The cpuidle subsystem

I Need some questions on CPUIDLE SUBSYSTEM

1) As per Linux Power States Concepts we have C0 to C7 states are their, but in linux kernel we are using only c0 to c3....can any one please clarify what is the reason?

2) How the processor will move from one state to another state...and once we from one state to another state what about the devices states in kernel...how it control

We are Doing Power management on ARM Cortex-A9. I am new to power management. can any one help me on CPUIDLE SUBSYSTEM. How it works in Linux kernel governors to Hardware level how the flow is going on...

Thanks & Regards,
Siva Krishna.