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Date: Fri, 5 Apr 2024 09:01:25 +0300
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Subject: Re: [PATCH 1/2] dt-bindings: mfd: Add ROHM BD71879
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To: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>,
 Andreas Kemnade <andreas@kemnade.info>
Cc: lee@kernel.org, robh+dt@kernel.org, krzysztof.kozlowski+dt@linaro.org,
 conor+dt@kernel.org, devicetree@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20240402193515.513713-1-andreas@kemnade.info>
 <20240402193515.513713-2-andreas@kemnade.info>
 <6420ac43-f200-459c-8c38-7cd55c64a155@linaro.org>
 <20240404123010.69454fda@aktux>
 <8f37211a-57ed-48ab-8de8-cd5a0d4c6609@linaro.org>
From: Matti Vaittinen <mazziesaccount@gmail.com>
In-Reply-To: <8f37211a-57ed-48ab-8de8-cd5a0d4c6609@linaro.org>
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On 4/4/24 15:04, Krzysztof Kozlowski wrote:
> On 04/04/2024 12:30, Andreas Kemnade wrote:
>> On Thu, 4 Apr 2024 08:59:54 +0200
>> Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org> wrote:
>>
>>> On 02/04/2024 21:35, Andreas Kemnade wrote:
>>>> As this chip was seen in several devices in the wild, add it.
>>>>
>>>> Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
>>>> Suggested-by: Matti Vaittinen <mazziesaccount@gmail.com>
>>>> ---
>>>>   Documentation/devicetree/bindings/mfd/rohm,bd71828-pmic.yaml | 4 +++-
>>>>   1 file changed, 3 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/Documentation/devicetree/bindings/mfd/rohm,bd71828-pmic.yaml b/Documentation/devicetree/bindings/mfd/rohm,bd71828-pmic.yaml
>>>> index 0b62f854bf6b..e4df09e8961c 100644
>>>> --- a/Documentation/devicetree/bindings/mfd/rohm,bd71828-pmic.yaml
>>>> +++ b/Documentation/devicetree/bindings/mfd/rohm,bd71828-pmic.yaml
>>>> @@ -17,7 +17,9 @@ description: |
>>>>   
>>>>   properties:
>>>>     compatible:
>>>> -    const: rohm,bd71828
>>>> +    enum:
>>>> +      - rohm,bd71828
>>>> +      - rohm,bd71879
>>>
>>> In your second commit you claim they are compatible, so why they are not
>>> marked as such?
>>>
>> so you mean allowing
>>
>> compatible = "rohm,bd71828"
>> and
>> compatible = "rohm,bd71879", "rohm,bd71828"


This makes me slightly nervous. It wouldn't be the first time when I've 
been told "they are similar", and later the reality has turned out to be 
"they are similar, except...". Furthermore, even if these devices seem 
similar to software (which is what the comment in the MFD driver is 
referring to), it does not mean these devices are 100% electrically 
compatible so that they could be used as a "drop-in" replacement to each 
others. I wouldn't guarantee that.

Furthermore, my current understanding is that the BD71828 was a model 
that was used for a limited purposes. So, maybe creating an dt-entry like:
compatible = "rohm,bd71879", "rohm,bd71828"

might not prove to be too useful. (But I'm not 100% certain on this).

> Yes. If there are reasons against, please briefly mention them in commit
> msg.

I would like to understand the rationale for allowing:
compatible = "rohm,bd71879", "rohm,bd71828".

Is the intention to:
1) allow boards which tell the software that "the hardware may be 
bd71828 or bd71879", or
2) to tell a binding reader that these ICs are likely to be usable as 
replacements to each others?
(Or, is there some other rationale beyond these?)

If it's 1), then I see limited sense in doing so, while I expect that 
not so many bd71828 variants will be seen out there - and at least not 
in that many different products. If it's the 2), then I wouldn't say we 
have the facts to do this.
And, as always, if there is 3), 4), ... - I am keen to learn :)

Yours,
	-- Matti

-- 
Matti Vaittinen
Linux kernel developer at ROHM Semiconductors
Oulu Finland

~~ When things go utterly wrong vim users can always type :help! ~~