Belkin F8E211 Betriebsanweisung PDF herunterladen (Seite 21)

The USB stack and library

USB (Universal Serial Bus) is a hardware and protocol speciﬁcation for

interconnecting various devices to a host controller. We supply a USB stack that

implements the USB protocol and allows user-written class drivers to communicate

with USB devices.

We also supply a USB driver library (usbd

*()) for class drivers to use in order to

communicate with the USB stack. Note that a class driver can be considered a “client”

of the USB stack.

The stack is implemented as a standalone process that registers the pathname of

/dev/io-usb/io-usb (by default). Currently, the stack contains the hub class driver

within it.

Host Controller Interface (HCI) types

The stack supports the three industry-standard HCI types:

• Open Host Controller Interface (OHCI)

• Universal Host Controller Interface (UHCI)

• Enhanced Host Controller Interface (EHCI)

We provide separate servers for each type (

devu-ohci.so, devu-uhci.so, and

devu-ehci.so). Note that USB devices don’t care whether a computer has an OHCI,

UHCI, or an EHCI controller.

Data buffers

The client library provides functions to allocate data buffers in shared memory; the

stack manages these data buffers and gives the client library access to them. This

means that all data transfers must use the provided buffers.

As a result, a class driver must reside on the same physical node as the USB stack. The

clients of the class driver, however, can be network-distributed. The advantage of this

approach is that no additional memory copy occurs between the time that the data is

received by the USB stack and the time that it’s delivered to the class driver (and vice

versa).

USB enumerator

With the QNX Neutrino OS, the USB enumerator attaches to the USB stack and waits

for device insertions. When a device insertion is detected, the enumerator looks in the

conﬁguration manager’s database to see which class driver it should start. It then starts

the appropriate driver, which provides for that class of device. For example, a USB

Ethernet class driver would register with

io-net and bring the interface up.

For small, deeply embedded systems, the enumerator isn’t required. The class drivers

can be started individually — they’ll wait around for their particular devices to be

detected by the stack. At that point, they’ll provide the appropriate services for that

September 10, 2007 Chapter 2 • Overview 7

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