ((timeout - pretimeout) / tick) might be
> rounded up to the same value as (timeout / tick). As a result, the
> number of PHASE2 ticks may be zero, causing an underflow when
> subtracting 1 to configure the hardware. While this results in a
> longer-than-expected time to system reset, the duration of PHASE1 and
> minimum ping interval for the watchdog would still be correct.
> 
> As the watchdog core ensures pretimeout is strictly less than timeout,
> ceil(timeout / tick) is strictly greater than floor(pretimeout / tick)
> and the number of PHASE1 ticks cannot be 0. So instead of rounding up
> the number of PHASE1 ticks, we can round down the number of PHASE2
> ticks, maintaining the current behavior while avoiding underflows.
> 
> The original helper function is now inlined, as it doesn't save any
> duplication anymore.
> 
> Signed-off-by: Sander Vanheule <sander@xxxxxxxxxxxxx>

Applied.

Thanks,
Guenter

> ---
>  drivers/watchdog/realtek_otto_wdt.c | 12 +++---------
>  1 file changed, 3 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/watchdog/realtek_otto_wdt.c b/drivers/watchdog/realtek_otto_wdt.c
> index 2c30ddd574c5..1495e8a8cbb5 100644
> --- a/drivers/watchdog/realtek_otto_wdt.c
> +++ b/drivers/watchdog/realtek_otto_wdt.c
> @@ -114,12 +114,6 @@ static int otto_wdt_tick_ms(struct otto_wdt_ctrl *ctrl, int prescale)
>   * the value stored in those fields. This means each phase will run for at least
>   * one tick, so small values need to be clamped to correctly reflect the timeout.
>   */
> -static inline unsigned int div_round_ticks(unsigned int val, unsigned int tick_duration,
> -		unsigned int min_ticks)
> -{
> -	return max(min_ticks, DIV_ROUND_UP(val, tick_duration));
> -}
> -
>  static int otto_wdt_determine_timeouts(struct watchdog_device *wdev, unsigned int timeout,
>  		unsigned int pretimeout)
>  {
> @@ -140,9 +134,9 @@ static int otto_wdt_determine_timeouts(struct watchdog_device *wdev, unsigned in
>  			return -EINVAL;
>  
>  		tick_ms = otto_wdt_tick_ms(ctrl, prescale);
> -		total_ticks = div_round_ticks(timeout_ms, tick_ms, 2);
> -		phase1_ticks = div_round_ticks(timeout_ms - pretimeout_ms, tick_ms, 1);
> -		phase2_ticks = total_ticks - phase1_ticks;
> +		total_ticks = max(2, DIV_ROUND_UP(timeout_ms, tick_ms));
> +		phase2_ticks = max(1, pretimeout_ms / tick_ms);
> +		phase1_ticks = total_ticks - phase2_ticks;
>  
>  		prescale_next++;
>  	} while (phase1_ticks > OTTO_WDT_PHASE_TICKS_MAX