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

[Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>]

On 10/27/25 6:29 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
> 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
> 
> [1] https://github.com/LineageOS/android_kernel_sony_msm8960t/blob/93319b1e5aa343ec1c1aabcb028c5e88c7df7c01/drivers/hwmon/pm8xxx-adc.c#L407-L408
> 
> Signed-off-by: Antony Kurniawan Soemardi <linux@smankusors.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,

The PM8921 datasheet suggests a single valid value of BIT(5)=1, BIT(6)=0
for a "1K" (1/1024?) ratio, although a comment in this file suggests
BIT(5)|BIT(6) is also valid and corresponds to 1/4096.. I wouldn't be
surprised if that is the case

The previously set bits are a field called DECI_SEL but are otherwise left
undescribed

Hope this is useful

Konrad