linux-kernel - Re: [PATCH v4 1/6] dt-bindings: adc: ad7173: add support for ad411x

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Message-ID: <0f0c0b92-af0d-4e68-9880-bacfd53d726f@gmail.com>
Date: Wed, 5 Jun 2024 09:54:31 +0300
From: "Ceclan, Dumitru" <mitrutzceclan@...il.com>
To: Jonathan Cameron <jic23@...nel.org>
Cc: Dumitru Ceclan via B4 Relay
 <devnull+dumitru.ceclan.analog.com@...nel.org>, dumitru.ceclan@...log.com,
 Lars-Peter Clausen <lars@...afoo.de>,
 Michael Hennerich <Michael.Hennerich@...log.com>,
 Rob Herring <robh@...nel.org>,
 Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
 Conor Dooley <conor+dt@...nel.org>, David Lechner <dlechner@...libre.com>,
 linux-iio@...r.kernel.org, devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/6] dt-bindings: adc: ad7173: add support for ad411x

On 03/06/2024 23:00, Jonathan Cameron wrote:
> On Mon, 3 Jun 2024 12:46:10 +0300
> "Ceclan, Dumitru" <mitrutzceclan@...il.com> wrote:
> 
>> On 01/06/2024 21:35, Jonathan Cameron wrote:
>>> On Fri, 31 May 2024 22:42:27 +0300
>>> Dumitru Ceclan via B4 Relay <devnull+dumitru.ceclan.analog.com@...nel.org> wrote:
>>>   
>>>> From: Dumitru Ceclan <dumitru.ceclan@...log.com>

...

>>>> +          Supported only by AD7172-2, AD7172-4, AD7175-2, AD7175-8, AD7177-2:
>>>> +            19: ((AVDD1 − AVSS)/5)+
>>>> +            20: ((AVDD1 − AVSS)/5)−  
>>>
>>> That's what it says on the datasheet (so fine to copy that here) but I'm curious, what does
>>> that mean in practice?  How can we have negative and postive signals of the difference
>>> between two power supply voltages where I'm fairly sure AVDD1 always greater than AVSS.
>>>  
>>
>> I have not tested that as I do not have a model that supports this wired up.
>> If I had to guess they are the same signal but one should be connected to the
>> positive input, one to the negative input...but I could be wrong.
> 
> If they are, then as far as I we are concerned is this one channel with two
> representations depending on whether it is 1st or 2nd in the list?
> Can we use one number and hide that detail in the driver?
> 
> Seems odd though if that is the case.
> 
> I guess if we find out later this is the case we can tighten the binding to
> enforce the right one instead of squashing them to one value, but that
> is a bit ugly.  Any chance of digging out the info?  If not we can go ahead
> but ideally answering things like this make a our life easier in the long run.
> 
> Jonathan
> 

"(Avdd1/Avss)/5+ as positive input and (Avdd/Avss)/5- as negative
  this is used for monitoring power supplies, the inputs must be selected in pair"
Perhaps it's an internal voltage divider...? I dunno

So it seems like this cannot be used as a common mode voltage input.
I'll restrict the driver to only allow these inputs paired together
and rename the define for these selections.