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Date:   Mon, 18 May 2020 17:44:17 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Qii Wang <qii.wang@...iatek.com>
Cc:     Wolfram Sang <wsa@...-dreams.de>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, srv_heupstream@...iatek.com,
        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

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?

> +       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!)

noreply@...erman.id.au reports:

    ERROR: modpost: "__udivdi3" [drivers/i2c/busses/i2c-mt65xx.ko] undefined!
    ERROR: modpost: "__divdi3" [drivers/i2c/busses/i2c-mt65xx.ko] undefined!

for 32-bit builds.

> +       max_step_cnt = mtk_i2c_max_step_cnt(check_speed);
> +       if ((2 * step_cnt) > low_cnt && low_cnt < max_step_cnt) {
> +               if (low_cnt > step_cnt) {
> +                       high_cnt = 2 * step_cnt - low_cnt;
> +               } else {
> +                       high_cnt = step_cnt;
> +                       low_cnt = step_cnt;
> +               }
> +       } else {
> +               return -2;
> +       }
> +
> +       sda_max = spec->max_hd_dat_ns / sample_ns;
> +       if (sda_max > low_cnt)
> +               sda_max = 0;
> +
> +       sda_min = DIV_ROUND_UP(spec->min_su_dat_ns, sample_ns);

One more.

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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