[ltp] T400 does not enter C4/C6 sleep state with USB devices connected

[Pedro Ribeiro]

Hi all,

I spent lots of hours trying to figure out why my T400 would not enter
C4 and C6 sleep states.

Here are my findings for anyone which is interested:
- When a USB device is connected, even if it supports USB autosuspend,
the cpu will never go down more than C3 (apparently the mode is
disabled by the BIOS).
- After removing all the USB devices, in AC it enters C4 and in
battery it enters C6.

So one more reason not to use USB devices while on battery...


Regards,
Pedro