Linux 5.11 Gets New Framework To Help Avoid Burning Your Skin On Hot Devices

Written by Michael Larabel in Hardware on 1 January 2021 at 04:00 PM EST. 13 Comments
While the Linux 5.11 merge window has been over for one week where new features are normally added, a power management pull request sent in today for mainline is adding some tardy features including the Dynamic Thermal Power Management (DTPM) framework that in part is designed to help ensure users don't burn themselves with hot devices.

Rafael Wysocki sent in the New Year's Day power management updates for Linux 5.11. The changes include a crash fix for P-State, new C-states table for Intel Snow Ridge processors within the Intel Idle driver, and the new DTPM Framework.

One of the driving features for the Dynamic Thermal Power Management framework is trying to ensure users don't burn themselves on hot laptops / devices. There are regulations in some regions to ensure a device's case temperature always stays below 45 degrees Celsius. Besides the legal requirements, users normally don't want to burn themselves anyhow. DTPM is part of the Linux PowerCap framework.

Linaro's Daniel Lezcano has been driving along this framework with interest from ARM device vendors. From one of the earlier patch series he summed up DTPM as:
The density of components greatly increased the last decade bringing a numerous number of heating sources which are monitored by more than 20 sensors on recent SoC. The skin temperature, which is the case temperature of the device, must stay below approximately 45°C in order to comply with the legal requirements.

The skin temperature is managed as a whole by an user space daemon, which is catching the current application profile, to allocate a power budget to the different components where the resulting heating effect will comply with the skin temperature constraint.

This technique is called the Dynamic Thermal Power Management.

The Linux kernel does not provide any unified interface to act on the power of the different devices. Currently, the thermal framework is changed to export artificially the performance states of different devices via the cooling device software component with opaque values. This change is done regardless of the in-kernel logic to mitigate the temperature. The user space daemon uses all the available knobs to act on the power limit and those differ from one platform to another.

This series provides a Dynamic Thermal Power Management framework to provide an unified way to act on the power of the devices.
Via sysfs, user-space can query the instantaneous power consumption, the power range and maximum power consumption, and various constraints. With these initial patches for Linux 5.11 is an initial DPTM CPU Energy Model implementation.

The new DPTM framework is pending with this pull request.

Update: Linus Torvalds has decided against accepting DTPM for Linux 5.11. Thus the new framework will be postponed to Linux 5.12.
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