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What: /sys/bus/pci/drivers/ehci_hcd/.../companion
Date: January 2007
KernelVersion: 2.6.21
Contact: Alan Stern <>
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
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).