Re: [PATCH v6 10/14] arm64: dts: freescale: imx8qxp: Remove unnecessary clock related entries

[Krzysztof Kozlowski <krzysztof.kozlowski@xxxxxxxxxx>]

On 30/06/2022 10:36, Viorel Suman wrote:
> On 22-06-29 20:04:43, Krzysztof Kozlowski wrote:
>> On 29/06/2022 18:44, Viorel Suman (OSS) wrote:
>>> From: Viorel Suman <viorel.suman@xxxxxxx>
>>>
>>> "clocks" and "clock-names" are not used the driver, so
>>> remove them in order to match the yaml definition.
>>
>> So this explains the unexpected change in the bindings... but actually
>> it does not explain whether it is correct or not. Just because driver
>> does not use it, is not a proof that clocks are not there. In different
>> OS/implementation this DTS might break stuff, so basically it is ABI
>> break. DTS should describe the hardware fully, so if the clocks are
>> there, should be in DTS regardless of the driver.
> 
> Hi Krzysztof,
> 
> Both XTAL clocks - 24MHz and 32kHz - are still defined in DTSI files, see for instance in
> arch/arm64/boot/dts/freescale/imx8qxp.dtsi :
> ---------------
>         xtal32k: clock-xtal32k {
>                 compatible = "fixed-clock";
>                 #clock-cells = <0>;
>                 clock-frequency = <32768>;
>                 clock-output-names = "xtal_32KHz";
>         };
> 
>         xtal24m: clock-xtal24m {
>                 compatible = "fixed-clock";
>                 #clock-cells = <0>;
>                 clock-frequency = <24000000>;
>                 clock-output-names = "xtal_24MHz";
>         };
> ---------------
> Both can be seen in /sys/kernel/debug/clk/clk_summary once boot is complete, both can be referenced
> in any DTS node, so there is no ABI break.

ABI break is not relevant to the fixed clocks being or not being defined
in the DTS. You have a device which was taking the clock inputs, so the
clocks stayed enabled.

Now, you don't take these inputs, so for example the clocks are getting
disabled as not used.

> 
> "DTS should describe the hardware fully" - this is true in case the OS is supposed to controll the
> hardware fully. i.MX8 System Controller Unit concept implies resources being allocated and managed
> by SCU, there is no direct OS access to some hardware. SCU actually defines the hardware environment
> the OS is being able to see and run within. SCU is able to define several such isolated hardware
> environments, each having its own OS running. So, in this particular case - i.MX8 SCU concept -
> DTS should describe the hardware from the perspective of the hardware environment exposed by SCU to
> OS.

OK, that sounds good, but the question about these clocks remain - are
they inputs to the SCU or not.

Regardless whether they are actual input or not, you used not
appropriate argument here - that Linux OS implementation does not use
them. The proper argument is - whether the hardware environment has them
connected or not.

Best regards,
Krzysztof