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+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).
+