Re: [PATCH 2/4] dt-bindings: arm: mediatek: Add new bindings of MediaTek frequency hopping

[Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>]

On 01/09/2022 11:04, AngeloGioacchino Del Regno wrote:
> Il 31/08/22 15:19, Krzysztof Kozlowski ha scritto:
>> On 31/08/2022 15:48, Johnson Wang wrote:
>>> Add the new binding documentation for MediaTek frequency hopping
>>> and spread spectrum clocking control.
>>>
>>> Co-developed-by: Edward-JW Yang <edward-jw.yang@mediatek.com>
>>> Signed-off-by: Edward-JW Yang <edward-jw.yang@mediatek.com>
>>> Signed-off-by: Johnson Wang <johnson.wang@mediatek.com>
>>> ---
>>>   .../bindings/arm/mediatek/mediatek,fhctl.yaml | 49 +++++++++++++++++++
>>>   1 file changed, 49 insertions(+)
>>>   create mode 100644 Documentation/devicetree/bindings/arm/mediatek/mediatek,fhctl.yaml
>>>
>>> diff --git a/Documentation/devicetree/bindings/arm/mediatek/mediatek,fhctl.yaml b/Documentation/devicetree/bindings/arm/mediatek/mediatek,fhctl.yaml
>>> new file mode 100644
>>> index 000000000000..c5d76410538b
>>> --- /dev/null
>>> +++ b/Documentation/devicetree/bindings/arm/mediatek/mediatek,fhctl.yaml
>>> @@ -0,0 +1,49 @@
>>> +# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
>>> +%YAML 1.2
>>> +---
>>> +$id: http://devicetree.org/schemas/arm/mediatek/mediatek,fhctl.yaml#
>>> +$schema: http://devicetree.org/meta-schemas/core.yaml#
>>> +
>>> +title: MediaTek frequency hopping and spread spectrum clocking control
>>> +
>>> +maintainers:
>>> +  - Edward-JW Yang <edward-jw.yang@mediatek.com>
>>> +
>>> +description: |
>>> +  Frequency hopping control (FHCTL) is a piece of hardware that control
>>> +  some PLLs to adopt "hopping" mechanism to adjust their frequency.
>>> +  Spread spectrum clocking (SSC) is another function provided by this hardware.
>>> +
>>> +properties:
>>> +  compatible:
>>> +    const: mediatek,fhctl
>>
>> You need SoC/device specific compatibles. Preferably only SoC specific,
>> without generic fallback, unless you can guarantee (while representing
>> MediaTek), that generic fallback will cover all of their SoCs?
>>
>>> +
>>> +  reg:
>>> +    maxItems: 1
>>> +
>>> +  mediatek,hopping-ssc-percents:
>>> +    description: |
>>> +      Determine the enablement of frequency hopping feature and the percentage
>>> +      of spread spectrum clocking for PLLs.
>>> +    $ref: /schemas/types.yaml#/definitions/uint32-matrix
>>> +    items:
>>> +      items:
>>> +        - description: PLL id that is expected to enable frequency hopping.
>>
>> So the clocks are indices from some specific, yet unnamed
>> clock-controller? This feels hacky. You should rather take here clock
>> phandles (1) or integrate it into specific clock controller (2). The
>> reason is that either your device does something on top of existing
>> clocks (option 1, thus it takes clock as inputs) or it modifies existing
>> clocks (option 2, thus it is integral part of clock-controller).
>>
> 
> FHCTL is a MCU that handles (some, or all, depending on what's supported on the
> SoC and what's needed by the board) PLL frequency setting, doing it in steps and
> avoiding overshooting and other issues.
> 
> We had a pretty big conversation about this a while ago and the indices instead
> of phandles is actually my fault, that happened because I initially proposed your
> option 2 but then for a number of reasons we went with this kind of solution.
> 
> I know it's going to be a long read, but the entire conversation is on the list [1]
> 

Sorry, but it's a hacky architecture where one device (which is a clock
provider) and second device have no relationship in hardware description
but both play with each other resources. That's simply not a proper
hardware description, so again:

1. If this is separate device (as you indicated), then it needs
expressing the dependencies and uses of other device resources.

2. If this is not a separate device, but integral part of clock
controller, then it would be fine but then probably should be child of
that device.

Best regards,
Krzysztof