Date: Thu, 30 May 2024 09:12:22 +0200
From: Nuno Sá <noname.nuno@...il.com>
To: David Lechner <dlechner@...libre.com>, "Ceclan, Dumitru"
	 <mitrutzceclan@...il.com>, Conor Dooley <conor@...nel.org>
Cc: dumitru.ceclan@...log.com, Lars-Peter Clausen <lars@...afoo.de>, Michael
 Hennerich <Michael.Hennerich@...log.com>, Jonathan Cameron
 <jic23@...nel.org>, Rob Herring <robh@...nel.org>, Krzysztof Kozlowski
 <krzysztof.kozlowski+dt@...aro.org>,  Conor Dooley <conor+dt@...nel.org>,
 linux-iio@...r.kernel.org, devicetree@...r.kernel.org, 
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/6] dt-bindings: adc: ad7173: add support for ad411x

On Wed, 2024-05-29 at 17:04 -0500, David Lechner wrote:
> On 5/29/24 8:38 AM, Ceclan, Dumitru wrote:
> > On 28/05/2024 20:52, Conor Dooley wrote:
> > > On Tue, May 28, 2024 at 03:16:07PM +0300, Ceclan, Dumitru wrote:
> > > > On 27/05/2024 20:48, Conor Dooley wrote:
> > > > > On Mon, May 27, 2024 at 08:02:34PM +0300, Dumitru Ceclan via B4 Relay
> > > > > wrote:
> > > > > > From: Dumitru Ceclan <dumitru.ceclan@...log.com>
> > > > > > 
> > > > > > Add support for: AD4111, AD4112, AD4114, AD4115, AD4116.
> > > > > > 
> > > > > > AD411x family ADCs support a VCOM pin, dedicated for single-ended usage.
> > > > > > AD4111/AD4112 support current channels, usage is implemented by
> > > > > >  specifying channel reg values bigger than 15.
> > > > > > 
> > > > > > Signed-off-by: Dumitru Ceclan <dumitru.ceclan@...log.com>
> > > > > > ---
> > > > > >  .../devicetree/bindings/iio/adc/adi,ad7173.yaml    | 122
> > > > > > ++++++++++++++++++++-
> > > > > >  1 file changed, 120 insertions(+), 2 deletions(-)
> > > > > > 
> > > > > > diff --git a/Documentation/devicetree/bindings/iio/adc/adi,ad7173.yaml
> > > > > > b/Documentation/devicetree/bindings/iio/adc/adi,ad7173.yaml
> > > > > > index ea6cfcd0aff4..5b1af382dad3 100644
> > > > > > --- a/Documentation/devicetree/bindings/iio/adc/adi,ad7173.yaml
> > > > > > +++ b/Documentation/devicetree/bindings/iio/adc/adi,ad7173.yaml
> > > > > > @@ -19,7 +19,18 @@ description: |
> > > > > >    primarily for measurement of signals close to DC but also delivers
> > > > > >    outstanding performance with input bandwidths out to ~10kHz.
> > > > > >  
> > > > > > +  Analog Devices AD411x ADC's:
> > > > > > +  The AD411X family encompasses a series of low power, low noise, 24-
> > > > > > bit,
> > > > > > +  sigma-delta analog-to-digital converters that offer a versatile range
> > > > > > of
> > > > > > +  specifications. They integrate an analog front end suitable for
> > > > > > processing
> > > > > > +  fully differential/single-ended and bipolar voltage inputs.
> > > > > > +
> > > > > >    Datasheets for supported chips:
> > > > > > +   
> > > > > > https://www.analog.com/media/en/technical-documentation/data-sheets/AD4111.pdf
> > > > > > +   
> > > > > > https://www.analog.com/media/en/technical-documentation/data-sheets/AD4112.pdf
> > > > > > +   
> > > > > > https://www.analog.com/media/en/technical-documentation/data-sheets/AD4114.pdf
> > > > > > +   
> > > > > > https://www.analog.com/media/en/technical-documentation/data-sheets/AD4115.pdf
> > > > > > +   
> > > > > > https://www.analog.com/media/en/technical-documentation/data-sheets/AD4116.pdf
> > > > > >     
> > > > > > https://www.analog.com/media/en/technical-documentation/data-sheets/AD7172-2.pdf
> > > > > >     
> > > > > > https://www.analog.com/media/en/technical-documentation/data-sheets/AD7172-4.pdf
> > > > > >     
> > > > > > https://www.analog.com/media/en/technical-documentation/data-sheets/AD7173-8.pdf
> > > > > > @@ -31,6 +42,11 @@ description: |
> > > > > >  properties:
> > > > > >    compatible:
> > > > > >      enum:
> > > > > > +      - adi,ad4111
> > > > > > +      - adi,ad4112
> > > > > > +      - adi,ad4114
> > > > > > +      - adi,ad4115
> > > > > > +      - adi,ad4116
> > > > > >        - adi,ad7172-2
> > > > > >        - adi,ad7172-4
> > > > > >        - adi,ad7173-8
> > > > > > @@ -129,10 +145,36 @@ patternProperties:
> > > > > >          maximum: 15
> > > > > >  
> > > > > >        diff-channels:
> > > > > > +        description: |
> > > > > > +          For using current channels specify select the current inputs
> > > > > > +           and enable the adi,current-channel property.
> > > > > > +
> > > > > > +          Family AD411x supports a dedicated VINCOM voltage input.
> > > > > > +          To select it set the second channel to 16.
> > > > > > +            (VIN2, VINCOM) -> diff-channels = <2 16>
> > > > > > +
> > > > > > +          There are special values that can be selected besides the
> > > > > > voltage
> > > > > > +          analog inputs:
> > > > > > +            21: REF+
> > > > > > +            22: REF−
> > > > > > +          Supported only by AD7172-2, AD7172-4, AD7175-2, AD7175-8,
> > > > > > AD7177-2:
> > > > > > +            19: ((AVDD1 − AVSS)/5)+
> > > > > > +            20: ((AVDD1 − AVSS)/5)−
> > > > > > +
> > > > > >          items:
> > > > > >            minimum: 0
> > > > > >            maximum: 31
> > > > > >  
> > > > > > +      single-channel:
> > > > > > +        description: |
> > > > > > +          Models AD4111 and AD4112 support single-ended current
> > > > > > channels.
> > > > > > +          To select the desired current input, specify the desired input
> > > > > > pair:
> > > > > > +            (IIN2+, IIN2−) -> single-channel = <2>
> > > > > > +
> > > > > > +        items:
> > > > > > +          minimum: 1
> > > > > > +          maximum: 16
> > > > > > +
> > > > > >        adi,reference-select:
> > > > > >          description: |
> > > > > >            Select the reference source to use when converting on
> > > > > > @@ -154,9 +196,26 @@ patternProperties:
> > > > > >            - avdd
> > > > > >          default: refout-avss
> > > > > >  
> > > > > > +      adi,current-channel:
> > > > > > +        description: |
> > > > > > +          Signal that the selected inputs are current channels.
> > > > > > +          Only available on AD4111 and AD4112.
> > > > > > +        type: boolean
> > > > > > +
> > > > > > +      adi,channel-type:
> > > > > > +        description:
> > > > > > +          Used to differentiate between different channel types as the
> > > > > > device
> > > > > > +           register configurations are the same for all usage types.
> > > > > > +          Both pseudo-differential and single-ended channels will use
> > > > > > the
> > > > > > +           single-ended specifier.
> > > > > > +        $ref: /schemas/types.yaml#/definitions/string
> > > > > > +        enum:
> > > > > > +          - single-ended
> > > > > > +          - differential
> > > > > > +        default: differential
> > > > > 
> > > > > I dunno if my brain just ain't workin' right today, or if this is not
> > > > > sufficiently explained, but why is this property needed? You've got
> > > > > diff-channels and single-channels already, why can you not infer the
> > > > > information you need from them? What should software do with this
> > > > > information?
> > > > > Additionally, "pseudo-differential" is not explained in this binding.
> > > > 
> > > > In previous thread we arrived to the conclusion single-ended and
> > > > pseudo-differential channels should be marked with the flag
> > > > "differential=false" in the IIO channel struct. This cannot
> > > > really be inferred as any input pair could be used in that
> > > > manner and the only difference would be in external wiring.
> > > > 
> > > > Single-channels cannot be used to define such a channel as
> > > > two voltage inputs need to be selected. Also, we are already
> > > > using single-channel to define the current channels.
> > > 
> > > If I understand correctly, the property could be simplified to a flag
> > > then, since it's only the pseudo differential mode that you cannot be
> > > sure of?
> > > You know when you're single-ended based on single-channel, so the
> > > additional info you need is only in the pseudo-differential case.
> > > 
> > Yes, it could just be a boolean flag. The only thing I have against
> > that is the awkwardness of having both diff-channels and
> > differential=false within a channel definition.
> > 
> > No, there is no uncertainty regarding pseudo-differential, it's
> > basically single-ended.
> > 
> > We cannot use single-channel for voltage channels, two voltage
> > inputs need to be specified. And again, single-channel will be
> > used here for the current channels. 
> 
> Instead of using diff-channels for single-ended/pseudo-differential
> plus a property that says "actually not differential" could we just
> add a second common-mode-channel property to specify the second
> input pin that is connected to the common mode voltage source?
> 

