linux-kernel - Re: [PATCH v2] iio: adc: max9611: Fix too short conversion time delay

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Message-ID: <20191202092328.GE1266@kunai>
Date:   Mon, 2 Dec 2019 10:23:29 +0100
From:   Wolfram Sang <wsa@...-dreams.de>
To:     Geert Uytterhoeven <geert+renesas@...der.be>
Cc:     Jonathan Cameron <jic23@...nel.org>,
        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 v2] iio: adc: max9611: Fix too short conversion time delay

On Mon, Dec 02, 2019 at 09:55:46AM +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.  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>

One minor nit, though:

>  	/*
>  	 * need a delay here to make register configuration
> -	 * stabilize. 1 msec at least, from empirical testing.
> +	 * stabilize.

This could be a one line comment now?


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