linux-kernel - Re: [PATCH] iio: adc: max9611: Fix too short conversion time delay

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Message-ID: <20191113093828.vk5qqtlr7bs5z5fb@uno.localdomain>
Date:   Wed, 13 Nov 2019 10:38:28 +0100
From:   Jacopo Mondi <jacopo@...ndi.org>
To:     Geert Uytterhoeven <geert+renesas@...der.be>
Cc:     Jacopo Mondi <jacopo+renesas@...ndi.org>,
        Jonathan Cameron <jic23@...nel.org>,
        Hartmut Knaack <knaack.h@....de>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>,
        linux-iio@...r.kernel.org, linux-renesas-soc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iio: adc: max9611: Fix too short conversion time delay

Hi Geert,

On Wed, Nov 13, 2019 at 10:21:33AM +0100, Geert Uytterhoeven wrote:
> As of commit b9ddd5091160793e ("iio: adc: max9611: Fix temperature
> reading in probe"), max9611 initialization sometimes fails on the
> Salvator-X(S) development board with:
>
>     max9611 4-007f: Invalid value received from ADC 0x8000: aborting
>     max9611: probe of 4-007f failed with error -5
>
> The max9611 driver tests communications with the chip by reading the die
> temperature during the probe function, which returns an invalid value.
>
> According to the datasheet, the typical ADC conversion time is 2 ms, but
> no minimum or maximum values are provided.  However, the driver assumes
> a 1 ms conversion time.  Usually the usleep_range() call returns after
> more than 1.8 ms, hence it succeeds.  When it returns earlier, the data
> register may be read too early, and the previous measurement value will
> be returned.  After boot, this is the temperature POR (power-on reset)
> value, causing the failure above.
>
> Fix this by increasing the delay from 1000-2000 µs to 2000-2200 µs.
>
> Note that this issue has always been present, but it was exposed by the
> aformentioned commit.
>
> Fixes: 69780a3bbc0b1e7e ("iio: adc: Add Maxim max9611 ADC driver")
> Signed-off-by: Geert Uytterhoeven <geert+renesas@...der.be>
> ---
> This problem was exposed in v5.3.
>
> After this patch, probing of the two max9611 sensors succeeded during
> ca. 3000 boot cycles on Salvator-X(S) boards, equipped with various
> R-Car H3/M3-W/M3-N SoCs.
> ---
>  drivers/iio/adc/max9611.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/iio/adc/max9611.c b/drivers/iio/adc/max9611.c
> index da073d72f649f829..b0755f25356d700d 100644
> --- a/drivers/iio/adc/max9611.c
> +++ b/drivers/iio/adc/max9611.c
> @@ -89,6 +89,11 @@
>  #define MAX9611_TEMP_SCALE_NUM		1000000
>  #define MAX9611_TEMP_SCALE_DIV		2083
>
> +/*
> + * Conversion time is 2 ms (typically)
> + */
> +#define MAX9611_CONV_TIME_US_RANGE	2000, 2200
> +

Is a 20% sleep range enough or should it be slightly lengthen ?

Apart from this, thanks a lot for finding the issue root cause!

Reviewed-by: Jacopo Mondi <jacopo+renesas@...ndi.org>

Thanks
  j

>  struct max9611_dev {
>  	struct device *dev;
>  	struct i2c_client *i2c_client;

> @@ -238,9 +243,9 @@ static int max9611_read_single(struct max9611_dev *max9611,
>
>  	/*
>  	 * need a delay here to make register configuration
> -	 * stabilize. 1 msec at least, from empirical testing.
> +	 * stabilize.
>  	 */
> -	usleep_range(1000, 2000);
> +	usleep_range(MAX9611_CONV_TIME_US_RANGE);
>
>  	ret = i2c_smbus_read_word_swapped(max9611->i2c_client, reg_addr);
>  	if (ret < 0) {
> @@ -507,7 +512,7 @@ static int max9611_init(struct max9611_dev *max9611)
>  			MAX9611_REG_CTRL2, 0);
>  		return ret;
>  	}
> -	usleep_range(1000, 2000);
> +	usleep_range(MAX9611_CONV_TIME_US_RANGE);
>
>  	return 0;
>  }
> --
> 2.17.1
>

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