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