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 2006, QNX Software Systems GmbH & Co. KG. The USB stack and library

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).

November 2, 2006 Chapter 2 Overview 9

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