Re: [PATCH 09/37] clk: renesas: rzg2l: fix computation formula

[Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>]

Hi Claudiu,

On Tue, Sep 26, 2023 at 4:44 PM Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> wrote:
> On Tue, Sep 26, 2023 at 1:47 PM claudiu beznea <claudiu.beznea@xxxxxxxxx> wrote:
> > On 14.09.2023 15:55, Geert Uytterhoeven wrote:
> > > On Tue, Sep 12, 2023 at 6:52 AM Claudiu <claudiu.beznea@xxxxxxxxx> wrote:
> > >> From: Claudiu Beznea <claudiu.beznea.uj@xxxxxxxxxxxxxx>
> > >>
> > >> According to hardware manual of RZ/G2L (r01uh0914ej0130-rzg2l-rzg2lc.pdf)
> > >> the computation formula for PLL rate is as follows:
> > >>
> > >> Fout = ((m + k/65536) * Fin) / (p * 2^s)
> > >>
> > >> and k has values in range [-32768, 32767]. Dividing k by 65536 with
> > >> integer variables leads all the time to zero. Thus we may have slight
> > >> differences b/w what has been set vs. what is displayed. Thus,
> > >> get rid of this and decompose the formula before dividing k by 65536.
> > >>
> > >> Fixes: ef3c613ccd68a ("clk: renesas: Add CPG core wrapper for RZ/G2L SoC")
> > >> Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@xxxxxxxxxxxxxx>
> > >
> > > Thanks for your patch!
> > >
> > >> --- a/drivers/clk/renesas/rzg2l-cpg.c
> > >> +++ b/drivers/clk/renesas/rzg2l-cpg.c
> > >> @@ -696,18 +696,22 @@ static unsigned long rzg2l_cpg_pll_clk_recalc_rate(struct clk_hw *hw,
> > >>         struct pll_clk *pll_clk = to_pll(hw);
> > >>         struct rzg2l_cpg_priv *priv = pll_clk->priv;
> > >>         unsigned int val1, val2;
> > >> -       unsigned int mult = 1;
> > >> -       unsigned int div = 1;
> > >> +       unsigned int div;
> > >> +       u64 rate;
> > >> +       s16 kdiv;
> > >>
> > >>         if (pll_clk->type != CLK_TYPE_SAM_PLL)
> > >>                 return parent_rate;
> > >>
> > >>         val1 = readl(priv->base + GET_REG_SAMPLL_CLK1(pll_clk->conf));
> > >>         val2 = readl(priv->base + GET_REG_SAMPLL_CLK2(pll_clk->conf));
> > >> -       mult = MDIV(val1) + KDIV(val1) / 65536;
> > >> +       kdiv = KDIV(val1);
> > >>         div = PDIV(val1) << SDIV(val2);
> > >>
> > >> -       return DIV_ROUND_CLOSEST_ULL((u64)parent_rate * mult, div);
> > >> +       rate = (u64)MDIV(val1) * parent_rate;
> > >> +       rate += ((long long)parent_rate * kdiv) / 65536;
> > >
> > > As the division is a binary shift, you can use the mul_u64_u32_shr() helper,
> > > and incorporate the sdiv shift at the same time:
> > >
> > >     rate += mul_u64_u32_shr(parent_rate, KDIV(val1), 16 + SDIV(val2));
>
>  [1]^
>
> > >
> > > You can save a multiplication by premultiplying mdiv by 65536:
> > >
> > >     rate = mul_u64_u32_shr(parent_rate, (MDIV(val1) << 16)) + KDIV(val1),
> > >                            16 + SDIV(val2));
>
> [2]^
>
> >
> > Looking again at this: KDIV (aka DIV_K) could have negative values thus
> > mul_u64_u32_shr() cannot be used here.
>
> That means you can indeed not use [1].
>
> But you can still use [2], as MDIV() must be in the range 64..533[3],
> so "(MDIV(val1) << 16)) + (s16)KDIV(val1)" is always positive.
> Note that you do need the cast to s16 (which I had missed before), or
> the intermediate variable kdiv of type s16 (like in your patch).

Or include the cast to a signed type in the definition of KDIV().

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