Below is Matthew Garrett's description about this new patch.
Right now we forcibly clear ASPM state on all devices if the BIOS indicates that the feature is not supported. Based on the Microsoft presentation "PCI Express In Depth for Windows Vista and Beyond", I'm starting to think that this may be an error. The implication is that unless the platform grants full control via _OSC, Windows will not touch any PCIe features - including ASPM. In that case clearing ASPM state would be an error unless the platform has granted us that control.
This patch reworks the ASPM disabling code such that the actual clearing of state is triggered by a successful handoff of PCIe control to the OS. The general ASPM code undergoes some changes in order to ensure that the ability to clear the bits isn't overridden by ASPM having already been disabled. Further, this theoretically now allows for situations where only a subset of PCIe roots hand over control, leaving the others in the BIOS state.
It's difficult to know for sure that this is the right thing to do - there's zero public documentation on the interaction between all of these components. But enough vendors enable ASPM on platforms and then set this bit that it seems likely that they're expecting the OS to leave them alone.
Measured to save around 5W on an idle Thinkpad X220.
This ~60 line kernel patch changes the ASPM behavior and from his testing, plus my initial testing, it seems to be relatively sane. From the various notebooks (a half-dozen so far) I have been trying out, there have not been any problems and ASPM is working without having to manually force it.
On the next page is a test of this patch and the system power consumption from one of the ASPM affected systems under the Linux 2.6.37, 2.6.38, 3.2, and 3.2 patched kernels.