linux-kernel - Re: [PATCH v3] iio: adc: max9611: Fix too short conversion time delay

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Message-ID: <20191207111156.7e5139f0@archlinux>
Date:   Sat, 7 Dec 2019 11:11:56 +0000
From:   Jonathan Cameron <jic23@...nel.org>
To:     Geert Uytterhoeven <geert+renesas@...der.be>
Cc:     Hartmut Knaack <knaack.h@....de>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        Jacopo Mondi <jacopo+renesas@...ndi.org>,
        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 v3] iio: adc: max9611: Fix too short conversion time
 delay

On Fri,  6 Dec 2019 14:19:44 +0100
Geert Uytterhoeven <geert+renesas@...der.be> 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.  Maxim Technical Support
> confirmed this was tested with temperature Ta=25 degreeC, and promised
> to inform me if a maximum/minimum value is available (they didn't get
> back to me, so I assume it is not).
> 
> 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 3000-3300 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>
> Reviewed-by: Jacopo Mondi <jacopo+renesas@...ndi.org>
> Reviewed-by: Wolfram Sang <wsa+renesas@...g-engineering.com>
Ah. I picked up v2 and did the tidy up.  Oh well, same result ;)

Jonathan

> ---
> 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.
> 
> v3:
>   - Add Reviewed-by,
>   - Join split comment line,
> 
> v2:
>   - Add Reviewed-by,
>   - Add feedback from Maxim Technical Support,
>   - Increase delay from 2000-2200 µs to 3000-3300 µs to play safe.
> ---
>  drivers/iio/adc/max9611.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/iio/adc/max9611.c b/drivers/iio/adc/max9611.c
> index bf76dfb3f2c9530b..6250d4bf46dc9642 100644
> --- a/drivers/iio/adc/max9611.c
> +++ b/drivers/iio/adc/max9611.c
> @@ -89,6 +89,12 @@
>  #define MAX9611_TEMP_SCALE_NUM		1000000
>  #define MAX9611_TEMP_SCALE_DIV		2083
>  
> +/*
> + * Conversion time is 2 ms (typically) at Ta=25 degreeC
> + * No maximum value is known, so play it safe
> + */
> +#define MAX9611_CONV_TIME_US_RANGE	3000, 3300
> +
>  struct max9611_dev {
>  	struct device *dev;
>  	struct i2c_client *i2c_client;
> @@ -223,10 +229,9 @@ 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.
> +	 * need a delay here to make register configuration 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) {
> @@ -493,7 +498,7 @@ 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;
>  }