According to Matthew Garrett, who developed the patch, the change can
reduce the power consumption of a Thinkpad X220 by 5 watts. The H's
associates at c't magazine in Germany tested systems in their laboratory
and experienced savings in the range of 1 to 3 watts which resulted in
noticeably longer battery life. The patch only has this effect on
systems with the firmware problem detailed above, which can be identified by using dmesg to display kernel messages and looking for the message "ACPI FADT declares the system doesn't support PCIe ASPM, so disable it".
The change fixes one of two problems which garnered serious attention last year as a result of reports on the Phoronix web site. Several weeks ago, the patch was merged into the main Linux development tree, which will form the basis of March's Linux 3.3 release. Patches like this, which could potentially cause problems on isolated systems, are not usually merged into long-term and stable kernels. The Fedora Project has, however, been testing the patch as part of its current distribution for some time, and Kroah-Hartman is therefore confident that the change will not cause any major problems. In view of the margin for error in this prediction, however, this is the only change in Linux 3.0.20 and 3.2.5.
Another problem which probably affects many more systems remains,
however. By default, current Linux kernels do not activate the RC6
graphics power-saving feature on Intel's popular Sandy Bridge processors
(which include a graphics core), as it has caused crashes and graphics
errors on isolated systems. According to measurements made by c't, RC6
typically saves 3 to 5 watts when idling, which for laptops means
reduced fan noise and longer battery life. It can be enabled for testing
purposes using the kernel argument i915.i915_enable_rc6=1.
Source:http://www.h-online.com/open/news/item/New-Linux-kernel-fixes-power-saving-issues-1429482.html
