[arch-general] usb-storage and processor C-states

Hello, I have two boxes: Pentium M / i915 / ICH7 and Via C7 / CN 700 / VT8237R. Both processors support C-states 0 through 3. Both southbridges support USB 2.0 (ehci). Yet when I plug in an usb flash drive into the Via C7 box the processor can no longer go into the C3 sleep state (C2 becomes the lowest possible) while the Pentium M box does not display this behavior and still happily idles in the C3 state. I'd like to poke upstream about it but I need to know more. What is to blame? Could this be a Southbridge/CPU/BIOS limitation, or is it driver's fault? How do I debug this further? Any help would be appreciated. Regards, JM