Intel Haswell/Broadwell Power Use On Linux Still Moving Lower
The area that Matthew Garrett has been focusing on is on making the various components of modern Intel Haswell/Broadwell systems better cooperate and all work towards getting in the lowest power-state possible. With the PCH, CPU, and GPU being tightly interconnected on modern processors, if one isn't in its optimal power state, it can often prevent the other components from hitting their ideal levels for power efficiency.
Matthew partially reworked the ACHI LPM handling in order to make SATA power management on Linux used by more systems. With SATA power management enabled, the rest of the hardware isn't blocked from achieving a lower power state. From his tests, he found that his idle power consumption on one of the systems dropped from 8.5 Watts to 5 Watts as a result.
More details can be found on Matthew's blog. These patches haven't yet landed in the mainline kernel but will likely be power management material for Linux 4.2.