Re: [PATCH v2 07/11] math64: New DIV_U64_ROUND_CLOSEST helper

[Pali Rohár]

On Friday 25 June 2021 17:22:31 Geert Uytterhoeven wrote:
> Hi Pali,
> 
> On Fri, Jun 25, 2021 at 4:37 PM Pali Rohár <pali@xxxxxxxxxx> wrote:
> > Provide DIV_U64_ROUND_CLOSEST helper which uses div_u64 to perform
> > division rounded to the closest integer using unsigned 64bit
> > dividend and unsigned 32bit divisor.
> >
> > Signed-off-by: Pali Rohár <pali@xxxxxxxxxx>
> 
> Thanks for your patch!
> 
> > --- a/include/linux/math64.h
> > +++ b/include/linux/math64.h
> > @@ -281,6 +281,19 @@ u64 mul_u64_u64_div_u64(u64 a, u64 mul, u64 div);
> >  #define DIV64_U64_ROUND_CLOSEST(dividend, divisor)     \
> >         ({ u64 _tmp = (divisor); div64_u64((dividend) + _tmp / 2, _tmp); })
> >
> > +/*
> > + * DIV_U64_ROUND_CLOSEST - unsigned 64bit divide with 32bit divisor rounded to nearest integer
> > + * @dividend: unsigned 64bit dividend
> > + * @divisor: unsigned 32bit divisor
> > + *
> > + * Divide unsigned 64bit dividend by unsigned 32bit divisor
> > + * and round to closest integer.
> > + *
> > + * Return: dividend / divisor rounded to nearest integer
> > + */
> > +#define DIV_U64_ROUND_CLOSEST(dividend, divisor)       \
> > +       ({ u32 _tmp = (divisor); div_u64((u64)(dividend) + _tmp / 2, _tmp); })
> 
> Given "dividend" should already be an unsigned 64-bit value, I don't
> think the cast to "u64" is needed. Similar macros in this file also
> don't have the cast.

It is just to ensure that plus operation between dividend and _tmp is
evaluated in 64-bit context to prevent overflow. Just a case when user
calls this macro with 32-bit dividend param. As it is a macro (and not
inline function) type is not automatically enforced.

DIV_S64_ROUND_CLOSEST macro assigns its argument into temporary 64-bit
variable which then ensures usage of 64-bit arithmetic operations. Same
applies for DIV64_U64_ROUND_CLOSEST and DIV64_U64_ROUND_UP macros.

So this is reason why I added explicit cast to u64.

> 
> > +
> >  /*
> >   * DIV_S64_ROUND_CLOSEST - signed 64bit divide with 32bit divisor rounded to nearest integer
> >   * @dividend: signed 64bit dividend
> 
> With the above nit fixed:
> 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