power management

I am working on BBB AM335x for power management stuff as i am very new to linux power management i need a bit help
i am trying to achieve following using sysfs but i didnt got sucess

i want to put my whole system into sleep state /idle state except WiFi and backlight,
when i am writting $echo mem > /sys/power/state my whole system is going into sleep state similarly i have checked with /sys/class/backlight but didnt worth.

Can you help me how i can do it ? How i can suspend and resume particular device only ??