Coreboot Gets Support For Haswell Power Limiting

After landing hardware support improvements last week for Coreboot, the open-source BIOS firmware replacement now has another new feature: ACPI power limiting and it's been implemented for Intel Haswell CPUs.

Google engineers continue to be hard at work on improving Coreboot's support for the latest-generation Intel Haswell processors, since there's now the Acer C720 Chromebook for sale that uses a Haswell-based Celeron processor. With the latest Intel hardware there's support to place power limits on individual components such as the CPU package and to ensure the system never exceeds those defined power consumption limits.

In this area we've seen the Linux kernel gain support for a power capping framework and a Intel Run-Time Average Power-Limiting driver (RAPL) for leveraging the new hardware capabilities. The Linux power limiting support was part of the ACPI improvements in Linux 3.13. Now within the Coreboot project we're seeing this support taken advantage of at the firmware level.

Landing in Coreboot on Friday was Haswell ACPI support for a controllable TDP. The new Haswell Coreboot code adds ACPI methods for limiting the package's TDP Wattage.

In another commit there's now Coreboot power limiting in place for thermal control. With the Acer C720 "Peppy" Chromebook, Coreboot is limiting the Celeron CPU's power to 12 Watts at 73 Celsius but places no power restrictions if the CPU's temperature is below 68 Celsius.

This change is specific to the "Peppy" Chromebook and is hard-coded inside Coreboot. The Acer C720 uses the Intel Celeron 2955U CPU that's a dual-core Haswell part clocked at 1.40GHz and without any form of Turbo Boost. The TDP of the processor with integrated HD Graphics is 15 Watts.

I have been testing out the Acer C720 for the past few days at Phoronix and I have been rather pleased with the performance of the Chromebook and Haswell Celeron processor. Full benchmarks on the C720 running Ubuntu Linux will be published in the next week or two but for now you can see some sample Linux benchmark results via 1312065-SO-CELERONHA14 and 1312067-SO-ACERCHROM69.