C-States lower than C2 impossible with USB?

[Peter Ganzhorn]

Hi,

with powertop I was able to find out that my system won't use the C3
state at all:

C0 (cpu running)        ( 6.5%)
C1                0.0ms ( 0.0%)                   0.0ms
C2                6.8ms (93.5%)                   6.6ms
C3                0.0ms ( 0.0%)                   0.0ms

After searching for possible causes for my problem I identified USB to
be the problem - disconnecting all USB devices from my system will make
the CPU enter C3 state a lot, about 50% with more than 20ms are reported
by powertop.
Unfortunately I have a usb mouse, usb keyboard und two external usb hard
drives - and I don't want to miss any of them. Using USB-to-PS2 adapters
may be an imperfect solution for mouse and keyboard, but the hard drives
will remain an issue for sure.
Is there any possible way to make the system to go to C3 with USB
devices attached???
On Google I read that Microsoft released a hot-fix for Windows which is
said to fix this issue, so I hope there is a way to do this with Linux
as well.

Peter