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

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Message-ID: <7c461931-3b04-4354-a892-52f469511c5a@kernel.org>
Date: Tue, 2 Sep 2025 14:26:53 +0200
From: Krzysztof Kozlowski <krzk@...nel.org>
To: Laurent Pinchart <laurent.pinchart@...asonboard.com>,
 Frank Li <Frank.li@....com>
Cc: Guoniu Zhou <guoniu.zhou@....com>, Rui Miguel Silva <rmfrfs@...il.com>,
 Martin Kepplinger <martink@...teo.de>, Purism Kernel Team <kernel@...i.sm>,
 Mauro Carvalho Chehab <mchehab@...nel.org>, Rob Herring <robh@...nel.org>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
 <conor+dt@...nel.org>, Shawn Guo <shawnguo@...nel.org>,
 Sascha Hauer <s.hauer@...gutronix.de>,
 Pengutronix Kernel Team <kernel@...gutronix.de>,
 Fabio Estevam <festevam@...il.com>, Philipp Zabel <p.zabel@...gutronix.de>,
 linux-media@...r.kernel.org, devicetree@...r.kernel.org,
 imx@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 1/4] media: dt-bindings: nxp,imx8mq-mipi-csi2: Add
 i.MX8ULP compatible string

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