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

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Message-ID: <8e228d2d-d22f-4092-8c6d-94ce989b4a84@baylibre.com>
Date: Sun, 17 Aug 2025 11:51:22 -0500
From: David Lechner <dlechner@...libre.com>
To: 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 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 have VSENSE wired on my board so I can work on those interrupts and
> register support in a later patch series.
>