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Date:   Tue, 19 May 2020 09:14:25 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Qii Wang <qii.wang@...iatek.com>
Cc:     Joe Perches <joe@...ches.com>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, srv_heupstream@...iatek.com,
        Wolfram Sang <wsa@...-dreams.de>, leilk.liu@...iatek.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mediatek@...ts.infradead.org,
        Linux I2C <linux-i2c@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2 2/2] i2c: mediatek: Add i2c ac-timing adjust support

Hi Qii,

On Tue, May 19, 2020 at 4:59 AM Qii Wang <qii.wang@...iatek.com> wrote:
> On Mon, 2020-05-18 at 17:44 +0200, Geert Uytterhoeven wrote:
> > On Thu, May 14, 2020 at 3:13 PM Qii Wang <qii.wang@...iatek.com> wrote:
> > > This patch adds a algorithm to calculate some ac-timing parameters
> > > which can fully meet I2C Spec.
> > >
> > > Signed-off-by: Qii Wang <qii.wang@...iatek.com>
> > > ---
> > >  drivers/i2c/busses/i2c-mt65xx.c | 328 +++++++++++++++++++++++++++++++++-------
> > >  1 file changed, 277 insertions(+), 51 deletions(-)
> > >
> > > diff --git a/drivers/i2c/busses/i2c-mt65xx.c b/drivers/i2c/busses/i2c-mt65xx.c
> > > index 0ca6c38a..7020618 100644
> > > --- a/drivers/i2c/busses/i2c-mt65xx.c
> > > +++ b/drivers/i2c/busses/i2c-mt65xx.c
> >
> > > +/*
> > > + * Check and Calculate i2c ac-timing
> > > + *
> > > + * Hardware design:
> > > + * sample_ns = (1000000000 * (sample_cnt + 1)) / clk_src
> > > + * xxx_cnt_div =  spec->min_xxx_ns / sample_ns
> > > + *
> > > + * Sample_ns is rounded down for xxx_cnt_div would be greater
> > > + * than the smallest spec.
> > > + * The sda_timing is chosen as the middle value between
> > > + * the largest and smallest.
> > > + */
> > > +static int mtk_i2c_check_ac_timing(struct mtk_i2c *i2c,
> > > +                                  unsigned int clk_src,
> > > +                                  unsigned int check_speed,
> > > +                                  unsigned int step_cnt,
> > > +                                  unsigned int sample_cnt)
> > > +{
> > > +       const struct i2c_spec_values *spec;
> > > +       unsigned int su_sta_cnt, low_cnt, high_cnt, max_step_cnt;
> > > +       unsigned int sda_max, sda_min, clk_ns, max_sta_cnt = 0x3f;
> > > +       long long sample_ns = (1000000000 * (sample_cnt + 1)) / clk_src;
> >
> > So sample_ns is a 64-bit value. Is that really needed?
> >
>
> (1000000000 * (sample_cnt + 1)) / clk_src value is a 32-bit, (1000000000
> * (sample_cnt + 1)) will over 32-bit if sample_cnt is 7.

The intermediate value will indeed not fit in 32-bit.
But that doesn't mean the end result won't fit in 32-bit.
As you divide spec->min_low_ns and spec->min_su_dat_ns (which I assume
are small numbers) by sample_ns below, sample_ns cannot be very large,
or the quotient will be zero anyway.
So just doing the multiplication in 64-bit, followed by a 64-by-32
division is probably fine:

    unsigned int sample_ns = div_u64(1000000000ULL * (sample_cnt + 1), clk_src);

You may want to take precautions for the case where the passed value of
clk_src is a small number (can that happen?).

BTW, clk_get_rate() returns "unsigned long", while mtk_i2c_set_speed()
takes an "unsigned int" parent_clk, which may cause future issues.
You may want to change that to "unsigned long", along the whole
propagation path, and use div64_ul() instead of div_u64() above.

> I think 1000000000 and clk_src is too big, maybe I can reduce then with
> be divided all by 1000.
> example:
>
> unsigned int sample_ns;
> unsigned int clk_src_khz = clk_src / 1000;

That may cause too much loss of precision.

>
> if(clk_src_khz)
>         sample_ns = (1000000 * (sample_cnt + 1)) / clk_src_khz;
> else
>         return -EINVAL;
>
> > > +       if (!i2c->dev_comp->timing_adjust)
> > > +               return 0;
> > > +
> > > +       if (i2c->dev_comp->ltiming_adjust)
> > > +               max_sta_cnt = 0x100;
> > > +
> > > +       spec = mtk_i2c_get_spec(check_speed);
> > > +
> > > +       if (i2c->dev_comp->ltiming_adjust)
> > > +               clk_ns = 1000000000 / clk_src;
> > > +       else
> > > +               clk_ns = sample_ns / 2;
> > > +
> > > +       su_sta_cnt = DIV_ROUND_UP(spec->min_su_sta_ns, clk_ns);
> > > +       if (su_sta_cnt > max_sta_cnt)
> > > +               return -1;
> > > +
> > > +       low_cnt = DIV_ROUND_UP(spec->min_low_ns, sample_ns);
> >
> > So this is a 32-bit by 64-bit division (indeed, not 64-by-32!)

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

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