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authorAlan Stern <stern@rowland.harvard.edu>2011-09-14 12:33:16 -0400
committerGreg Kroah-Hartman <gregkh@suse.de>2011-09-18 01:48:37 -0700
commit25aa55488c3644b19e33ba79c564191b8e33f477 (patch)
tree13a18f3e4a4b20791e4d8db29789a797e4b3559e /Documentation/ABI
parent073b85469308b41468149f4aa7144c1e1bf4f0d7 (diff)
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: Alan Stern <stern@rowland.harvard.edu> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Diffstat (limited to 'Documentation/ABI')
-rw-r--r--Documentation/ABI/testing/sysfs-bus-pci-drivers-ehci_hcd46
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
index 000000000000..60c60fa624b2
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-bus-pci-drivers-ehci_hcd
@@ -0,0 +1,46 @@
+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).
+