linux-kernel - Re: [PATCH] iio: adc: ad7124: fix temperature channel

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Message-ID: <h2rof27omrhv4l6pjisdsnvkpb35ovy7e6m4soeltfun5rafk5@oriv7e3egh3p>
Date: Wed, 24 Sep 2025 13:03:43 +0200
From: Uwe Kleine-König <u.kleine-koenig@...libre.com>
To: David Lechner <dlechner@...libre.com>
Cc: Michael Hennerich <Michael.Hennerich@...log.com>, 
	Jonathan Cameron <jic23@...nel.org>, Nuno Sá <nuno.sa@...log.com>, 
	Andy Shevchenko <andy@...nel.org>, Jonathan Cameron <Jonathan.Cameron@...wei.com>, 
	linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iio: adc: ad7124: fix temperature channel

Hello David,

On Tue, Sep 23, 2025 at 03:18:02PM -0500, David Lechner wrote:
> Fix temperature channel not working due to gain and offset not being
> initialized. This was causing the raw temperature readings to be always
> 8388608 (0x800000).
> 
> To fix it, we just make sure the gain and offset values are set to the
> default values and still return early without doing an internal
> calibration.
> 
> While here, add a comment explaining why we don't bother calibrating
> the temperature channel.
> 
> Fixes: 47036a03a303 ("iio: adc: ad7124: Implement internal calibration at probe time")
> Signed-off-by: David Lechner <dlechner@...libre.com>
> ---
>  drivers/iio/adc/ad7124.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/iio/adc/ad7124.c b/drivers/iio/adc/ad7124.c
> index 374e39736584f55c1290db3e257dff2c60f884d2..94d90a63987c0f9886586db0c4bc1690855be2c1 100644
> --- a/drivers/iio/adc/ad7124.c
> +++ b/drivers/iio/adc/ad7124.c
> @@ -1518,10 +1518,6 @@ static int __ad7124_calibrate_all(struct ad7124_state *st, struct iio_dev *indio
>  	int ret, i;
>  
>  	for (i = 0; i < st->num_channels; i++) {
> -
> -		if (indio_dev->channels[i].type != IIO_VOLTAGE)
> -			continue;
> -
>  		/*
>  		 * For calibration the OFFSET register should hold its reset default
>  		 * value. For the GAIN register there is no such requirement but
> @@ -1531,6 +1527,13 @@ static int __ad7124_calibrate_all(struct ad7124_state *st, struct iio_dev *indio
>  		st->channels[i].cfg.calibration_offset = 0x800000;
>  		st->channels[i].cfg.calibration_gain = st->gain_default;
>  
> +		/*
> +		 * Only the main voltage input channels are important enough
> +		 * to be automatically calibrated here.
> +		 */
> +		if (indio_dev->channels[i].type != IIO_VOLTAGE)
> +			continue;
> +

I don't understand a detail of the problem. The commit log suggests that
there is a general problem, but looking at the patch I suspect there is
only a problem if at probe time the OFFSET and GAIN register for the
temperature channel are different from their reset default setting.

I think the patch is fine, but if my understanding is right the commit
log is more dramatic than the issue really is, as it needs some fiddling
with the ADC's registers between poweron and driver loading to trigger.

Best regards
Uwe

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