Yeah, I definitely don't like having diff-channels and then go "oh, not really a
differential channel". So, if could avoid that, it would be great!

> Just to make sure I'm understanding, single-ended means common mode
> voltage is 0V (or AVSS for this chip, I guess) and pseudo-differential
> means the common mode voltage is something else other than that.
> In other words, single-ended is just a special case of pseudo-differential.
> So effectively, no difference that we need to describe?
> 
> So something like this could work?
> 
> 
>         /* a fully differential voltage input channel */
>         channel@0 {
>           reg = <0>;
>           bipolar;
>           diff-channels = <0 1>; /* VIN0 is +, VIN1 is - */
>           adi,reference-select = "vref";
>         };
> 
>         /* a single-ended voltage input channel */
>         channel@1 {
>           reg = <1>;
>           /* no bipolar since common mode is 0V */
>           single-channel = <2>; /* VIN2 is input */
>           common-mode-channel = <3>; /* VIN3 connected to 0V */
>         };
> 
>         /* a pseudo-differential voltage input channel */
>         channel@2 {
>           reg = <2>;
>           bipolar; /* since common mode is not 0V */
>           single-channel = <4>; /* VIN4 is input */
>           common-mode-channel = <5>; /* VIN5 connected to Vref / 2 */
>           adi,reference-select = "vref";
>         };
> 
> 	/* a current input channel */
>         channel@3 {
>           reg = <3>;
>           bipolar;
>           /* 0 is not the same pin as channel@0 because of
>            * the adi,current-channel flag */
>           single-channel = <0>; /* using IIN0+ and IIN0- pins */
>           adi,current-channel;

Unless I'm missing something, this flag is also not being used anywhere in the
current series?

- Nuno Sá

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