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

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Date:   Tue, 26 Sep 2023 14:47:06 +0300
From:   claudiu beznea <claudiu.beznea@...on.dev>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
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, Geert,

On 14.09.2023 15:55, Geert Uytterhoeven wrote:
> Hi Claudiu,
> 
> 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));
> 
> 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));

Looking again at this: KDIV (aka DIV_K) could have negative values thus
mul_u64_u32_shr() cannot be used here.

> 
>> +
>> +       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
>