linux-kernel - Re: [PATCH v2 2/2] i2c: mediatek: Add i2c ac-timing adjust support

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Date:   Tue, 19 May 2020 10:57:53 +0800
From:   Qii Wang <qii.wang@...iatek.com>
To:     Joe Perches <joe@...ches.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>
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 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.

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;

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!)
> 
> 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