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

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Date: Wed, 3 Apr 2024 10:22:24 -0500
From: David Lechner <dlechner@...libre.com>
To: "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>, linux-iio@...r.kernel.org, devicetree@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/6] dt-bindings: adc: ad7173: add support for ad411x

On Wed, Apr 3, 2024 at 2:50 AM Ceclan, Dumitru <mitrutzceclan@...ilcom> wrote:
>
> On 02/04/2024 00:16, David Lechner wrote:
> > On Mon, Apr 1, 2024 at 2:37 PM David Lechner <dlechner@...librecom> wrote:
> >>
> >> On Mon, Apr 1, 2024 at 10:10 AM Dumitru Ceclan via B4 Relay
> >> <devnull+dumitru.ceclan.analog.com@...nel.org> wrote:
> >>>
> >>> From: Dumitru Ceclan <dumitru.ceclan@...log.com>
> >>>
>
> ...
>
> >>>
> >>>      properties:
> >>>        reg:
> >>> +        description:
> >>> +          Reg values 16-19 are only permitted for ad4111/ad4112 current channels.
> >>>          minimum: 0
> >>> -        maximum: 15
> >>> +        maximum: 19
> >>
> >> This looks wrong. Isn't reg describing the number of logical channels
> >> (# of channel config registers)?
> >>
> >> After reviewing the driver, I see that > 16 is used as a way of
> >> flagging current inputs, but still seems like the wrong way to do it.
> >> See suggestion below.
> >>
> >>>
> >>>        diff-channels:
> >>> +        description:
> >>> +          For using current channels specify only the positive channel.
> >>> +            (IIN2+, IIN2−) -> diff-channels = <2 0>
> >>
> >> I find this a bit confusing since 2 is already VIN2 and 0 is already
> >> VIN0. I think it would make more sense to assign unique channel
> >> numbers individually to the negative and positive current inputs.
> >> Also, I think it makes sense to use the same numbers that the
> >> registers in the datasheet use (8 - 11 for negative and 12 to 15 for
> >> positive).
> >>
> >> So: (IIN2+, IIN2−) -> diff-channels = <13 10>
> >
> > Thinking about this a bit more...
> >
> > Since the current inputs have dedicated pins and aren't mix-and-match
> > with multiple valid wiring configurations like the voltage inputs, do
> > we even need to describe them in the devicetree?
> >
> > In the driver, the current channels would just be hard-coded like the
> > temperature channel since there isn't any application-specific
> > variation.
>
>  Sure, but we still need to offer the user a way to configure which
> current inputs he wants and if they should use bipolar or unipolar coding.

>From the datasheet, it looks like only positive current input is
allowed so I'm not sure bipolar applies here. But, yes, if there is
some other variation in wiring or electrical signal that needs to be
describe here, then it makes sense to allow a channel configuration
node for it.

>
>  One other issue that arises is the fact that we will reserve 5
> (including temp) channels out of the 15 that the user has the option to
> configure. While the binding will configure only 11 channels for
> example, the driver probe will error out with the message "Too many
> channels specified".
>

Surely the driver could be changed to fix this, if needed. :-)