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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Thu, 17 Oct 2019 14:55:58 +0200
Message-ID: <CAMuHMdUH0LrZ6iEuN1aWCTt_-jpgp=EjxubMAVdp11HLL=ayyQ@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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Hi Jacopo,

CC i2c

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.

Wolfram: do you have any clues?

> Also, I opted for deferring probe instead of arbitrary repeat the
> temperature read. What's your opinion?

While this is probably OK if the max9611 driver is built-in, I'm afraid
this may lead to unbounded delays for a reprobe in case the driver
is modular.

> --- a/drivers/iio/adc/max9611.c
> +++ b/drivers/iio/adc/max9611.c
> @@ -80,6 +80,7 @@
>   * The complete formula to calculate temperature is:
>   *     ((adc_read >> 7) * 1000) / (1 / 480 * 1000)
>   */
> +#define MAX9611_TEMP_POR               0x8000
>  #define MAX9611_TEMP_MAX_POS           0x7f80
>  #define MAX9611_TEMP_MAX_NEG           0xff80
>  #define MAX9611_TEMP_MIN_NEG           0xd980
> @@ -480,8 +481,10 @@ static int max9611_init(struct max9611_dev *max9611)
>         if (ret)
>                 return ret;
>
> -       regval &= MAX9611_TEMP_MASK;
> +       if (regval == MAX9611_TEMP_POR)
> +               return -EPROBE_DEFER;
>
> +       regval &= MAX9611_TEMP_MASK;
>         if ((regval > MAX9611_TEMP_MAX_POS &&
>              regval < MAX9611_TEMP_MIN_NEG) ||
>              regval > MAX9611_TEMP_MAX_NEG) {

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