Re: [PATCH 2/2] arm64: dts: qcom: SM8750: Enable CPUFreq support

[Akhil P Oommen <akhilpo@oss.qualcomm.com>]

On 1/20/2026 8:13 PM, Konrad Dybcio wrote:
> On 1/20/26 12:25 PM, Akhil P Oommen wrote:
>> On 1/20/2026 3:44 PM, Konrad Dybcio wrote:
>>> On 1/19/26 8:00 PM, Akhil P Oommen wrote:
>>>> On 12/11/2025 12:32 AM, Jagadeesh Kona wrote:
>>>>> Add the cpucp mailbox, sram and SCMI nodes required to enable
>>>>> the CPUFreq support using the SCMI perf protocol on SM8750 SoCs.
>>>>>
>>>>> Signed-off-by: Jagadeesh Kona <jagadeesh.kona@oss.qualcomm.com>
>>>>
>>>> Just curious, does this patch enable thermal mitigation for CPU clusters
>>>> too?
>>>
>>> If nothing changed, we have lets-not-explode type mitigations via LMH,
>>> but lets-not-burn-the-user would require a skin temp sensor to be
>>> wired up, which then could be used to enable some cooling action
>>
>> In some chipsets, I have noticed that the gpu cooling device throttles
>> GPU to the lowest OPP even with not-so-heavy GPU workloads, making it
>> unusable-ly slow. My hypothesis was that it was due to unmitigated CPU
>> temperature tripping up GPU Tsens.
>>
>> So, I am wondering if there are any additional CPU cooling related
>> changes required to get a reasonable overall performance under thermal
>> constraints.
> 
> Yes, something like the aforementioned skin-temp sensor at least..

I suppose this sensor is off-chip and slow to react.

> 
> Today Linux will not throttle the CPUs at all (they're not even declared
> as cooling devices) and we sorta agreed that in general it's a good thing
> (tm), because otherwise we'd be coding in a cooling profile into the SoC
> DTSI without taking into account the cooling capabilities of a given end
> device (i.e. in an extreme case, a PC with SM8650 with a cooler that's
> 3kg of aluminium vs a Steam Frame headset where the SoC is centimeters
> away from your face)
> 
> Currently, we have cooling policies for devices with fans and the only
> other action is based on a skin temperature sensor (sc8280xp + x13s).
> Everything else is left up to the LMH defaults. AFAIK work is ongoing to
> create a more informed solution, that would have to (quite obviously)
> live in userland.

I can understand that the skin-temp based mitigation is influenced by
various design decision outside of the SoC die. But I think there should
an on-chip sensor based mitigation which is fast and usually for a short
duration which helps to avoid extreme temperature or violating the
maximum operating point of the chipset. I guess it should depend only on
the SoC characteristics as it is a last resort. It may be implemented in
SW (like cooling device for Adreno GPU) or in HW. Probably the LMH
hardware you mentioned offers this functionality for CPU clusters. I
have no clue. :(

I am hoping that if this on-chip mitigation is enabled and wired up
correctly for CPU clusters (probably DDR too), it would reduce the
unnecessary thermal trips on GPU Tsens and help to reach a performance
equilibrium which is reasonably good.

-Akhil.

> 
> Konrad