On Fri, 29 Aug 2025 21:40:24 -0300
Marcelo Schmitt <marcelo.schmitt@xxxxxxxxxx> wrote:
> Previously, the AD4030 driver was using the number of scan realbits for the
> voltage channel to derive the scale to millivolts. Though, when sample
> averaging is enabled (oversampling_ratio > 1), the number of scan realbits
> for the channel is set to 30 and doesn't match the amount of conversion
> precision bits. Due to that, the calculated channel scale did not correctly
> scale raw sample data to millivolt units in those cases. Use chip specific
> precision bits to derive the correct channel _scale on every and all
> channel configuration.
>
> Fixes: dc78e71d7c15 ("iio: adc: ad4030: remove some duplicate code")
> Signed-off-by: Marcelo Schmitt <marcelo.schmitt@xxxxxxxxxx>
Hi Marcelo
I was assuming that when this said 'averaging' it actually meant
summing (there is a note about using the upper precision bits to get the same
scaling which is what we'd expect it were simply summing over X samples).
So given that we don't divide back down to get the original scaling I'm
not following how this works.
E.g. If we 'averaged' just 2 values of 3 then we'd go from a value of 3 to
one of 6. Therefore I'd expect the scale to halve as each lsb represents
half the voltage it did when we weren't averaging those 2 samples.
I think that is what we'd see with the current code, so my reasoning is
clearly wrong, but why?
Jonathan
> ---
> This was probalby buggy since
> commit 949abd1ca5a4 ("iio: adc: ad4030: add averaging support")
> but I decided to set the fixes tag with dc78e71d7c15 because this patch will
> not apply cleanly over 949abd1ca5a4.
>
> drivers/iio/adc/ad4030.c | 9 ++++++++-
> 1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/iio/adc/ad4030.c b/drivers/iio/adc/ad4030.c
> index 1bc2f9a22470..82784593f976 100644
> --- a/drivers/iio/adc/ad4030.c
> +++ b/drivers/iio/adc/ad4030.c
> @@ -394,7 +394,14 @@ static int ad4030_get_chan_scale(struct iio_dev *indio_dev,
> else
> *val = st->vref_uv / MILLI;
>
> - *val2 = scan_type->realbits;
> + /*
> + * Even though the sample data comes in a 30-bit chunk when the ADC
> + * is averaging samples, the conversion precision is still 16-bit or
> + * 24-bit depending on the device. Thus, instead of scan_type->realbits,
> + * use chip specific precision bits to derive the correct scale to mV.
> + */
> + *val2 = scan_type->realbits == 30 ? st->chip->precision_bits
> + : scan_type->realbits;
>
> return IIO_VAL_FRACTIONAL_LOG2;
> }
