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Commit 25aa5548 authored by Alan Stern's avatar Alan Stern Committed by Greg Kroah-Hartman
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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: default avatarAlan Stern <stern@rowland.harvard.edu>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@suse.de>
parent 073b8546

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

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+46 −0
Original line number Diff line number Diff line 
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).