linux-kernel - Re: [PATCH RFT 2/2] i2c: rcar: improve accuracy for R-Car Gen3+

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Message-ID: <CAMuHMdXkiMH04DHFPonsyuV9O5RFPqA5dN8ny4UT9MmS_D9F0g@mail.gmail.com>
Date:   Tue, 19 Sep 2023 11:44:22 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Wolfram Sang <wsa+renesas@...g-engineering.com>
Cc:     linux-renesas-soc@...r.kernel.org,
        Andi Shyti <andi.shyti@...nel.org>, linux-i2c@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFT 2/2] i2c: rcar: improve accuracy for R-Car Gen3+

On Tue, Sep 19, 2023 at 11:42 AM Geert Uytterhoeven
<geert@...ux-m68k.org> wrote:
> On Wed, Sep 13, 2023 at 11:38 AM Wolfram Sang
> <wsa+renesas@...g-engineering.com> wrote:
> > With some new registers, SCL can be calculated to be closer to the
> > desired rate. Apply the new formula for R-Car Gen3 device types.
> >
> > Signed-off-by: Wolfram Sang <wsa+renesas@...g-engineering.com>
>
> > --- a/drivers/i2c/busses/i2c-rcar.c
> > +++ b/drivers/i2c/busses/i2c-rcar.c
>
> > -       /* keep icccr value */
> > -       priv->icccr = scgd << cdf_width | cdf;
> > +               priv->icccr = scgd << cdf_width | cdf;
> > +       } else {
> > +               u32 x, sum_ratio = RCAR_SCHD_RATIO + RCAR_SCLD_RATIO;
> > +               /*
> > +                * SCLD/SCHD ratio and SMD default value are explained above
> > +                * where they are defined. With these definitions, we can compute
> > +                * x as a base value for the SCLD/SCHD ratio:
> > +                *
> > +                * SCL = clkp / (8 + 2 * SMD + SCLD + SCHD + F[(ticf + tr + intd) * clkp])
> > +                * SCL = clkp / (8 + 2 * RCAR_DEFAULT_SMD + RCAR_SCLD_RATIO * x
> > +                *               + RCAR_SCHD_RATIO * x + F[...])
> > +                *
> > +                * with: sum_ratio = RCAR_SCLD_RATIO + RCAR_SCHD_RATIO
> > +                * and:  smd = 2 * RCAR_DEFAULT_SMD
> > +                *
> > +                * SCL = clkp / (8 + smd + sum_ratio * x + F[...])
> > +                * 8 + smd + sum_ratio * x + F[...] = SCL / clkp
> > +                * x = ((SCL / clkp) - 8 - smd - F[...]) / sum_ratio
>
> Woops, I missed that both "SCL / clkp" above should be "clkp / SCL".
>
> Fortunately I noticed your "[PATCH 2/2] i2c: rcar: add FastMode+
> support for Gen4" fixed that, but I guess it's better to fix that in
> this patch instead.

And while at it, please move the "2 *" to replace "smd" by "2 * smd", to
match the docs, and the result after FM+?

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