Chapter 13. USB Devices

Written by Nick Hibma.
Modifications for Handbook made by Murray Stokely.
Table of Contents
13.1. Introduction
13.2. Host Controllers
13.3. USB Device Information
13.4. Device Probe and Attach
13.5. USB Drivers Protocol Information

13.1. Introduction

The Universal Serial Bus (USB) is a new way of attaching devices to personal computers. The bus architecture features two-way communication and has been developed as a response to devices becoming smarter and requiring more interaction with the host. USB support is included in all current PC chipsets and is therefore available in all recently built PCs. Apple's introduction of the USB-only iMac has been a major incentive for hardware manufacturers to produce USB versions of their devices. The future PC specifications specify that all legacy connectors on PCs should be replaced by one or more USB connectors, providing generic plug and play capabilities. Support for USB hardware was available at a very early stage in NetBSD and was developed by Lennart Augustsson for the NetBSD project. The code has been ported to FreeBSD and we are currently maintaining a shared code base. For the implementation of the USB subsystem a number of features of USB are important.

Lennart Augustsson has done most of the implementation of the USB support for the NetBSD project. Many thanks for this incredible amount of work. Many thanks also to Ardy and Dirk for their comments and proofreading of this paper.

  • Devices connect to ports on the computer directly or on devices called hubs, forming a treelike device structure.

  • The devices can be connected and disconnected at run time.

  • Devices can suspend themselves and trigger resumes of the host system

  • As the devices can be powered from the bus, the host software has to keep track of power budgets for each hub.

  • Different quality of service requirements by the different device types together with the maximum of 126 devices that can be connected to the same bus, require proper scheduling of transfers on the shared bus to take full advantage of the 12Mbps bandwidth available. (over 400Mbps with USB 2.0)

  • Devices are intelligent and contain easily accessible information about themselves

The development of drivers for the USB subsystem and devices connected to it is supported by the specifications that have been developed and will be developed. These specifications are publicly available from the USB home pages. Apple has been very strong in pushing for standards based drivers, by making drivers for the generic classes available in their operating system MacOS and discouraging the use of separate drivers for each new device. This chapter tries to collate essential information for a basic understanding of the USB 2.0 implementation stack in FreeBSD/NetBSD. It is recommended however to read it together with the relevant 2.0 specifications and other developer resources:

13.1.1. Structure of the USB Stack

The USB support in FreeBSD can be split into three layers. The lowest layer contains the host controller driver, providing a generic interface to the hardware and its scheduling facilities. It supports initialisation of the hardware, scheduling of transfers and handling of completed and/or failed transfers. Each host controller driver implements a virtual hub providing hardware independent access to the registers controlling the root ports on the back of the machine.

The middle layer handles the device connection and disconnection, basic initialisation of the device, driver selection, the communication channels (pipes) and does resource management. This services layer also controls the default pipes and the device requests transferred over them.

The top layer contains the individual drivers supporting specific (classes of) devices. These drivers implement the protocol that is used over the pipes other than the default pipe. They also implement additional functionality to make the device available to other parts of the kernel or userland. They use the USB driver interface (USBDI) exposed by the services layer.

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