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 14:35, Laurent Pinchart wrote:
> On Tue, Sep 02, 2025 at 02:26:53PM +0200, Krzysztof Kozlowski wrote:
>> 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.
> 
> That's only the case if someone enables the clock, isn't it ? From a DT
> binding point of view, how can we know that the extra clock will be

We talk about software using the binding in this particular case. Can
the software use fallback? Yes, it can.

> enabled by a component separate from the driver (in this case by the
> fact that the devm_bulk_clk_get_all() function gets all clocks) ?

If you go that way, only 100% identical devices are compatible.

> 
>> 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
> 
> Quoting that slide, you wrote
> 
> "Two devices are compatible when the new device works with Linux drivers
> bound via fallback (old) compatible".
> 
> That is clearly the case here for the existing *Linux* driver. But what
> if the driver called devm_bulkd_clk_get() with a device-specific list of
> clocks ? Or what if the same DT bindings are used on an OS that has no
> clk_get_all() equivalent ? This is my concern with declaring those two
> devices as compatible: they may be from the point of view of the current
> implementation of the corresponding Linux kernel driver, but DT bindings
> are not Linux-specific.

It seems you think of compatibility as new device is compatible with old
kernel, e.g. one not requesting that clock. We don't talk about such case.

> 
> Or do DT bindings assume that drivers have to always enable all clocks
> declared in DT, even if they don't know what those clocks are ? That
> seems error-prone, in quite a few cases drivers need to handle separate
> clocks in a device-specific way, with for instance a particular
> ordering, preventing them from using devm_bulk_clk_get_all(). If all
> drivers are required to manage all clocks declared in DT, this would get
> messy quite quickly.
> 
I don't really want to dive into such specifics, because it is
impossible to create a generic rule of out. We decide here about
programming interface mostly. Can Linux use the one from fallback-device
to properly operate the new one? Can the same driver bind to fallback
and operate the new device?

If you enable clock by clock for whatever reason, e.g. very specific
programming power up sequence, then answer would be: no, Linux cannot
use fallback because handling clocks differ.

Best regards,
Krzysztof