linux-kernel - Re: [PATCH 2/2] iio: adc: qcom-pm8xxx-xoadc: fix incorrect calibration values

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Message-ID: <0eea7e4c-ec3b-421c-8522-aa3f52b5cb13@baylibre.com>
Date: Mon, 27 Oct 2025 13:35:41 -0500
From: David Lechner <dlechner@...libre.com>
To: Antony Kurniawan Soemardi <linux@...nkusors.com>,
 Jonathan Cameron <jic23@...nel.org>, Nuno Sá
 <nuno.sa@...log.com>, Andy Shevchenko <andy@...nel.org>
Cc: linux-arm-msm@...r.kernel.org, linux-iio@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] iio: adc: qcom-pm8xxx-xoadc: fix incorrect
 calibration values

On 10/27/25 12:29 PM, Antony Kurniawan Soemardi wrote:
> On msm8960 phones, the XOADC driver was using incorrect calibration
> values:
> absolute calibration dx = 625000 uV, dy = 4 units
> ratiometric calibration dx = 1800, dy = -29041 units
> 
> As a result, reading from the IIO bus returned unexpected results:
> in_voltage_7 (USB_VBUS): 0
> in_voltage_10 (125V): 0
> 
> The issue was caused by not setting the ratiometric scale (amux_ip_rsv)
> from the predefined channels. Additionally, the downstream code always

Mentioning downstream kernels is usually a red flag. :-) 

We can justify it here though by saying that there is no documentation
available other than downstream source code, so we are just using it
as a reference.

> set the ADC_ARB_USRP_DIG_PARAM register to PM8XXX_ADC_ARB_ANA_DIG [1].
> That value does not include the SEL_SHIFT0 and SEL_SHIFT1 bits. Enabling
> those bits caused calibration errors too, so they were removed.
> 
> With these fixes, calibration now uses the correct values:
> absolute calibration dx = 625000 uV, dy = 6307 units
> ratiometric calibration dx = 1800, dy = 18249 units
> 
> Reading from the IIO bus now returns expected results:
> in_voltage_7 (USB_VBUS): 4973836
> in_voltage_10 (125V): 1249405

Would be useful to mention which hardware you tested with in case
it turns out that there is some other hardware that does require the
SHIFT0/1 bits to be set.

> 
> [1] https://github.com/LineageOS/android_kernel_sony_msm8960t/blob/93319b1e5aa343ec1c1aabcb028c5e88c7df7c01/drivers/hwmon/pm8xxx-adc.c#L407-L408
> 

Since this is a fix, it should have a Fixes: tag. And it sounds like
possibly two separate fixes. In that case, it should be two separate
patches.

> Signed-off-by: Antony Kurniawan Soemardi <linux@...nkusors.com>
> ---
>  drivers/iio/adc/qcom-pm8xxx-xoadc.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/iio/adc/qcom-pm8xxx-xoadc.c b/drivers/iio/adc/qcom-pm8xxx-xoadc.c
> index 8555f34036fb13c41ac720dc02c1dc39876e9198..a53d361456ec36b66d258041877bd96ab37838c4 100644
> --- a/drivers/iio/adc/qcom-pm8xxx-xoadc.c
> +++ b/drivers/iio/adc/qcom-pm8xxx-xoadc.c
> @@ -503,10 +503,11 @@ static int pm8xxx_read_channel_rsv(struct pm8xxx_xoadc *adc,
>  		goto unlock;
>  
>  	/* Decimation factor */
> -	ret = regmap_write(adc->map, ADC_ARB_USRP_DIG_PARAM,
> -			   ADC_ARB_USRP_DIG_PARAM_SEL_SHIFT0 |
> -			   ADC_ARB_USRP_DIG_PARAM_SEL_SHIFT1 |
> -			   ch->decimation << ADC_DIG_PARAM_DEC_SHIFT);
> +	ret = regmap_update_bits(adc->map,
> +				 ADC_ARB_USRP_DIG_PARAM,
> +				 ADC_ARB_USRP_DIG_PARAM_DEC_RATE0 |
> +				 ADC_ARB_USRP_DIG_PARAM_DEC_RATE1,
> +				 ch->decimation << ADC_DIG_PARAM_DEC_SHIFT);

As a follow-up patch, it would be nice to update the driver to use FIELD_PREP().

I.e. remove ADC_ARB_USRP_DIG_PARAM_DEC_RATE0, ADC_ARB_USRP_DIG_PARAM_DEC_RATE1
and ADC_DIG_PARAM_DEC_SHIFT macros and replace them with one macro:

#define ADC_ARB_USRP_DIG_PARAM_DEC_RATE		GENMASK(6, 5)

Then use it like:

	ret = regmap_update_bits(adc->map,
				 ADC_ARB_USRP_DIG_PARAM,
				 ADC_ARB_USRP_DIG_PARAM_DEC_RATE,
				 FIELD_PREP(ADC_ARB_USRP_DIG_PARAM_DEC_RATE,
					    ch->decimation));

This should be done for all of the similar multi-bit fields.

>  	if (ret)
>  		goto unlock;
>  
> @@ -783,6 +784,7 @@ static int pm8xxx_xoadc_parse_channel(struct device *dev,
>  	ch->calibration = VADC_CALIB_ABSOLUTE;
>  	/* Everyone seems to use default ("type 2") decimation */
>  	ch->decimation = VADC_DEF_DECIMATION;
> +	ch->amux_ip_rsv = hwchan->amux_ip_rsv;
>  
>  	if (!fwnode_property_read_u32(fwnode, "qcom,ratiometric", &rsv)) {
>  		ch->calibration = VADC_CALIB_RATIOMETRIC;
>