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

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Message-ID: <CAPLW+4kVMo68DAO0y_=m3k81Xeh4wYV9+KX3fg=5S7xwOh0O7Q@mail.gmail.com>
Date: Tue, 5 Aug 2025 00:03:23 -0500
From: Sam Protsenko <semen.protsenko@...aro.org>
To: Guenter Roeck <linux@...ck-us.net>
Cc: sw617.shin@...sung.com, 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 Mon, Aug 4, 2025 at 11:47 PM Guenter Roeck <linux@...ck-us.net> wrote:
>
> On 8/4/25 21:22, sw617.shin@...sung.com wrote:
> > On Saturday, August 2, 2025 at 1:12 PM Sam Protsenko <semen.protsenko@...aro.org> wrote:
> >
> >> 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
> >
> > Thank you for your feedback.
> > I'll make the code changes as follows in the next patch set:
> >
> > static inline unsigned int s3c2410wdt_max_timeout(struct s3c2410_wdt *wdt)
> >   {
> >          const unsigned long freq = s3c2410wdt_get_freq(wdt);
> > +       const u64 div_max = (S3C2410_WTCON_PRESCALE_MAX + 1) *
> > +                       S3C2410_WTCON_MAXDIV;
> > +       const u64 n_max = S3C2410_WTCNT_MAXCNT * div_max;
>
> Not sure if splitting this expression adds any value. Why not just the following ?
>
> const u64 n_max = (u64)(S3C2410_WTCON_PRESCALE_MAX + 1) *
>                     S3C2410_WTCON_MAXDIV * S3C2410_WTCNT_MAXCNT;
>
> Or just use a define ?
>

Oh, that code snippet above is just a suggestion, please treat it as
pseudo-code for this purpose, -- I just wanted to illustrate the idea,
and should've been more clear! Yes, definitely should be revised and
re-tested. And yeah, I agree, one single const or define would be
enough, no need to make it too verbose. Also, the naming may be not
the best, e.g. cnt_max might be better than n_max for example. But
I'll leave it to Sangwook Shin to decide on the implementation, just
wanted to communicate the idea.

> > +       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.

> > -       return S3C2410_WTCNT_MAXCNT / (freq / (S3C2410_WTCON_PRESCALE_MAX + 1)
> > -                                      / S3C2410_WTCON_MAXDIV);
> > +       if (t_max > UINT_MAX)
> > +               t_max = UINT_MAX;
> > +
> > +       return (unsigned int)t_max;
>
> I am quite sure that this typecast is unnecessary.
>
> Guenter
>