linux-kernel - Re: [PATCH v2 09/29] dt-bindings: clock: mediatek: Describe MT8196 peripheral clock controllers

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Message-ID: <29eeae4f-59ed-4781-88b1-4fd76714ecb6@kernel.org>
Date: Wed, 25 Jun 2025 13:05:45 +0200
From: Krzysztof Kozlowski <krzk@...nel.org>
To: AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>,
 Laura Nao <laura.nao@...labora.com>, mturquette@...libre.com,
 sboyd@...nel.org, robh@...nel.org, krzk+dt@...nel.org, conor+dt@...nel.org,
 matthias.bgg@...il.com, p.zabel@...gutronix.de, richardcochran@...il.com
Cc: guangjie.song@...iatek.com, wenst@...omium.org,
 linux-clk@...r.kernel.org, devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 linux-mediatek@...ts.infradead.org, netdev@...r.kernel.org,
 kernel@...labora.com
Subject: Re: [PATCH v2 09/29] dt-bindings: clock: mediatek: Describe MT8196
 peripheral clock controllers

On 25/06/2025 11:45, AngeloGioacchino Del Regno wrote:
> Il 25/06/25 10:57, Krzysztof Kozlowski ha scritto:
>> On 25/06/2025 10:20, AngeloGioacchino Del Regno wrote:
>>> Il 24/06/25 18:02, Krzysztof Kozlowski ha scritto:
>>>> On 24/06/2025 16:32, Laura Nao wrote:
>>>>> +  '#reset-cells':
>>>>> +    const: 1
>>>>> +    description:
>>>>> +      Reset lines for PEXTP0/1 and UFS blocks.
>>>>> +
>>>>> +  mediatek,hardware-voter:
>>>>> +    $ref: /schemas/types.yaml#/definitions/phandle
>>>>> +    description:
>>>>> +      On the MT8196 SoC, a Hardware Voter (HWV) backed by a fixed-function
>>>>> +      MCU manages clock and power domain control across the AP and other
>>>>> +      remote processors. By aggregating their votes, it ensures clocks are
>>>>> +      safely enabled/disabled and power domains are active before register
>>>>> +      access.
>>>>
>>>> Resource voting is not via any phandle, but either interconnects or
>>>> required opps for power domain.
>>>
>>> Sorry, I'm not sure who is actually misunderstanding what, here... let me try to
>>> explain the situation:
>>>
>>> This is effectively used as a syscon - as in, the clock controllers need to perform
>>> MMIO R/W on both the clock controller itself *and* has to place a vote to the clock
>>> controller specific HWV register.
>>
>> syscon is not the interface to place a vote for clocks. "clocks"
>> property is.
>>
>>>
>>> This is done for MUX-GATE and GATE clocks, other than for power domains.
>>>
>>> Note that the HWV system is inside of the power domains controller, and it's split
>>> on a per hardware macro-block basis (as per usual MediaTek hardware layout...).
>>>
>>> The HWV, therefore, does *not* vote for clock *rates* (so, modeling OPPs would be
>>> a software quirk, I think?), does *not* manage bandwidth (and interconnect is for
>>> voting BW only?), and is just a "switch to flip".
>>
>> That's still clocks. Gate is a clock.
>>
>>>
>>> Is this happening because the description has to be improved and creating some
>>> misunderstanding, or is it because we are underestimating and/or ignoring something
>>> here?
>>>
>>
>> Other vendors, at least qcom, represent it properly - clocks. Sometimes
>> they mix up and represent it as power domains, but that's because
>> downstream is a mess and because we actually (at upstream) don't really
>> know what is inside there - is it a clock or power domain.
>>
> 
> ....but the hardware voter cannot be represented as a clock, because you use it
> for clocks *or* power domains (but at the same time, and of course in different
> drivers, and in different *intertwined* registers).
> 
> So the hardware voter itself (and/or bits inside of its registers) cannot be
> represented as a clock :\
> 
> In the context of clocks, it's used for clocks, (and not touching power domains at
> all), but in the context of power domains it's used for power domains (and not
> touching clocks at all).

I don't understand this. Earlier you mentioned "MUX-GATE and GATE
clocks", so these are clocks, right? How these clocks are used in other
places as power domains? If they are, this either has to be fixed or
apparently this is a power domain and use it as power domain also here.

Really, something called as hardware voter is not that uncommon and it
does fit existing bindings.

> 
> I'm not sure what qcom does - your reply makes me think that they did it such that
> the clocks part is in a MMIO and the power domains part is in a different MMIO,
> without having clock/pd intertwined voting registers...

No, you just never have direct access to hardware. You place votes and
votes go to the firmware. Now depending on person submitting it or
writing internal docs, they call it differently, but eventually it is
the same. You want to vote for some specific signal to be active or
running at some performance level.

> 
> Still not sure what to do here, then...
> 
> Cheers,
> Angelo


Best regards,
Krzysztof