linux-kernel - Re: [PATCH v6 2/5] watchdog: s3c2410_wdt: Fix max

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Message-ID: <f7f009c8-3f7b-4493-9120-d45d42a8c5e9@roeck-us.net>
Date: Thu, 21 Aug 2025 08:34:46 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: Sangwook Shin <sw617.shin@...sung.com>
Cc: krzk@...nel.org, alim.akhtar@...sung.com, wim@...ux-watchdog.org,
	semen.protsenko@...aro.org, dongil01.park@...sung.com,
	khwan.seo@...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 v6 2/5] watchdog: s3c2410_wdt: Fix max_timeout being
 calculated larger

On Mon, Aug 18, 2025 at 11:18:23AM +0900, Sangwook Shin 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.
> To resolve this issue, the order of calculations in the computation process
> has been adjusted.
> 
> Reviewed-by: Sam Protsenko <semen.protsenko@...aro.org>
> Signed-off-by: Sangwook Shin <sw617.shin@...sung.com>

Reviewed-by: Guenter Roeck <linux@...ck-us.net>

> ---
>  drivers/watchdog/s3c2410_wdt.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/watchdog/s3c2410_wdt.c b/drivers/watchdog/s3c2410_wdt.c
> index 95f7207e390a..1e8cf0299713 100644
> --- a/drivers/watchdog/s3c2410_wdt.c
> +++ b/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)
> -- 
> 2.25.1
> 
>