Hi Uwe,
On Thu, Apr 14, 2022 at 12:27 PM Geert Uytterhoeven
<geert@xxxxxxxxxxxxxx> wrote:
> On Wed, Apr 13, 2022 at 10:51 AM Uwe Kleine-König
> <u.kleine-koenig@xxxxxxxxxxxxxx> wrote:
> > Dividing by the result of a division looses precision. Consider for example
> > clk_rate = 33000000 and period_ns = 500001. Then
> >
> > clk_rate / (NSEC_PER_SEC / period_ns)
> >
> > has the exact value 16500.033, but in C this evaluates to 16508. It gets
> > worse for even bigger values of period_ns, so with period_ns = 500000001,
> > the exact result is 16500000.033 while in C we get 33000000.
> >
> > For that reason use
> >
> > clk_rate * period_ns / NSEC_PER_SEC
> >
> > instead which doesn't suffer from this problem. To ensure this doesn't
> > overflow add a safeguard check for clk_rate.
> >
> > Incidentally this fixes a division by zero if period_ns > NSEC_PER_SEC.
> > Another side effect is that values bigger than INT_MAX for period and
> > duty_cyle are not wrongly discarded any more.
>
> You forgot to mention that pwm_state.period is no longer truncated to u32.
Please ignore this bogus comment.
Sorry for the fuzz.
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
