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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Sun, 10 Nov 2019 19:45:36 +0100
Message-ID: <CAMuHMdULSOOmcoKrQ40TJaNVOAuBsCA=bgC3pMB8jOGpRCAMhg@mail.gmail.com>
Subject: Re: [PATCH] iio: adc: max9611: Defer probe on POR read
To:     Jacopo Mondi <jacopo+renesas@jmondi.org>
Cc:     linux-iio@vger.kernel.org,
        Linux-Renesas <linux-renesas-soc@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Wolfram Sang <wsa+renesas@sang-engineering.com>,
        Linux I2C <linux-i2c@vger.kernel.org>
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On Thu, Oct 17, 2019 at 2:55 PM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> On Wed, Oct 16, 2019 at 12:23 PM Jacopo Mondi <jacopo+renesas@jmondi.org> wrote:
> > The max9611 driver tests communications with the chip by reading the die
> > temperature during the probe function. If the temperature register
> > POR (power-on reset) value is returned from the test read, defer probe to
> > give the chip a bit more time to properly exit from reset.
> >
> > Reported-by: Geert Uytterhoeven <geert+renesas@glider.be>
> > Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
>
> Thanks for your patch!
>
> > Geert,
> >   I've not been able to reproduce the issue on my boards (M3-N
> > Salvator-XS and M3-W Salvator-X). As you reported the issue you might be
> > able to reproduce it, could you please test this?
>
> I can reproduce it on Salvator-XS with R-Car H3 ES2.0.
> According to my logs, I've seen the issue on all Salvator-X(S) boards,
> but not with the same frequency.  Probability is highest on H3 ES2.0
> (ca. 5% of the boots since I first saw the issue), followed by H3 ES1.0,
> M3-W, and M3-N.
>
> After more investigation, my findings are:
>   1. I cannot reproduce the issue if the max9611 driver is modular.
>      Is it related to using max9611 "too soon" after i2c bus init?
>      How can "i2c bus init" impact a slave device?
>      Perhaps due to pin configuration, e.g. changing from another pin
>      function or GPIO to function i2c4?
>   2. Adding a delay at the top of max9611_init() fixes the issue.
>      This would explain why the issue is less likely to happy on slower
>      SoCs like M3-N.
>   3. Disabling all other i2c slaves on i2c4 in DTS fixes the issue.
>      Before, max9611 was initialized last, so this moves init earlier,
>      contradicting theory #1.
>   4. Just disabling the adv7482 (which registers 11 dummies i2c slaves)
>      in DTS does not fix the issue.
>
> Unfortunately i2c4 is exposed on a 60-pin Samtec QSH connector only,
> for which I have no breakout adapter.

Some soldering fixed that. Still investigating.
Here's a status update:

  A. I can reproduce the issue at 100 kHz instead of 400 kHz.
  B. 3 above doesn't seem to be true: I can reproduce it with all other
     slaves disabled.
  C. The code says:

        /*
         * need a delay here to make register configuration
         * stabilize. 1 msec at least, from empirical testing.
         */
        usleep_range(1000, 2000);

     However, the datasheet says:

        Parameter            MIN     TYP     MAX
        Conversion Time      -       2 ms    -

     So 1 ms is definitely too short.
     Unfortunately the datasheet has no maximum value.

  D. For 2: msleep(1) is sufficient, usleep_range(200, 500) is not.
     And this is still not explained by C.
     I also don't know yet who's resetting the chip on reboot, as it
     does not have a reset line, but all registers are zeroed (except
     for the POR temperature value).

To be investigated more...

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-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