linux-kernel - Re: [PATCH v3 1/6] dt-bindings: adc: ad7173: add support for ad411x

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Message-ID: <e0be3356bf809035963c4801f05b9db2675c111e.camel@gmail.com>
Date: Thu, 30 May 2024 09:50:51 +0200
From: Nuno Sá <noname.nuno@...il.com>
To: Conor Dooley <conor@...nel.org>, "Ceclan, Dumitru"
 <mitrutzceclan@...il.com>
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>,
 David Lechner <dlechner@...libre.com>, 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 +0100, Conor Dooley wrote:
> On Wed, May 29, 2024 at 04:38:53PM +0300, 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>
> > > > > > +      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.
> 
> What I was suggesting was more like "adi,pseudo-differential" (you don't
> need to set the =false or w/e, flag properties work based on present/not
> present). I think that would avoid the awkwardness?
> 

Yeah, that was also my understanding of your reply... But I think you're also
mentioning to have this flag together with the single-channel property? 

I'm a bit confused because it seems to me that single-channel only gets one input
while we need to select two for pseudo-differential/single-ended. Is this correct
Dumitru?

FWIW, I think we already have that awkwardness in the current form. If I'm not
missing anything, what we have in the driver is pretty much:

if (not diff && single-channel)
	// then current channel
else
	// relies on the channel-type stuff

So, effectively with the above we have

diff-channels = <1 0>;

but then wait, not so fast

adi,channel-type = "single-ended"

To me the above is equally awkward (not sure if there's any precedence in using diff-
channels like this though)...

I would like for this to be explicit... If we have diff-channels, then it's surely
differential.

If not we could use the single-channel property and instead of using an extra flag we
could make the property having either 1 or 2 items. If we have 1, then it's a current
channel. If we have 2, then it's voltage single-ended/pseudo-differential. 

David's suggestion is also pretty good (and I like it's more explicit about what's
going on) so I would likely go with it...

- Nuno Sá


> > > >