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

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Message-ID: <39e2bd90-8566-42d9-8ecd-6c5ba0f9747d@oss.qualcomm.com>
Date: Wed, 21 Jan 2026 13:52:26 +0100
From: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
To: Akhil P Oommen <akhilpo@....qualcomm.com>,
        Jagadeesh Kona <jagadeesh.kona@....qualcomm.com>
Cc: Ajit Pandey <ajit.pandey@....qualcomm.com>,
        Imran Shaik <imran.shaik@....qualcomm.com>,
        Taniya Das <taniya.das@....qualcomm.com>,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org, Sibi Sankar <sibi.sankar@....qualcomm.com>,
        Jassi Brar <jassisinghbrar@...il.com>, Rob Herring <robh@...nel.org>,
        Krzysztof Kozlowski <krzk+dt@...nel.org>,
        Conor Dooley
 <conor+dt@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konradybcio@...nel.org>
Subject: Re: [PATCH 2/2] arm64: dts: qcom: SM8750: Enable CPUFreq support

On 1/21/26 1:47 PM, Akhil P Oommen wrote:
> On 1/21/2026 5:06 PM, Konrad Dybcio wrote:
>> On 1/20/26 9:54 PM, Akhil P Oommen wrote:
>>> 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@....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.
>>
>> Yes, this would be placed somewhere on the chassis of the device to
>> reflect the actual temperature that the user could experience (since
>> there are regulations about maximum values of that)
>>
>>>> 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. :(
>>
>> Yes, the CPUs are covered.
> 
> Does this LMH based thermal migitation require any initialization from
> Linux? If yes, could you please share a link to a patch which enables it
> for any recent chipset for my reference?

It used to, see drivers/thermal/qcom/lmh.c

I believe it went away roughly with 8250 where the firmware now takes
care of all that.

FYI I think it first appeared with 8994

Konrad