22 Overview of USB

Host Software

The Previous Chapter

The previous chapter introduced the concept of device classes and discussed their role within the USB. The chapter introduced the class types and provided a more detailed summary of the audio, mass storage, monitor, and communications classes. These classes were discussed to provide the reader with a sense of the information defined for each class and the USB mechanisms that they use. A detailed discussion of device classes requires in-depth knowledge in the associated field such as telephony and audio, and is outside the scope of this book.

This Chapter

Host software consists of three types of components: the USB Device Drivers, the USB Driver, and the host controller driver. This chapter discusses the role of each of these layers and describes the requirements of their programming interface.

USB Software

Host software provides the interface between USB device drivers (or client drivers) and the devices that they must communicate with. USB device drivers are unaware of the USB implementation. That is, they have no knowledge of the characteristics, capabilities, or limitations of the USB nor of the USB device. Consequently, USB host software must accept transfer requests made by USB device drivers and perform the requested transfers based on the requirements of the USB. The following general capabilities are supported by a USB system.

•USB Interface Control

•Configuration Services

•Bus and Device Management

•Power Control

421

USB System Architecture

•Device Data Access

•Event Notification

•Collection of Status and Activity Statistics

•Error Detection and Handling

USB software is based on the device framework established by the USB specification. This framework describes the logical view that each software element has of the USB devices. The relationships between the USB software layers and their view of USB devices are reviewed below. Refer to Figure 22-1 on page 423.

Function Layer

Transactions performed over the USB are initiated by USB device drivers (client drivers). During configuration, the hub client accesses the bus, and when accessing other USB devices, the client drivers may be class-specific or vendorspecific. No matter which USB client driver wishes to access a given USB device, it must use host software to request its I/O transfer be performed over USB (via an I/O request packet, or IRP). These clients only have knowledge of the device interface (consisting of a collection of endpoints) that they wish to manipulate. Therefore a USB client driver’s visibility to USB is limited to:

•the interface within their device

•class-specific descriptors that have been pre-defined to help them determine specific characteristics of their interface

•the mechanisms provided by host software to access and control their function

Device Layer

Client initiated transfers must be performed according to the characteristics of the USB and the capabilities of the target USB device. USB host software exists to support USB clients by providing services that they can use to initiate transfers and control their devices.

Host software also ensures that the bus can support all devices attached to the USB. The host software views each device through its standard device descriptors. These descriptors provide the necessary information to determine which driver will use this device and how much of the bus bandwidth is required. With this knowledge, host software can establish the communications pipes that the clients will later use when they access their interface.

422

Chapter 22: Overview of USB Host Software

Host software must also forward IRPs to the host controller driver (HCD), which has specific knowledge of the host controller design. The HCD passes the IRPs on to the host controller in a form that it understands.

Interface Layer

This layer is represented by the host controller (including the root hub), the USB cable(s), and the device’s USB interface. These components actually transmit control and data information to/from the USB devices.

Figure 22-1: Device Framework — Software’s View of Hardware

423

USB System Architecture

The Software Components

The three software components that compose the host USB software solution are illustrated in Figure 22-2 on page 425. The primary functions associated with each layer include the following (note, however, that the exact division of responsibility is not precisely defined by the specification):

USB Client Drivers — client drivers are the software entities that control a given USB functional device. Client drivers must exist for each type (class) of function attached to the USB. These drivers are unaware of the details associated with the USB transfer mechanisms and must rely on USB host software to manage their transfer requests based on the capabilities and limitations of the USB. The intended implementation of client drivers is based on device class definitions. Client drivers view the USB devices as a collection of endpoints that can be accessed to control and communicate with their function.

USB Driver (USBD) — The USB driver has knowledge of the devices requirements (via device descriptors), as well as knowledge of the USB’s capabilities. With this knowledge the USBD must divide IRPs into USBand device-sized chunks. The USBD also supports USB device configuration by ensuring that the USB resources required by each device can be supported. It also establishes communications pipes for each endpoint detected during configuration, but only if the necessary USB bandwidth is available. The USBD provides a programming interface called USBDI (USB driver interface), giving client drivers a way to request transfers be performed to or from their USB function. A variety of client services are provided by the USB driver to assist the USB client in controlling and accessing its function.

USB Host Controller Driver (HCD) — two implementations of USB Host Controllers have been defined: the Open Host Controller and Universal Host Controller. Consequently, two Host Controller Drivers must be implemented if each controller is to be supported by host software. The Host Controller Driver provides the low level support for the USB by converting IRPs into individual transactions to be performed via the USB. The programming interface between the USB Driver and the Host Controller Driver is implementation specific and is not addressed by the USB specification.

Note that the USB specification does not define the exact division of duties that the USBD and HCD are responsible for. Subsequent chapters define the basic requirements of their programming interfaces; however, the exact implementation of these interfaces is operating system dependent.

424

Chapter 22: Overview of USB Host Software

Figure 22-2: Software Layers

425