linux-kernel - Re: [PATCH 1/5] dt-bindings: iio: mcp9600: Add compatible for microchip,mcp9601

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Message-ID: <2025081713-wooden-clam-aee35a@boujee-and-buff>
Date: Sun, 17 Aug 2025 13:34:14 -0400
From: Ben Collins <ben.collins@...ux.dev>
To: David Lechner <dlechner@...libre.com>
Cc: Jonathan Cameron <jic23@...nel.org>, Ben Collins <bcollins@...ter.com>, 
	Nuno Sá <nuno.sa@...log.com>, Andy Shevchenko <andy@...nel.org>, Rob Herring <robh@...nel.org>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, 
	Andrew Hepp <andrew.hepp@...pp.dev>, linux-iio@...r.kernel.org, devicetree@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] dt-bindings: iio: mcp9600: Add compatible for
 microchip,mcp9601

On Sun, Aug 17, 2025 at 11:51:22AM -0500, David Lechner wrote:
> On 8/17/25 11:37 AM, Ben Collins wrote:
> > On Sat, Aug 16, 2025 at 01:55:31PM -0500, David Lechner wrote:
> >> On 8/16/25 4:58 AM, Jonathan Cameron wrote:
> >>> On Fri, 15 Aug 2025 16:46:03 +0000
> >>> Ben Collins <bcollins@...ter.com> wrote:
> >>>
> >>>> The mcp9600 driver supports the mcp9601 chip, but complains about not
> >>>> recognizing the device id on probe. A separate patch...
> >>>>
> >>>> 	iio: mcp9600: Recognize chip id for mcp9601
> >>>>
> >>>> ...addresses this. This patch updates the dt-bindings for this chip to
> >>>> reflect the change to allow explicitly setting microchip,mcp9601 as
> >>>> the expected chip type.
> >>>>
> >>>> The mcp9601 also supports features not found on the mcp9600, so this
> >>>> will also allow the driver to differentiate the support of these
> >>>> features.
> >>>
> >>> If it's additional features only then you can still use a fallback
> >>> compatible.  Intent being that a new DT vs old kernel still 'works'.
> >>>
> >>> Then for the driver on new kernels we match on the new compatible and
> >>> support those new features.  Old kernel users get to keep the ID
> >>> mismatch warning - they can upgrade if they want that to go away ;)
> >>>
> >>> Krzysztof raised the same point on v2 but I'm not seeing it addressed
> >>> in that discussion.
> >>
> >> One could make the argument that these are not entirely fallback
> >> compatible since bit 4 of the STATUS register has a different
> >> meaning depending on if the chip is MCP9601/L01/RL01 or not.
> > 
> > There are some nuances to this register between the two, but it can be
> > used generically as "not in range" for both.
> > 
> > My understanding from the docs is if VSENSE is connected on mcp9601,
> > then it is explicitly open-circuit detection vs. short-circuit, which
> > is bit 5.
> > 
> >> Interestingly, the existing bindings include interrupts for
> >> open circuit and short circuit alert pins. But these pins
> >> also only exist on MCP9601/L01/RL01. If we decide these aren't
> >> fallback compatible, then those properties should have the
> >> proper constraints added as well.
> > 
> > In my v4 patch, I'm going to remove the short/open circuit interrupts
> > since they are not implemented, yet.
> 
> Don't remove them from the devicetree bindings. Even if the Linux driver
> doesn't use it, the bindings should be as complete as possible.
> 
> https://docs.kernel.org/devicetree/bindings/writing-bindings.html
> 

I couldn't find anything that would easily describe this type of layout:

properties:
...
  interrupts:
    minItems: 1
    maxItems: 4
  interrupt-names:
    minItems: 1
    items:
      - const: alert1
      - const: alert2
      - const: alert3
      - const: alert4

allOf:
  - if:
      properties:
        compatible:
          contains:
            const: microchip,mcp9601
    then:
      # Override maxItems
      interrupts:
        maxItems: 6
      # XXX Add items to existing list???
      interrupt-names:
        items:
          - const: open-circuit
          - const: short-circuit

-- 
 Ben Collins
 https://libjwt.io
 https://github.com/benmcollins
 --
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