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

[AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>]

Il 27/06/25 10:37, Krzysztof Kozlowski ha scritto:
> On Wed, Jun 25, 2025 at 02:42:15PM +0200, AngeloGioacchino Del Regno wrote:
>> Il 25/06/25 13:05, Krzysztof Kozlowski ha scritto:
>>> 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?
>>
>> I think you've misread, or I've explained badly enough to make you misread...
>> let me describe some more to try to let you understand this properly.
>>
>> The hardware voter is a unit that is used to vote for "flipping various switches",
>> in particular, you can vote for, *either*:
>>   - Enabling or disabling a *clock*; or
>>   - Enabling or disabling a *power domain*.
>>
>> There may be multiple (by hardware, in-silicon) copies of the Hardware Voter; in
>> the specific case of the MediaTek Dimensity 9400 MT6991 and of the MediaTek MT8196
>> Chromebook SoC, there is only one instance.
> 
> Everything so far very similar to qcom... They do exactly like that.
> 
>>
>> The Hardware Voter, there, is located in the SCPSYS macro-block.
>>
>> The SCPSYS macro-block contains:
>>   - A system controller
>>   - A Hardware Voter IP (new in MT6991/MT8196)
>>   - A power domains controller
>>   - Other hardware that is not relevant for this discussion
>>
>> The HWV is MMIO-accessible, and there is one (small, for now) set of registers,
>> allowing to vote for turning on/off one (or maybe multiple too, not sure about
>> that as there's no documentation and when I tried with multi-votes it didn't work)
>> clk/pd at a time.
> 
> Sure, the only difference against qcom is interface - qcom uses
> remoteprocs channels, here you have MMIO. The interface does not matter
> though.
> 
>>
>> Probably not important but worth mentioning: the HWV can vote for clocks or for
>> power domains in macro-blocks outside of its own (so, outside of the SCPSYS block,
>> for example - it can vote to turn on a clock or a power domain in HFRPSYS as well).
> 
> Same for qcom.
> 
>>
>> The register set in the HWV is *not* split between clock voters and PDs voters,
>> in the sense that the register set of clock voters is *not contiguous*; trying
>> to be as clear as possible, you have something like (mock register names ahead):
>>   0x0 - CLOCK_VOTER_0 (each bit is a clock)
>>   0x4 - PD_VOTER_0 (each bit is a power domain)
>>   0x8 - SECURE_WORLD_CLOCK_VOTER_1
>>   0xc - PD_VOTER_1
>>   0x10 - SECURE_WORLD_PD_VOTER_0
>>
>> ...etc etc.
> 
> OK
> 
>>
>>>> If they are, this either has to be fixed or
>>> apparently this is a power domain and use it as power domain also here.
>>
>> So no, clocks are not used as power domains, and power domains are not used as
>> clocks; we are talking purely about something that aggregates votes internally
> 
> OK
> 
>> and decides to turn on/off "whatever thing it is" (one of the clocks, or one of
>> the power domains) - and to do that, you flip a bit in a register, and then you
>> read another two registers to know the status of the internal state machine....
> 
> Sure. This is 100% not syscon, though. You must not do it via syscon,
> because you will be flopping bits of other devices in this driver.
> What's more, the actual implementation - registers for voting - is
> irrelevant to this device here. This device here wants:
> power domain
> or
> clock
> 
> Hm... don't we have bindings for this? Wait, we have!
> 
>>
>> ....and you do that atomically, this can't sleep, the system has to lock up
>> until HWV is done (I think I know what you're thinking, and yes, it's really
>> like this) otherwise you're surely racing.
> 
> Sure, no problems here.
> 
>>
>>>
>>> Really, something called as hardware voter is not that uncommon and it
>>> does fit existing bindings.
>>>
>>
>> Do you mean the interconnect/qcom/bcm-voter.c?
> 
> This and many others - all rpm/rpmh/rsc are for that.
> 
>>
>> That one seems to aggregate votes in software to place a vote in a hardware voter
>> (the Bus Clock Manager) and I see it as being really convoluted.
> 
> I do not say that drivers are example to follow. Actually, I do not
> recommend even DT bindings!
> 
>>
>> For MediaTek's HWV, you don't need to aggregate anything - actually, the HWV itself
>> is taking care of aggregating requests internally...
>>
>> Also, checking sdx75 and x1e80100 DTs, I see a virtual clock controller described,
>> placing votes through the bcm-voter, and with clocks that looks like being kind
>> of disguised/faked as interconnects?
> 
> Don't remember exactly, but I don't think it matters. What matters is
> you need to choose appropriate representation for your votes.
>>
>> That's a bit unclear, and even if I'm wrong about those being disguised as icc,
>> and not virtual, purely looking at the usage of the clk_virt and bcm-voters, I
>> seriously don't think that any similar structure with interconnect would fit
>> MediaTek SoCs in any way...
> 
> 
>>
>>>>
>>>> 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.
>>>
>>
>> Okay then there is one similarity, but it's different; MTK HWV is only arbitering
>> a on/off request; Nothing else.
> 
> Does not matter, still the same concept.
> 
> In 2026 or 2027 you will do other votes as well...
> 

Yeah, okay I think I got your point.

Now that I can understand the sense I think I can come up with some nicer solution.

Thanks for all :-)
Angelo

>>
>> No RATE votes.
>> No performance levels.
>> Literally, that's it.
> 
> Best regards,
> Krzysztof
>