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

[Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>]

Hi Claudiu,

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));

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));

> +
> +       return DIV_ROUND_CLOSEST_ULL(rate, div);

return DIV_ROUND_CLOSEST_ULL(rate, PDIV(val1));

>  }
>
>  static const struct clk_ops rzg2l_cpg_pll_ops = {

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