On Mon, Aug 4, 2025 at 11:47 PM Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
>
> On 8/4/25 21:22, sw617.shin@xxxxxxxxxxx wrote:
> > On Saturday, August 2, 2025 at 1:12 PM Sam Protsenko <semen.protsenko@xxxxxxxxxx> 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
>
