- Split calls into 
  - device_pm_suspend() [ Saving device state. ]
  - device_pm_power_down() [ Powering devices down. ]
  - device_pm_power_up() [ Powering devices up. ]
  - device_pm_resume() [ Restoring device state. ]

- Walk local dpm_active list when suspending devices, and move devices to 
  dpm_suspended list when ->suspend() is called. 
- Walk dpm_suspended list to power down devices (with interrrupts enabled.)
  - Try to power down devices with IRQs on. 
  - If they succeed, move them to dpm_off list.
  - If they return -EAGAIN, move them to dpm_off_irq list. 
- Disable interrupts and suspend devices needed interrupts off. 

- Do converse on resume 
  - power on devices that need interrupts off and move them to 
    dpm_suspended.
  - Enable interrupts.
  - Power on all other devices and move them to dpm_suspended.
  - Restore state of all devices and move them to dpm_active.