Re: [PATCH v5 6/6] arm64: dts: qcom: Enable cpu cooling devices for QCS9075 platforms

[Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>]

On 10.01.2025 12:54 AM, Dmitry Baryshkov wrote:
> On Wed, Jan 08, 2025 at 09:38:19PM +0530, Manaf Meethalavalappu Pallikunhi wrote:
>>
>> Hi Dmitry,
>>
>>
>> On 1/8/2025 6:16 PM, Dmitry Baryshkov wrote:
>>> On Wed, Jan 08, 2025 at 05:57:06PM +0530, Manaf Meethalavalappu Pallikunhi wrote:
>>>> Hi Dmitry,
>>>>
>>>>
>>>> On 1/3/2025 11:21 AM, Dmitry Baryshkov wrote:
>>>>> On Tue, Dec 31, 2024 at 05:31:41PM +0530, Manaf Meethalavalappu Pallikunhi wrote:
>>>>>> Hi Dmitry,
>>>>>>
>>>>>> On 12/30/2024 9:10 PM, Dmitry Baryshkov wrote:
>>>>>>> On Sun, Dec 29, 2024 at 08:53:32PM +0530, Wasim Nazir wrote:
>>>>>>>> From: Manaf Meethalavalappu Pallikunhi <quic_manafm@quicinc.com>
>>>>>>>>
>>>>>>>> In QCS9100 SoC, the safety subsystem monitors all thermal sensors and
>>>>>>>> does corrective action for each subsystem based on sensor violation
>>>>>>>> to comply safety standards. But as QCS9075 is non-safe SoC it
>>>>>>>> requires conventional thermal mitigation to control thermal for
>>>>>>>> different subsystems.
>>>>>>>>
>>>>>>>> The cpu frequency throttling for different cpu tsens is enabled in
>>>>>>>> hardware as first defense for cpu thermal control. But QCS9075 SoC
>>>>>>>> has higher ambient specification. During high ambient condition, even
>>>>>>>> lowest frequency with multi cores can slowly build heat over the time
>>>>>>>> and it can lead to thermal run-away situations. This patch restrict
>>>>>>>> cpu cores during this scenario helps further thermal control and
>>>>>>>> avoids thermal critical violation.
>>>>>>>>
>>>>>>>> Add cpu idle injection cooling bindings for cpu tsens thermal zones
>>>>>>>> as a mitigation for cpu subsystem prior to thermal shutdown.
>>>>>>>>
>>>>>>>> Add cpu frequency cooling devices that will be used by userspace
>>>>>>>> thermal governor to mitigate skin thermal management.
>>>>>>> Does anything prevent us from having this config as a part of the basic
>>>>>>> sa8775p.dtsi setup? If HW is present in the base version but it is not
>>>>>>> accessible for whatever reason, please move it the base device config
>>>>>>> and use status "disabled" or "reserved" to the respective board files.
>>>>>> Sure,  I will move idle injection node for each cpu to sa8775p.dtsi and keep
>>>>>> it disabled state. #cooling cells property for CPU, still wanted to keep it
>>>>>> in board files as we don't want to enable any cooling device in base DT.
>>>>> "we don't want" is not a proper justification. So, no.
>>>> As noted in the commit, thermal cooling mitigation is only necessary for
>>>> non-safe SoCs. Adding this cooling cell property to the CPU node in the base
>>>> DT (sa8775p.dtsi), which is shared by both safe and non-safe SoCs, would
>>>> violate the requirements for safe SoCs. Therefore, we will include it only
>>>> in non-safe SoC boards.
>>> "is only necessary" is fine. It means that it is an optional part which
>>> is going to be unused / ignored / duplicate functionality on the "safe"
>>> SoCs. What kind of requirement is going to be violated in this way?
>>
>> From the perspective of a safe SoC, any software mitigation that compromises
>> the safety subsystem’s compliance should not be allowed. Enabling the
>> cooling device also opens up the sysfs interface for userspace, which we may
>> not fully control.
> 
> THere are a lot of interfaces exported to the userspace.
> 
>> Userspace apps or partner apps might inadvertently use
>> it. Therefore, we believe it is better not to expose such an interface, as
>> it is not required for that SoC and helps to avoid opening up an interface
>> that could potentially lead to a safety failure.
> 
> How can thermal mitigation interface lead to safety failure? Userspace
> can possibly lower trip points, but it can not override existing
> firmware-based mitigation.
> And if there is a known problem with the interface, it should be fixed
> instead.

I think the intended case to avoid is where a malicious actor would set
the trips too low, resulting in throttling down the CPU to FMIN / Linux
throttling CPUs to try and escape what it believes to be possible thermal
runaway / a system reboot. Not something desired in a car.

Konrad