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Date:   Mon, 24 Jul 2023 17:11:19 +0300
From:   Serge Semin <fancer.lancer@gmail.com>
To:     Geert Uytterhoeven <geert@linux-m68k.org>
Cc:     wuyonggang001@208suo.com, mturquette@baylibre.com,
        sboyd@kernel.org, linux-clk@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] clk: baikal-t1: Using div64_ Ul replaces do_ Div()
 function
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On Mon, Jul 24, 2023 at 03:38:49PM +0200, Geert Uytterhoeven wrote:
> Hi Serge,
> 
> On Mon, Jul 24, 2023 at 3:13 PM Serge Semin <fancer.lancer@gmail.com> wrote:
> > On Mon, Jul 24, 2023 at 12:04:19PM +0200, Geert Uytterhoeven wrote:
> > > On Wed, Jun 14, 2023 at 8:07 AM <wuyonggang001@208suo.com> wrote:
> > > > Fix the following coccicheck warning:
> > > >
> > > > drivers/clk/baikal-t1/ccu-pll.c:81:1-7: WARNING: do_div() does a
> > > > 64-by-32 division, please consider using div64_ul instead.
> > > >
> > > > Signed-off-by: Yonggang Wu <wuyonggang001@208suo.com>
> > >
> > > Thanks for your patch, which is now commit b93d1331ea266dea
> > > ("clk: baikal-t1: Using div64_ Ul replaces do_ Div() function")
> > > in clk/clk-next.
> > >
> > > > b/drivers/clk/baikal-t1/ccu-pll.c
> > > > index 13ef28001439..d41735c6956a 100644
> > > > --- a/drivers/clk/baikal-t1/ccu-pll.c
> > > > +++ b/drivers/clk/baikal-t1/ccu-pll.c
> 
> > > > @@ -78,9 +78,9 @@ static inline unsigned long ccu_pll_calc_freq(unsigned
> > > > long ref_clk,
> > > >   {
> > > >       u64 tmp = ref_clk;
> > > >
> >
> > > > -    do_div(tmp, nr);
> > > > +    div64_ul(tmp, nr);
> > > >       tmp *= nf;
> > > > -    do_div(tmp, od);
> > > > +    div64_ul(tmp, od);
> > > >
> > > >       return tmp;
> > >
> > > Likewise.
> >
> > Right. This will also break the driver.
> >
> > > But as ref_clk is unsigned long, there is no need to use div64_ul()
> > > for the first division, and this can be simplified to:
> > >
> > >     u64 tmp = (u64)(ref_clk / nr) * nf;
> > >     return div64_ul(tmp, od);
> >
> > Absolutely right. My intention of using the do_div() anyway was for
> > the sake of the code unification.
> >
> > >
> > > To avoid loss of precision, it might be better to reverse the order
> > > of the division and multiplication:
> > >
> >
> > >     u64 tmp = (u64)ref_clk * nf / nr;
> >
> > Alas exactly this code will cause the compilation error on the 32-bit
> > platform:
> > ccu-pll.c:(.text+0x458): undefined reference to `__udivdi3'
> >
> > That's why I am using the do_div() here. I would have rather used the
> > div64_ul() instead as this patch suggests, but I haven't known about its
> > existence up to this moment.
> 
> Bummer, that was a silly mistake on my side...
> (Initially, I didn't write the cast to u64 there, as all of ref_clk, nf, and nr
>  are unsigned long.  Then I realized "ref_clk * nf" might overflow on
>  32-bit, thus requiring a 64-bit result. And I added the cast...)
> 
> > Anyway my intention of dividing before multiplying had twofold
> > justification. Firstly I didn't want to use the "/" operator and
> > do_div() macro in the statements used to implement the same formulae.
> > Since I couldn't use the operator I decided to use the macro only for
> > the code unification. Secondly the PLL is designed in a way so the
> > signal is first divided by NR, then multiplied by NF and then divided
> > by OD. That's why I decided to preserve the same order in the
> > calculations here. I assumed back then that the NR-divider performs
> > the integer division in the analog circuitry. I have doubts now that
> > my assumption was correct since it's analog device and most likely
> > divides the source signal with no integer rounding-up. So using the
> > order suggested by you would have likely given a more exact result.
> >
> > >
> > > But doing that requires intimate knowledge about the range of nf to
> > > avoid overflow, so I leave that to Serge.
> >
> > nr: 1 - 2^6
> > nf: 1 - 2^13
> > ref_clk: normally 25'000'000 Hz.
> > Using "unsigned long"/u32 multiplication will give the integer
> > overflow. Meanwhile the u64 arithmetics will be more than enough here.
> >
> > So to speak the next alteration seems more correct here:
> > +return div64_ul(div64_ul((u64)ref_clk * nf, nr), od);
> >
> > What do you think?
> 

> Given the ranges above, nr and nf can be u32 instead of unsigned long.
> So perhaps it makes sense to use the mul_u64_u32_div() helper?
> 
>     return div64_ul(mul_u64_u32_div(ref_clk, nf, nr), of);

Just a day of discoveries today.) Didn't know about the
mul_u64_u32_div() existence either. Thanks for suggestion. Anyway
seeing "unsigned long" is 32-bits wide on my platform, nr/nf/od will
always be within the specified ranges, why not. Although using two
div64_ul()'s seems a bit more readable. But it might be just because
of me not being used to the mul_u64_u32_div() prototype notation.

-Serge(y)

> 
> Gr{oetje,eeting}s,
> 
>                         Geert
> 
> -- 
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-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