USB: document ehci-hcd's "companion" sysfs attribute

This patch (as1484) adds documentation for ehci-hcd's "companion"
sysfs attribute, which was added to the kernel over four years ago but
never documented.
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

Documentation/ABI/testing/sysfs-bus-pci-drivers-ehci_hcd

+What:		/sys/bus/pci/drivers/ehci_hcd/.../companion
+		/sys/bus/usb/devices/usbN/../companion
+Date:		January 2007
+KernelVersion:	2.6.21
+Contact:	Alan Stern <stern@rowland.harvard.edu>
+Description:
+		PCI-based EHCI USB controllers (i.e., high-speed USB-2.0
+		controllers) are often implemented along with a set of
+		"companion" full/low-speed USB-1.1 controllers.  When a
+		high-speed device is plugged in, the connection is routed
+		to the EHCI controller; when a full- or low-speed device
+		is plugged in, the connection is routed to the companion
+		controller.
+
+		Sometimes you want to force a high-speed device to connect
+		at full speed, which can be accomplished by forcing the
+		connection to be routed to the companion controller.
+		That's what this file does.  Writing a port number to the
+		file causes connections on that port to be routed to the
+		companion controller, and writing the negative of a port
+		number returns the port to normal operation.
+
+		For example: To force the high-speed device attached to
+		port 4 on bus 2 to run at full speed:
+
+			echo 4 >/sys/bus/usb/devices/usb2/../companion
+
+		To return the port to high-speed operation:
+
+			echo -4 >/sys/bus/usb/devices/usb2/../companion
+
+		Reading the file gives the list of ports currently forced
+		to the companion controller.
+
+		Note: Some EHCI controllers do not have companions; they
+		may contain an internal "transaction translator" or they
+		may be attached directly to a "rate-matching hub".  This
+		mechanism will not work with such controllers.  Also, it
+		cannot be used to force a port on a high-speed hub to
+		connect at full speed.
+
+		Note: When this file was first added, it appeared in a
+		different sysfs directory.  The location given above is
+		correct for 2.6.35 (and probably several earlier kernel
+		versions as well).
+