USB

• USB-IF(USB Implementers Forum)
• USB Protocol Specification
• Linux USB
  • The Linux USB sub-system
• USB Central Information, tools, and links to material about the Universal Serial Bus (USB)

• Introduction
  • USB in a NutShell
  • An Introduction to USB Development
  • What is a USB?

• /proc/bus/usb/devices
  • usbview - Graphical display
  • USBView - Linux USB stuff
  • usbtree

• USB On-The-Go(USB OTG)
  • USB On-The-Go Basics

Session Request Protocol(SRP)

• control when the link's power session is active

Host Negotiation Protocol (HNP)

• exchange their Host/Device roles

• Transaction

Bulk Transaction

• Token(IN/OUT/PING[high speed])
• Data(DATA0/DATA1)
• Handshake(ACK/NAK/STALL/NYET[high speed])

Setup Transaction

• Token(SETUP)
• Data(DATA0)
• Handshake(ACK)

Interrupt Transaction

• Token(IN/OUT)
• Data(DATA0/DATA1)
• Handshake(ACK/NAK/STALL)

Isochronous Transaction

• Token(IN/OUT)
• Data(DATAx)

• Data Flow Types

Control transfers

• Used to configure the device, get information about it, send commands to it, retrieve status information.
• reliable short data packets
• Control Read/Write Sequences
  • setup stage(Setup Transaction)
  • data stage(Bulk transaction)
  • status stage(Bulk Transaction)

Bulk transfers

• reliable data packets up to the full bus bandwidth
• used for printers, storage or network devices

Interrupt transfers

• Transfer small amounts of data at a fixed rate each time the hosts asks the device for data.
• Guaranteed, reserved bandwidth.
• similar to bulk transfers which are polled periodically
• such as USB mice and keyboards
• Require constant polling from the host

Isochronous transfers

• with guaranteed bus bandwidth but without any reliability
• for audio and video devices

• USB Descriptor Hierarchy
  • <kernel src>/include/linux/usb/ch9.h
    • defined in chapter 9 of the USB 2.0 specification and in the Wireless USB 1.0 (spread around).

Device Descriptor

• Represent the devices connected to the USB bus
• A USB device has only one device descriptor
• struct usb_device
• struct usb_device_descriptor
  • bMaxPacketSize0
    • the maximum packet size for the Default Control Pipe

Configuration Descriptor

• Represent the state of the device
• Examples
  • Active
  • Standby
  • Initialization
• A USB device has one or more configuration descriptors
• struct usb_host_config
• struct usb_config_descriptor

Interface Descriptor

• Represent logical devices; Each interface encapsulates a single highlevel function (USB logical connection)
• Example
  • USB webcam's possible interfaces
    • video stream
    • audio stream
    • keyboard (control buttons)
• A configuration provides one or more interfaces
• One usb_class_driver is needed for each interface.
  • register/unregister by usb_register_dev/usb_deregister_dev
• each USB interface may have different parameter settings. Example: different bandwidth settings for an audio interface. The initial state is in the first setting, (number 0).
• struct usb_interface
  • struct usb_host_interface *cur_altsetting;
  • struct usb_host_interface *altsetting;
    • interface->alsetting[i]->endpoint[j]->desc (to access the usb_endpoint_descriptor structure for each of its endpoints)
  • unsigned int num_altsetting;
  • int minor;
    • Minor number this interface is bound to.
• struct usb_interface_descriptor

Endpoint Descriptor

• Represent unidirectional communication pipes
• A interface has zero or more endpoint descriptors describing a unique set of endpoints within the configuration.
• An endpoint represents a logical data source or sink of a USB device
• Endpoint zero/Control endpoint
  • is used for all standard control transfers and there is never a descriptor for this endpoint
  • The pipe that consists of the two endpoints with endpoint number zero is called the Default Control Pipe.
  • used to configure the device at insertion time

• struct usb_endpoint_descriptor
  • wMaxPacketSize