Re: [PATCH v5 1/4] media: dt-bindings: nxp,imx8mq-mipi-csi2: Add i.MX8ULP compatible string

[Krzysztof Kozlowski <krzk@kernel.org>]

On 02/09/2025 10:35, Laurent Pinchart wrote:
>>>>          compatible:
>>>>            contains:
>>>>              enum:
>>>> -              - fsl,imx8qxp-mipi-csi2
>>>> +              - fsl,imx8ulp-mipi-csi2
>>>> +    then:
>>>> +      properties:
>>>> +        reg:
>>>> +          minItems: 2
>>>> +        resets:
>>>> +          minItems: 2
>>>> +          maxItems: 2
>>>> +        clocks:
>>>> +          minItems: 4
>>>> +        clock-names:
>>>> +          minItems: 4
>>>
>>> But according to this, the ULP version requires more clocks than the QXP
>>> version.
>>
>> If only clock number difference, generally, it is still compatible and can
>> be fallback, especialy driver use devm_bulk_clk_get_all().
> 
> That's a driver-specific implementation decision, so I don't think it
> should be taken into account to decide on compatibility.

The clock inputs do not restrict compatibility. If Linux can use
fallback to bind and operate properly, then it's a strong indication
devices are compatible.

Imagine exactly the same registers, so same programming interface, but
one device takes one more clock which just needs to be enabled through
its lifetime. Such devices are fully compatible, even though clock
inputs differ.

I also wanted to express exactly that case on my slides from OSSE -
slide 28:
https://osseu2025.sched.com/event/25Vsl/dts-101-from-roots-to-trees-aka-devicetree-for-beginners-krzysztof-kozlowski-linaro

(although I focused on reversed case when devices are not compatible,
because that is decisive case).

Best regards,
Krzysztof