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+What: /sys/devices/.../power_state
+Date: January 2013
+Contact: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
+ The /sys/devices/.../power_state attribute is only present for
+ device objects representing ACPI device nodes that provide power
+ management methods.
+ If present, it contains a string representing the current ACPI
+ power state of the given device node. Its possible values,
+ "D0", "D1", "D2", "D3hot", and "D3cold", reflect the power state
+ names defined by the ACPI specification (ACPI 4 and above).
+ If the device node uses shared ACPI power resources, this state
+ determines a list of power resources required not to be turned
+ off. However, some power resources needed by the device node in
+ higher-power (lower-number) states may also be ON because of
+ some other devices using them at the moment.
+ This attribute is read-only.