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

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Message-ID: <CAPLW+4mNejXUMW0VqZW8hieNUFmDqS_-qqE=p+bV4TnwM_TWgw@mail.gmail.com>
Date: Tue, 5 Aug 2025 17:53:12 -0500
From: Sam Protsenko <semen.protsenko@...aro.org>
To: sw617.shin@...sung.com
Cc: Guenter Roeck <linux@...ck-us.net>, krzk@...nel.org, alim.akhtar@...sung.com, 
	wim@...ux-watchdog.org, 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 Tue, Aug 5, 2025 at 2:26 AM <sw617.shin@...sung.com> wrote:
>
> On Tuesday, August 5, 2025 at 2:03 PM Sam Protsenko <semen.protsenko@...aro.org> wrote:
>
> >
> > > > +       u64 t_max = n_max / freq;
> > > >
> > >
> > > Make sure this compiles on 32-bit builds.
> > >
> >
> > Can you please elaborate what might be the possible problem -- just
> > curious? I admit I never though about 32-bit case when writing that code,
> > but don't see any immediate issues with that too.
> >
>
> In my opinion, it seems that Gunter Reck's explanation is correct.
> I've found out that the error of "undefined reference to '__aeabi_uldivmod'" may occur when compiling new code on a 32-bit architecture.

Indeed. I was able to reproduce that behavior when building for ARM32 too.

> If you don't mind, I would like to proceed with maintaining the previous revision below.
> From my perspective, this approach appears to be the most reasonable solution for supporting both 32-bit and 64-bit architectures.
>
> @@ -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);
>  }
>

I don't mind, although it's quite easy to fix the code I suggested by
replacing this line:

    u64 t_max = n_max / freq;

with this one:

    u64 t_max = div64_ul(n_max, freq);

from <math64.h>, as Guenter suggested. But I'm totally fine with your
implementation as well.