On Wed, Apr 20, 2022 at 2:12 PM 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.
>
> Note that duty > period can never happen, so the respective check can be
> dropped.
>
> 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.
>
> Fixes: 99b82abb0a35 ("pwm: Add Renesas TPU PWM driver")
> Signed-off-by: Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx>
Reviewed-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>
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
