Last month I wrote that Intel Sandy Bridge RC6 Is Good To Go after the Intel OTC developers were spending months trying to figure out why for some Sandy Bridge systems this feature, which can significantly conserve power while the system is idling by dropping the supplied power to the integrated graphics processor, was causing stability issues and problems for a small number of systems. Intel has tried at several different times to enable RC6-by-default, but in the end it was always reverted due to such problems.
Intel remains confident that RC6 should be safe to have by default for the latest-generation Intel hardware (RC6 for Ivy Bridge is already on-by-default as they're confident it's working well there), at least using the "plain" RC6 that avoids the deepest power states. While they have been comfortable with RC6 for Sandy Bridge, the change to enable this hardware power-savings feature wasn't found in the exciting DRM pull for the Linux 3.4 kernel.
Eugeni Dodonov of Intel has now sent two patches to the mailing list for having this support merged.
Daniel Vetter and Chris Wilson noticed that the latest rc6 patches went missing in the latest drm merge requests. So I updated them to the latest Linus' tree, and improved the description a bit.Meanwhile, Ubuntu 12.04 LTS with its Linux 3.2-based kernel continues to have RC6 enabled. Aside from the RC6 power-savings benchmarks shown in last month's article, there's some more results in Intel RC6 Support On The Sandy Bridge Desktop.
Those settings have received a huge testing on Ubuntu (thanks a lot Canonical!!), and so far it looks like they solve all the known issues on SNB with RC6 which were out there for the past months.
However, one can never be too sure, so the earlier we get them in the main kernel, the earlier we'll find out if there are still some bugs in the wild (with any luck, there won't be any). And with this, we'll complete the lets-try-rc6-by-default-one-more-time saga.
Eugeni Dodonov (2):
drm/i915: allow to select rc6 modes via kernel parameter
drm/i915: enable plain RC6 on Sandy Bridge by default