Diffstat (limited to 'Documentation/ABI/testing/sysfs-bus-pci-drivers-ehci_hcd')
1 files changed, 46 insertions, 0 deletions
diff --git a/Documentation/ABI/testing/sysfs-bus-pci-drivers-ehci_hcd b/Documentation/ABI/testing/sysfs-bus-pci-drivers-ehci_hcd
new file mode 100644
@@ -0,0 +1,46 @@
+Date: January 2007
+Contact: Alan Stern <firstname.lastname@example.org>
+ 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).