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+What: /sys/devices/.../online
+Date: April 2013
+Contact: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
+Description:
+ The /sys/devices/.../online attribute is only present for
+ devices whose bus types provide .online() and .offline()
+ callbacks. The number read from it (0 or 1) reflects the value
+ of the device's 'offline' field. If that number is 1 and '0'
+ (or 'n', or 'N') is written to this file, the device bus type's
+ .offline() callback is executed for the device and (if
+ successful) its 'offline' field is updated accordingly. In
+ turn, if that number is 0 and '1' (or 'y', or 'Y') is written to
+ this file, the device bus type's .online() callback is executed
+ for the device and (if successful) its 'offline' field is
+ updated as appropriate.
+
+ After a successful execution of the bus type's .offline()
+ callback the device cannot be used for any purpose until either
+ it is removed (i.e. device_del() is called for it), or its bus
+ type's .online() is exeucted successfully.