RE: [RFC PATCH 0/3] thermal: Add CPU hotplug cooling driver

[Biju Das]

Hi John,

Thanks for the patch.

> -----Original Message-----
> From: John Madieu <john.madieu.xa@xxxxxxxxxxxxxx>
> Sent: 09 March 2025 12:13
> Subject: [RFC PATCH 0/3] thermal: Add CPU hotplug cooling driver
> 
> MIME-Version: 1.0
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> 
> This patch series introduces a new thermal cooling driver that implements CPU hotplug-based thermal
> management. The driver dynamically takes CPUs offline during thermal excursions to reduce power
> consumption and prevent overheating, while maintaining system stability by keeping at least one CPU
> online.
> 
> 1- Problem Statement
> 
> Modern SoCs require robust thermal management to prevent overheating under heavy workloads. Existing
> cooling mechanisms like frequency scaling may not always provide sufficient thermal relief, especially
> in multi-core systems where per-core thermal contributions can be significant.
> 
> 2- Solution Overview
> 
> The driver:
> 
>  - Integrates with the Linux thermal framework as a cooling device
>  - Registers per-CPU cooling devices that respond to thermal trip points
>  - Uses CPU hotplug operations to reduce thermal load
>  - Maintains system stability by preserving the boot CPU from being put offline,  regardless the CPUs
> that are specified in cooling device list.
>  - Implements proper state tracking and cleanup
> 
> Key Features:
> 
>  - Dynamic CPU online/offline management based on thermal thresholds
>  - Device tree-based configuration via thermal zones and trip points
>  - Hysteresis support through thermal governor interactions
>  - Safe handling of CPU state transitions during module load/unload
>  - Compatibility with existing thermal management frameworks
> 
> Testing
> 
>  - Verified on Renesas RZ/G3E platforms with multi-core CPU configurations
>  - Validated thermal response using artificial load generation (emul_temp)
>  - Confirmed proper interaction with other cooling devices
>  - Verified support for 'plug' type trace events
>  - Tested with step_wise governor
> 
> As the 'hot' type is already used for user space notification, I've choosen 'plug' for this new type.
> suggestions on this are welcome. Here is an example of 'thermal-zone' that integrate 'plug' type:
> 
> ```
> thermal-zones {
> 	cpu-thermal {
> 		polling-delay = <1000>;
> 		polling-delay-passive = <250>;
> 		thermal-sensors = <&tsu>;
> 
> 		cooling-maps {
> 			map0 {
> 				trip = <&target>;
> 				cooling-device = <&cpu0 0 3>, <&cpu3 0 3>;
> 				contribution = <1024>;
> 			};

Is it not possible here to make cpu1 and cpu2 as well for DVFS passive cooling?

> 
> 			map1 {
> 				trip = <&trip_emergency>;
> 				cooling-device = <&cpu1 0 1>, <&cpu2 0 1>;
> 				contribution = <1024>;
> 			};
> 
> 		};

Is it not possible here to make cpu3 as well as hot pluggable device for cooling?

Cheers,
Biju