linux-kernel - Re: [RFC PATCH] math.h: Account for 64-bit division on i386

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Message-ID: <CAMuHMdU5F1nEpF9+jJSP0oap6GdP9mQSPp0c3zSmXeOPrCzA9A@mail.gmail.com>
Date: Wed, 23 Apr 2025 09:02:10 +0200
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Guenter Roeck <linux@...ck-us.net>
Cc: cem@...nel.org, linux-kernel@...r.kernel.org, Hans.Holmberg@....com, 
	oe-kbuild-all@...ts.linux.dev, hch@....de, lukas@...ner.de, 
	angelogioacchino.delregno@...labora.com, Jonathan.Cameron@...wei.com
Subject: Re: [RFC PATCH] math.h: Account for 64-bit division on i386

On Sun, 20 Apr 2025 at 19:42, Guenter Roeck <linux@...ck-us.net> wrote:
> On Sun, Apr 20, 2025 at 08:40:27AM -0700, Guenter Roeck wrote:
> > On Sat, Apr 19, 2025 at 01:51:46PM +0200, cem@...nel.org wrote:
> > > From: Carlos Maiolino <cem@...nel.org>
> > >
> > > Building linux on i386 might fail if a 64bit type is passed to
> >
> > i386 actually builds. Its compiler is probably able to convert
> > the offending mult_frac() without helpers since the divisor is
> > a constant. I see the problem with openrisc and parisc, with
> > gcc 13.3.0.
> >
> > > mult_fract(). To prevent the failure, use do_div() for the division
> > > calculation instead of hardcoding a / b.
> > >
> > > Signed-off-by: Carlos Maiolino <cmaiolino@...hat.com>
> > > Reported-by: kernel test robot <lkp@...el.com>
> > > Closes: https://lore.kernel.org/oe-kbuild-all/202504181233.F7D9Atra-lkp@intel.com/
> > > ---
> > >
> > > I'm sending it as a RFC because I didn't to extensive testing on this
> > > patch, also I'm not sure if mult_frac() was intended to work on 32-bit
> > > only types. If that's the case, perhaps, a new mult_frac64() might be a
> > > better idea?!
> > >
> > >  include/linux/math.h | 19 ++++++++++---------
> > >  1 file changed, 10 insertions(+), 9 deletions(-)
> > >
> > > diff --git a/include/linux/math.h b/include/linux/math.h
> > > index 0198c92cbe3e..05ea853b75b4 100644
> > > --- a/include/linux/math.h
> > > +++ b/include/linux/math.h
> > > @@ -133,15 +133,16 @@ __STRUCT_FRACT(u32)
> > >  #undef __STRUCT_FRACT
> > >
> > >  /* Calculate "x * n / d" without unnecessary overflow or loss of precision. */
> > > -#define mult_frac(x, n, d) \
> > > -({                         \
> > > -   typeof(x) x_ = (x);     \
> > > -   typeof(n) n_ = (n);     \
> > > -   typeof(d) d_ = (d);     \
> > > -                           \
> > > -   typeof(x_) q = x_ / d_; \
> > > -   typeof(x_) r = x_ % d_; \
> > > -   q * n_ + r * n_ / d_;   \
> > > +#define mult_frac(x, n, d)         \
> > > +({                                 \
> > > +   typeof(x) x_ = (x);             \
> > > +   typeof(n) n_ = (n);             \
> > > +   typeof(d) d_ = (d);             \
> > > +                                   \
> > > +   typeof(x_) r = do_div(x_, d_);  \
> > > +   r *= n_;                        \
> > > +   do_div(r, d_);                  \
> > > +   x_ * n_ + r;                    \
> > >  })
> > >
> >
> > Unfortunately that doesn't work. I get build errors on parisc.
> >
>
> Turns out the first parameter of do_div needs to be u64, not s64,
> at least on parisc.

That is correct: the first parameter must be u64, the second must be u32:
https://elixir.bootlin.com/linux/v6.14.3/source/include/asm-generic/div64.h

As you must never use open-coded 64-bit divisions in the kernel,
simple helpers like mult_frac() can only be used for 32-bit values.
For anything involving 64-bit divisions, you must use the helpers from
<linux/math64.h>, e.g. mul_u64_u32_div().
https://elixir.bootlin.com/linux/v6.14.3/source/include/linux/math64.h#L257

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

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