Re: [PATCH v3 6/6] iio: adc: ad4851: add ad485x driver

[David Lechner <dlechner@baylibre.com>]

On 10/25/24 9:29 AM, David Lechner wrote:
> On 10/25/24 6:35 AM, Miclaus, Antoniu wrote:
>>>
> ...
> 
>>>
>>> See the ad7380 driver as an example of how to impelemt this. [2]
>>>
>>> [2]: https://urldefense.com/v3/__https://lore.kernel.org/linux-
>>> iio/20240530-iio-add-support-for-multiple-scan-types-v3-5-
>>> cbc4acea2cfa@baylibre.com/__;!!A3Ni8CS0y2Y!4LS7UI11XqIHRgT3ckx76VYn
>>> CyeikpTumyjO0qDTn7eF7Fd-
>>> jFFL8yqpYcMAxP_u3VC09bfIAB7gW_rvGoM_sEA$
>>>
>>> Also, I would expect the .sign value to depend on how the
>>> input is being used. If it is differential or single-ended
>>> bipolar, then it is signed, but if it is signle-ended unipoloar
>>> then it is unsiged.
>>>
>>> Typically, this is coming from the devicetree because it
>>> depends on what is wired up to the input.
>>
>> This topic is mentioned in the cover letter, maybe not argued enough there.
>> Yes, the go-to approach is to specify the unipolar/bipolar configuration in the devicetree.
>> But this is a request from the actual users of the driver: to have the softspan fully
>> controlled from userspace. That's why the offset and scale implementations were added.
>> Both these attributes are influencing the softspan.
>>
>>>> +	},								\
>>>> +}
>>>
> 
> The cover letter did not get sent, so we did not see this.

So please resend it so we can get the full explanation.

> 
> Still, I have doubts about using the offset attribute for
> this since a 0 raw value is always 0V for both unipolar
> and bipolar cases. There is never an offset to apply to
> the raw value.
> 
> So I think we will need to find a different way to control
> this other than the offset attribute.

I thought about this some more and I have an idea to solve the
issue without using devicetree or the offset attribute.

But we should see what Jonathan thinks before implementing this
in case it isn't a good idea.

We can expose each voltage input to userspace as two different
channels, a single-ended channel and a differential channel.

For an 8 channel chip, we would have 16 IIO channels (in order
of scan_index):

in_voltage0_raw
in_voltage0-voltage8_raw
in_voltage1_raw
in_voltage1-voltage9_raw
...
in_voltage7_raw
in_voltage7-voltage15_raw

If you read the voltage using in_voltageX_raw, then the SoftSpan
for that channel gets set to the 0V to +V value based on
in_voltageX_scale. Likewise, if you read the in_voltageX-voltageY_raw
attribute, the SoftSpan gets set to -V to +V according to
in_voltageX-voltageY_scale.

For buffered reads, only one of each in_voltageX_raw/in_voltageX-voltageY_raw
pair can be enabled at the same time (because the chip is simultaneous
sampling).