linux-kernel - Re: [PATCH v1 08/16] arm64: dts: mt8195: Add power domains controller

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Date:   Wed, 6 Jul 2022 16:35:17 +0200
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     Matthias Brugger <matthias.bgg@...il.com>,
        Tinghan Shen <tinghan.shen@...iatek.com>,
        Yong Wu <yong.wu@...iatek.com>, Joerg Roedel <joro@...tes.org>,
        Will Deacon <will@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Chun-Jie Chen <chun-jie.chen@...iatek.com>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>,
        Enric Balletbo i Serra <enric.balletbo@...labora.com>,
        Weiyi Lu <weiyi.lu@...iatek.com>
Cc:     iommu@...ts.linux-foundation.org,
        linux-mediatek@...ts.infradead.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Project_Global_Chrome_Upstream_Group@...iatek.com
Subject: Re: [PATCH v1 08/16] arm64: dts: mt8195: Add power domains controller

On 06/07/2022 15:41, Matthias Brugger wrote:
> 
> 
> On 04/07/2022 14:38, Krzysztof Kozlowski wrote:
>> On 04/07/2022 12:00, Tinghan Shen wrote:
>>> Add power domains controller node for mt8195.
>>>
>>> Signed-off-by: Weiyi Lu <weiyi.lu@...iatek.com>
>>> Signed-off-by: Tinghan Shen <tinghan.shen@...iatek.com>
>>> ---
>>>   arch/arm64/boot/dts/mediatek/mt8195.dtsi | 327 +++++++++++++++++++++++
>>>   1 file changed, 327 insertions(+)
>>>
>>> diff --git a/arch/arm64/boot/dts/mediatek/mt8195.dtsi b/arch/arm64/boot/dts/mediatek/mt8195.dtsi
>>> index 8d59a7da3271..d52e140d9271 100644
>>> --- a/arch/arm64/boot/dts/mediatek/mt8195.dtsi
>>> +++ b/arch/arm64/boot/dts/mediatek/mt8195.dtsi
>>> @@ -10,6 +10,7 @@
>>>   #include <dt-bindings/interrupt-controller/irq.h>
>>>   #include <dt-bindings/phy/phy.h>
>>>   #include <dt-bindings/pinctrl/mt8195-pinfunc.h>
>>> +#include <dt-bindings/power/mt8195-power.h>
>>>   
>>>   / {
>>>   	compatible = "mediatek,mt8195";
>>> @@ -338,6 +339,332 @@
>>>   			#interrupt-cells = <2>;
>>>   		};
>>>   
>>> +		scpsys: syscon@...06000 {
>>> +			compatible = "syscon", "simple-mfd";
>>
>> These compatibles cannot be alone.
>>
> 
> You mean we would need something like "mediatek,scpsys" as dummy compatible 
> that's not bound to any driver?

Yes. syscon (and simple-mfd) must always come with a specific compatible.

> 
>>> +			reg = <0 0x10006000 0 0x1000>;
>>> +			#power-domain-cells = <1>;
>>
>> If it is simple MFD, then probably it is not a power domain provider.
>> Decide.
> 
> The SCPSYS IP block of MediaTek SoCs group several functionality, one is the 
> power domain controller. Others are not yet implemented, but defining the scpsys 
> as a MFD will give us the possibility to do so in the future.

No, quite the opposite. Having simple-mfd prevents you from implementing
it correctly later as a driver, because you cannot remove it. It would
be ABI break.

It's fine to have one block being a simple MFD having several children,
but then it's not a power controller. Children could be such power
controller, but not simple-mfd. Rob explained this several times:
https://lore.kernel.org/all/YXhINE00HG6hbQI4@robh.at.kernel.org/
https://lore.kernel.org/all/20220701000959.GA3588170-robh@kernel.org/


Best regards,
Krzysztof