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

Thanks