Hi Uwe,
On Tue, Mar 12, 2024 at 8:20 AM Uwe Kleine-König
<u.kleine-koenig@xxxxxxxxxxxxxx> wrote:
> On Tue, Feb 20, 2024 at 07:43:17PM +0000, Biju Das wrote:
> > --- /dev/null
> > +++ b/drivers/pwm/pwm-rzg2l-gpt.c
> > +static u64 calculate_period_or_duty(struct rzg2l_gpt_chip *rzg2l_gpt, u32 val, u8 prescale)
> > +{
> > + u64 tmp, d;
> > +
> > + /*
> > + * Rate is in MHz and is always integer for peripheral clk
> > + * 2^32 * 2^10 (prescalar) * 10^9 > 2^64
> > + * 2^32 * 2^10 (prescalar) * 10^6 < 2^64
> > + * Multiply val with prescalar first, if the result is less than
> > + * 2^34, then multiply by 10^9. Otherwise divide nr and dr by 10^3
> > + * so that it will never overflow.
> > + */
> > +
> > + tmp = (u64)val << (2 * prescale);
> > + if (tmp <= (1ULL << 34)) {
>
> I would have written that as:
>
> if (tmp >> 34 == 0)
>
> (which implements tmp < (1ULL << 34), which doesn't matter much).
>
> > + tmp *= NSEC_PER_SEC;
> > + d = rzg2l_gpt->rate;
> > + } else {
> > + tmp *= div64_u64(NSEC_PER_SEC, KILO);
>
> I don't know if the compiler is clever enough to not calculate that
> every time?
Not on 32-bit when written that way.
> Also using div64_u64 is too heavy given that both values fit
> into an u32.
Indeed, so "NSEC_PER_SEC / KILO" should be fine.
I guess NSEC_PER_MSEC would be too obfuscating?
>
> > + d = div64_u64(rzg2l_gpt->rate, KILO);
>
> At first I thought you could better use 1024 as the common divisor here
> as it could be implemented using a shift operation. But I understood
> with the comment above that we're not losing precision here as both
> NSEC_PER_SEC and rate are a multiple of 1000.
>
> Maybe s/Rate is in MHz and is always integer for peripheral clk/Rate is
> a multiple of 1000000, and so dividing by 1000 is an exact operation./ ?
>
> > + }
> > +
> > + return DIV64_U64_ROUND_UP(tmp, d);
> > +}
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
