Date:   Tue, 12 Jul 2022 14:58:36 +0200
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.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>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Chun-Jie Chen <chun-jie.chen@...iatek.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 12/07/2022 14:54, AngeloGioacchino Del Regno wrote:
> Il 12/07/22 14:47, Krzysztof Kozlowski ha scritto:
>> On 12/07/2022 12:33, AngeloGioacchino Del Regno wrote:
>>> Il 12/07/22 11:03, Krzysztof Kozlowski ha scritto:
>>>> On 12/07/2022 10:53, AngeloGioacchino Del Regno wrote:
>>>>> Il 12/07/22 10:37, Krzysztof Kozlowski ha scritto:
>>>>>> On 12/07/2022 10:17, AngeloGioacchino Del Regno wrote:
>>>>>>> Il 06/07/22 17:18, Krzysztof Kozlowski ha scritto:
>>>>>>>> On 06/07/2022 14:00, Tinghan Shen wrote:
>>>>>>>>> Hi Krzysztof,
>>>>>>>>>
>>>>>>>>> After discussing your message with our power team,
>>>>>>>>> we realized that we need your help to ensure we fully understand you.
>>>>>>>>>
>>>>>>>>> On Mon, 2022-07-04 at 14:38 +0200, 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.
>>>>>>>>>
>>>>>>>>> the scpsys sub node has the compatible of the power domain driver.
>>>>>>>>> do you suggest that the compatible in the sub node should move to here?
>>>>>>>>
>>>>>>>> Not necessarily, depends. You have here device node representing system
>>>>>>>> registers. They need they own compatibles, just like everywhere in the
>>>>>>>> kernel (except the broken cases...).
>>>>>>>>
>>>>>>>> Whether this should be compatible of power-domain driver, it depends
>>>>>>>> what this device node is. I don't know, I don't have your datasheets or
>>>>>>>> your architecture diagrams...
>>>>>>>>
>>>>>>>>>
>>>>>>>>>>> +			reg = <0 0x10006000 0 0x1000>;
>>>>>>>>>>> +			#power-domain-cells = <1>;
>>>>>>>>>>
>>>>>>>>>> If it is simple MFD, then probably it is not a power domain provider.
>>>>>>>>>> Decide.
>>>>>>>>>
>>>>>>>>> this MFD device is the power controller on mt8195.
>>>>>>>>
>>>>>>>> Then it is not a simple MFD but a power controller. Do not use
>>>>>>>> "simple-mfd" compatible.
>>>>>>>>
>>>>>>>>> Some features need
>>>>>>>>> to do some operations on registers in this node. We think that implement
>>>>>>>>> the operation of these registers as the MFD device can provide flexibility
>>>>>>>>> for future use. We want to clarify if you're saying that an MFD device
>>>>>>>>> cannot be a power domain provider.
>>>>>>>>
>>>>>>>> MFD device is Linuxism, so it has nothing to do here. I am talking only
>>>>>>>> about simple-mfd. simple-mfd is a simple device only instantiating
>>>>>>>> children and not providing anything to anyone. Neither to children. This
>>>>>>>>      the most important part. The children do not depend on anything from
>>>>>>>> simple-mfd device. For example simple-mfd device can be shut down
>>>>>>>> (gated) and children should still operate. Being a power domain
>>>>>>>> controller, contradicts this usually.
>>>>>>>>
>>>>>>>
>>>>>>> If my interpretation of this issue is right, I have pushed a solution for it.
>>>>>>> Krzysztof, Matthias, can you please check [1] and give feedback, so that
>>>>>>> Tinghan can rewrite this commit ASAP?
>>>>>>>
>>>>>>> Reason is - I need the MT8195 devicetree to be complete to push the remaining
>>>>>>> pieces for Tomato Chromebooks, of course.
>>>>>>>
>>>>>>> [1]: https://patchwork.kernel.org/project/linux-mediatek/list/?series=658527
>>>>>>
>>>>>> I have two or three similar discussions, so maybe I lost the context,
>>>>>> but I don't understand how your fix is matching real hardware.
>>>>>>
>>>>>> In the patchset here, Tinghan claimed that power domain controller is a
>>>>>> child of 10006000. 10006000 is also a power domain controller. This was
>>>>>> explicitly described by the DTS code.
>>>>>>
>>>>>> Now you abandon this hierarchy in favor of syscon. If the hierarchy was
