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

   1 What:           /sys/bus/pci/drivers/ehci_hcd/.../companion
   2                 /sys/bus/usb/devices/usbN/../companion
   3 Date:           January 2007
   4 KernelVersion:  2.6.21
   5 Contact:        Alan Stern <stern@rowland.harvard.edu>
   6 Description:
   7                 PCI-based EHCI USB controllers (i.e., high-speed USB-2.0
   8                 controllers) are often implemented along with a set of
   9                 "companion" full/low-speed USB-1.1 controllers.  When a
  10                 high-speed device is plugged in, the connection is routed
  11                 to the EHCI controller; when a full- or low-speed device
  12                 is plugged in, the connection is routed to the companion
  13                 controller.
  14
  15                 Sometimes you want to force a high-speed device to connect
  16                 at full speed, which can be accomplished by forcing the
  17                 connection to be routed to the companion controller.
  18                 That's what this file does.  Writing a port number to the
  19                 file causes connections on that port to be routed to the
  20                 companion controller, and writing the negative of a port
  21                 number returns the port to normal operation.
  22
  23                 For example: To force the high-speed device attached to
  24                 port 4 on bus 2 to run at full speed::
  25
  26                         echo 4 >/sys/bus/usb/devices/usb2/../companion
  27
  28                 To return the port to high-speed operation::
  29
  30                         echo -4 >/sys/bus/usb/devices/usb2/../companion
  31
  32                 Reading the file gives the list of ports currently forced
  33                 to the companion controller.
  34
  35                 Note: Some EHCI controllers do not have companions; they
  36                 may contain an internal "transaction translator" or they
  37                 may be attached directly to a "rate-matching hub".  This
  38                 mechanism will not work with such controllers.  Also, it
  39                 cannot be used to force a port on a high-speed hub to
  40                 connect at full speed.
  41
  42                 Note: When this file was first added, it appeared in a
  43                 different sysfs directory.  The location given above is
  44                 correct for 2.6.35 (and probably several earlier kernel
  45                 versions as well).
  46