linux/Documentation/ABI/testing/sysfs-bus-pci-drivers-xhci_hcd

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What: /sys/bus/pci/drivers/xhci_hcd/.../dbc
Date: June 2017
Contact: Lu Baolu <baolu.lu@linux.intel.com>
Description:
xHCI compatible USB host controllers (i.e. super-speed
USB3 controllers) are often implemented with the Debug
Capability (DbC). It can present a debug device which
is fully compliant with the USB framework and provides
the equivalent of a very high performance full-duplex
serial link for debug purpose.
The DbC debug device shares a root port with xHCI host.
When the DbC is enabled, the root port will be assigned
to the Debug Capability. Otherwise, it will be assigned
to xHCI.
Writing "enable" to this attribute will enable the DbC
functionality and the shared root port will be assigned
to the DbC device. Writing "disable" to this attribute
will disable the DbC functionality and the shared root
port will roll back to the xHCI.
Reading this attribute gives the state of the DbC. It
can be one of the following states: disabled, enabled,
initialized, connected, configured and stalled.