Re: [PATCH v3 4/9] iio: adc: ti-ads1262: support per-channel reference and gain

["Kurt Borja" <kuurtb@gmail.com>]

On Sat Aug 8, 2026 at 1:39 PM -05, David Lechner wrote:
> On 8/7/26 10:58 PM, Kurt Borja wrote:
>> Allow each channel to select its voltage reference through the
>> "reference-sources" firmware property. Then, use the reference voltage
>> to calculate available scales.
>
> It looks like this is also implementing PGA gain at the same time, but
> isn't mentioned in ght commit messsage. I would also expect something
> here about how we should handle PGA bypass (even if it just says default
> works always and we can consdier controlling it later).

I forgot about the PGA bypass stuff. I do believe it should belong in
devicetree because it changes the voltage range of the analog inputs.
See datasheet section 10.3 and 7.3.

>
>> 
>> The ADS1262 allows single-ended supply configurations or bipolar supply
>> configurations. In single ended configurations both the analog and
>> digital rails share the same ground, i.e. AVSS = DGND = 0 V. In bipolar
>> supply configurations, AVSS can go below ground, e.g. AVSS = -2.5 V.
>> 
>> If AVSS is below ground, the ADC can achieve true bipolar measurements
>> and the external references can also have voltage levels below ground.
>> This is currently an issue because the regulator subsystem doesn't
>> support negative voltages.
>> 
>> The ad4170-4 driver faces this problem too and the same workaround is
>> used in this case: assume every regulator reports magnitudes (absolute
>> values). If the chip has a bipolar supply configuration, then assume
>> positive references are above ground (>= 0 V) and negative references
>> are below ground (<= 0 V). This is not a hardware constraint, but it is
>> the most common wiring.
>> 
>> Signed-off-by: Kurt Borja <kuurtb@gmail.com>
>> ---
>>  drivers/iio/adc/ti-ads1262.c | 417 +++++++++++++++++++++++++++++++++++++++++--
>>  1 file changed, 406 insertions(+), 11 deletions(-)
>> 
>> diff --git a/drivers/iio/adc/ti-ads1262.c b/drivers/iio/adc/ti-ads1262.c
>> index b3b7b1249102..360ce01a5871 100644
>> --- a/drivers/iio/adc/ti-ads1262.c
>> +++ b/drivers/iio/adc/ti-ads1262.c

[...]

>> @@ -688,6 +818,91 @@ static const struct regmap_bus ads1262_regmap_bus = {
>>  	.max_raw_write = ADS1262_MAX_REGMAP_WRITE,
>>  };
>>  
>> +static void ads1262_calculate_scales(int (*scales)[2], size_t num_scales,
>> +				     u32 full_scale, u64 mult,
>> +				     u32 resolution)
>> +{
>> +	unsigned int i;
>> +	s64 val;
>> +
>> +	for (i = 0; i < num_scales; i++) {
>
> This could use a comment explaining the relasionship of the index in the
> array to the PGA multipier.
>
>> +		val = mul_u64_u64_shr(full_scale, mult, resolution - 1 + i);
>> +		iio_val_s64_decompose(val, &scales[i][0], &scales[i][1]);
>> +	}
>> +}
>> +
>> +static int ads1262_populate_scales_resistance(struct ads1262 *st,
>> +					      const struct iio_chan_spec *spec)
>> +{
>> +	struct ads1262_channel *chan = &st->channels[spec->scan_index];
>> +	u32 full_scale;
>> +
>> +	if (WARN_ON(!ads1262_ref_is_external(chan->ref_p, chan->ref_n)))
>> +		return -EINVAL;
>
> WARN_ON() is a bit strong for something that is coming from the devicetree.
> I would just fail the parse() function with an appropriate error message
> so that we don't have to check here.

Actually, an IIO_RESISTANCE channel without external reference is just a
bug and would read past the end of buffer below, which is the only
reason I verify it one last time. We should never actually hit this
warning. I'll add a comment explaining that.


>
>> +
>> +	full_scale = st->rref_ohms[chan->ref_p - 1][chan->ref_n - 1];
>> +
>> +	chan->num_scales = ARRAY_SIZE(chan->scales);
>> +
>> +	ads1262_calculate_scales(chan->scales, chan->num_scales, full_scale,
>> +				 PICO, ADS1262_ADC1_RESOLUTION);
>> +
>> +	return 0;
>> +}

-- 
Thanks,
 ~ Kurt