What:		/sys/bus/usb/devices/.../power/autosuspend
Date:		March 2007
KernelVersion:	2.6.21
Contact:	Alan Stern <stern@rowland.harvard.edu>
Description:
		Each USB device directory will contain a file named
		power/autosuspend.  This file holds the time (in seconds)
		the device must be idle before it will be autosuspended.
		0 means the device will be autosuspended as soon as
		possible.  Negative values will prevent the device from
		being autosuspended at all, and writing a negative value
		will resume the device if it is already suspended.

		The autosuspend delay for newly-created devices is set to
		the value of the usbcore.autosuspend module parameter.

What:		/sys/bus/usb/devices/.../power/level
Date:		March 2007
KernelVersion:	2.6.21
Contact:	Alan Stern <stern@rowland.harvard.edu>
Description:
		Each USB device directory will contain a file named
		power/level.  This file holds a power-level setting for
		the device, one of "on", "auto", or "suspend".

		"on" means that the device is not allowed to autosuspend,
		although normal suspends for system sleep will still
		be honored.  "auto" means the device will autosuspend
		and autoresume in the usual manner, according to the
		capabilities of its driver.  "suspend" means the device
		is forced into a suspended state and it will not autoresume
		in response to I/O requests.  However remote-wakeup requests
		from the device may still be enabled (the remote-wakeup
		setting is controlled separately by the power/wakeup
		attribute).

		During normal use, devices should be left in the "auto"
		level.  The other levels are meant for administrative uses.
		If you want to suspend a device immediately but leave it
		free to wake up in response to I/O requests, you should
		write "0" to power/autosuspend.

What:		/sys/bus/usb/devices/.../power/persist
Date:		May 2007
KernelVersion:	2.6.23
Contact:	Alan Stern <stern@rowland.harvard.edu>
Description:
		If CONFIG_USB_PERSIST is set, then each USB device directory
		will contain a file named power/persist.  The file holds a
		boolean value (0 or 1) indicating whether or not the
		"USB-Persist" facility is enabled for the device.  Since the
		facility is inherently dangerous, it is disabled by default
		for all devices except hubs.  For more information, see
		Documentation/usb/persist.txt.

What:		/sys/bus/usb/device/.../power/connected_duration
Date:		January 2008
KernelVersion:	2.6.25
Contact:	Sarah Sharp <sarah.a.sharp@intel.com>
Description:
		If CONFIG_PM and CONFIG_USB_SUSPEND are enabled, then this file
		is present.  When read, it returns the total time (in msec)
		that the USB device has been connected to the machine.  This
		file is read-only.
Users:
		PowerTOP <power@bughost.org>
		http://www.lesswatts.org/projects/powertop/

What:		/sys/bus/usb/device/.../power/active_duration
Date:		January 2008
KernelVersion:	2.6.25
Contact:	Sarah Sharp <sarah.a.sharp@intel.com>
Description:
		If CONFIG_PM and CONFIG_USB_SUSPEND are enabled, then this file
		is present.  When read, it returns the total time (in msec)
		that the USB device has been active, i.e. not in a suspended
		state.  This file is read-only.

		Tools can use this file and the connected_duration file to
		compute the percentage of time that a device has been active.
		For example,
		echo $((100 * `cat active_duration` / `cat connected_duration`))
		will give an integer percentage.  Note that this does not
		account for counter wrap.
Users:
		PowerTOP <power@bughost.org>
		http://www.lesswatts.org/projects/powertop/