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

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Message-ID: <CAMuHMdXJ_gp5cdGpcK-kGk16YGDX8d9MEjQQkSobOGLphbJ5dQ@mail.gmail.com>
Date:   Tue, 26 Sep 2023 16:44:00 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     claudiu beznea <claudiu.beznea@...on.dev>
Cc:     mturquette@...libre.com, sboyd@...nel.org, robh+dt@...nel.org,
        krzysztof.kozlowski+dt@...aro.org, conor+dt@...nel.org,
        ulf.hansson@...aro.org, linus.walleij@...aro.org,
        gregkh@...uxfoundation.org, jirislaby@...nel.org,
        magnus.damm@...il.com, catalin.marinas@....com, will@...nel.org,
        prabhakar.mahadev-lad.rj@...renesas.com,
        biju.das.jz@...renesas.com, quic_bjorande@...cinc.com,
        arnd@...db.de, konrad.dybcio@...aro.org, neil.armstrong@...aro.org,
        nfraprado@...labora.com, rafal@...ecki.pl,
        wsa+renesas@...g-engineering.com,
        linux-renesas-soc@...r.kernel.org, linux-clk@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mmc@...r.kernel.org, linux-gpio@...r.kernel.org,
        linux-serial@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Claudiu Beznea <claudiu.beznea.uj@...renesas.com>
Subject: Re: [PATCH 09/37] clk: renesas: rzg2l: fix computation formula

Hi Claudiu,

On Tue, Sep 26, 2023 at 1:47 PM claudiu beznea <claudiu.beznea@...on.dev> wrote:
> On 14.09.2023 15:55, Geert Uytterhoeven wrote:
> > On Tue, Sep 12, 2023 at 6:52 AM Claudiu <claudiu.beznea@...on.dev> wrote:
> >> From: Claudiu Beznea <claudiu.beznea.uj@...renesas.com>
> >>
> >> 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@...renesas.com>
> >
> > 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).

[3] As the current PLL driver is read-only, there is no calculation or
    validation of the PLL parameters.

> >> +
> >> +       return DIV_ROUND_CLOSEST_ULL(rate, div);
> >
> > return DIV_ROUND_CLOSEST_ULL(rate, PDIV(val1));

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