EHCI spec中的描述：
A USB 2.0 Host Controller includes one high-speed mode host controller and 0 or more USB 1.1 host
controllers (see Figure 1-2). The high-speed host controller implements an EHCI interface. It is used for all
high-speed communications to high-speed-mode devices connected to the root ports of the USB 2.0 host
controller. This specification allows communications to Full- and Low-speed devices connected to the root
ports of the USB 2.0 host controller to be provided by companion USB 1.1 host controllers. If an
implementation does not include companion host controllers, the host controller must include a high-speed
device permanently attached to each of the EHCI ports the implementation is planning to utilize. The EHCI
controller cannot work with a Full- or Low-speed device.
High-speed devices are always routed to and controlled by the EHCI host controller (eHC). When running
and configured, the eHC is the default ownerof all the root ports. The eHC and its driver initially detect all
device attaches. It has additional control bits visible in each port register to manage the routing logic. For
example: if the attached device is not a high-speed device, the eHC driver releases ownership of the port
(and thus control of the device) to a companion host controller. For that port, enumeration starts over from
the initial attach detect point and the device is enumerated under the cHC. Otherwise, the eHC retains
ownership of the port and the device completes enumeration under the eHC.
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