>>>>>> correct, your patchset does not match the hardware, so it's a no-go.
>>>>>> Describe the hardware.
>>>>>>
>>>>>> However maybe this patch did not make any sense and there is no
>>>>>> relationship parent-child... so what do you guys send here? Bunch of
>>>>>> hacks and work-arounds?
>>>>>>
>>>>>
>>>>> For how I get it, hardware side, the SPM (System Power Manager) resides inside
>>>>> of the SCPSYS block (consequently, in that iospace).
>>>>>
>>>>> As Matthias pointed out earlier, SCPSYS provides more functionality, but the
>>>>> only one that's currently implemented upstream is the System Power Manager,
>>>>> responsible for managing the MTCMOS (power domains).
>>>>>
>>>>> In any case, now I'm a little confused on how to proceed, can you please give
>>>>> some suggestion?
>>>>
>>>> You should make SCPSYS (0x10006000, AFAIU) a proper driver with its own
>>>> compatible (followed by syscon if needed), even if now it is almost
>>>> empty stub. The driver should populate OF children and then you can
>>>> embed SPM inside and reference to parent's regmap. No need for
>>>> simple-mfd. Later the SCPSYS can grow, if you ever need it.
>>>>
>>>>
>>>
>>> I see an issue with such approach: the SCPSYS doesn't have a mailbox, doesn't
>>> need power management from the Linux side, doesn't have any register to check
>>> HW revision, it's always online (hence it doesn't need Linux to boot it), it
>>> doesn't need any root clock, nor regulator, nor mmu context, and there's no
>>> retrievable "boot log" of any sort.
>>
>> No problems, there are other drivers who do not need any resources,
>> except address space.
>>
>>>
>>> Hence, a driver with its own compatible would be an empty stub forever: it's
>>> not going to get any "scpsys root handling" at all, because there's none to do.
>>
>> But it is a power domain provider, so you need to embed it in some
>> dirver, don't you?
>>
>>
>>> Digging through some downstream kernels, the only other functionality that I
>>> can find in the SCPSYS is a MODULE_RESET (which is used to reset the SCP System)
>>> and a INFRA_IRQ_SET, used to set "wake locks" on the AP and CONNSYS (modem).
>>
>> So why was power domain provider added to SCPSYS? Guys, I don't know
>> your architecture, so I deduct it based on pieces of DTS code I see.
>>
>>>
>>> Both of these may only be used in the SCP mailbox driver (which is *not* SCPSYS)
>>> to perform an ipi_send operation (but currently we simply en/disable the clock
>>> and that's enough), or to perform a reset and firmware reload of the SCP (but
>>> currently we're simply powering off and back on: this may change in the future).
>>>
>>> So, at the end of the day, we would end up having a copy of simple-pm-bus and
>>> nothing else, which doesn't look like being optimal at all.
>>
>> No, because you need that power domain driver, don't you? If you don't
>> need power domain provider/driver, why the heck this is there:
>>
>> +		scpsys: syscon@...06000 {
>> +			compatible = "syscon", "simple-mfd";
>> +			reg = <0 0x10006000 0 0x1000>;
>> +			#power-domain-cells = <1>;
>>                          ^^^^^^^^^^^^^^^^^
>> Entire discussion started from this.
>>
> 
> Is this all a huge misunderstanding? It probably is, at least partially.
> 
> That node shouldn't have any #power-domain-cells, the only PD is the SPM node
> (mediatek,mt8195-power-controller), not the scpsys parent! Ugh...

My comment was quite clear I think:

  > +			#power-domain-cells = <1>;
  If it is simple MFD, then probably it is not a power domain provider.
  Decide.

and you keep telling me that it is a power domain provider and MFD and
something more.

Anyway neither syscon nor simple-mfd can be without specific compatible.

Best regards,
Krzysztof

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