Home Networking Tutorials

1
A Tour to USB Driver Development in Linux K.
Sebi Kumar Consultant Embedded Systems
Group Talent Transformation
2
OUTLINE

• Introduction
• Architecture Overview
• USB Data Flow Model
• USB Transfer Types
• USB Electrical Model
• Enumeration
• Writing USB Drivers

3
Introduction to USB

• Low Cost
• Ease of use
• Port expansion
• Cable Power
• Error Detection and Recovery
• Support 4 types of transfers

4
USB Device View
A
B
Hub
USB connectors
Dongle
5
Architecture Overview

• USB Bus Topology
• 7 Tiers ( Include HOST )
• Up to 127 Devices

6
USB Data Flow Model

• Endpoints
• Endpoint Zero
• Non-Endpoint Zero
• Pipes
• Message pipe
• Stream pipe
• Frames and MicroFrames
• Frame ? 1ms
• MicroFrame ? 125 us ( High Speed )

7
USB Data Flow Model
Client Software
Host
Buffers
Data Flows
Pipes
Endpoints
USB Device
Interface
8
Transfer types

• Isochronous (e.g. Audio, Telephony, Motion
  camera...)
• Interrupt (e.g. Mouse, Joystick....)
• Bulk (e.g. Printer, Scanner, Still Camera.....)
• Control (e.g. Configuration, Messages)
• Bursty, bi-directional, higher level protocol
• Used for bus management, configuration, device
  control

9
USB scheduling and priority

• up to 90 of bandwidth for isochronous and
  interrupt data
• up to 10 for control
• remaining go to bulk transfers
• Priority-based scheduling
• isochronous data has highest priority,
• interrupt,
• control,
• bulk has lowest priority.

10
USB Electrical Model
11
Enumeration steps -

• 1)The user plugs a device into USB port.
• 2)Hub detects the device
• 3)Host learns of new device
• 4)Hub resets the device.
• 5) Hub establishes a single path between device
  and a bus.

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Enumeration (cont.)

• 6) The hub detects the device speed
• 7) The host sends a GET_Descriptor request to
  learn the maximum packet size of default pipe.
• 8) The host assigns a unique address to device by
  sending a set_address request.
• 9) The host learns about device abilities -
• Hubs are also USB devices, and host enumerates
  the newly attached hub in the same way as device.

13
Linux USB Subsystem
14
Device Driver Framework

• USB device drivers are registered and
  deregistered at the subsystem.
• A driver must register 2 entry points and its
  name.
• For certain USB devices, a driver registers file
  operation and minor number.

15

• struct usb_driver
• const char name
• void (probe)(struct usb_device , unsigned
  int,
• const struct usb_device_id id_table)
• void (disconnect)(struct usb_device , void )
• struct list_head driver_list
• struct file_operations fops
• int minor
• struct semaphore serialize
• int (ioctl) (struct usb_device dev, unsigned
  int code,
• void buf)
• const struct usb_device_id id_table

16
Entry Points Framework

• USB driver has two entry points to normal
• drivers -
• void probe ( struct usb device dev, unsigned
• int interface, const struct usb_device_id
  id table)
• -This entry point is called whenever a new
  device is attached to the bus.
• void disconnect ( struct usb device dev,
• void drv context)
• - This function is called whenever a device is
  disconnected.

17
Registering a USB Driver

• We register our USB driver with USB
• subsystem using
• usb_register (struct usb_driver u_drv)
• - This is usually invoked in our init_module().
• Un-registering of the USB driver is usually
• performed using -
• usb_deregister (struct usb_driver drv)
• -This is usually invoked in our
  cleanup_module().

18
Hot plug

• static struct usb_device_id skel_table
• USB_DEVICE(USB_SKEL_
  VENDOR_ID,
• USB_SKEL_PRODUCT_ID)
  ,
• 
  / Terminating entry /
• MODULE_DEVICE_TABLE (usb, skel_table)

19
Descriptor Explained

• A USB device when attached will be
• enumerated.
• Enumeration means assigning the device a
• unique number.
• The unique number must range from 1-127.
• A descriptor is basically a structure containing
  information about the device and its properties.

20
USB Descriptors
21
USB Request Block

• A driver passes messages to the USB subsystem
  using URB
• A URB consists of relevant information for
  executing a USB transaction.
• It delivers the data and status back.
• Execution of an URB is an asynchronous operation.
  Ongoing transfer for a particular URB can be
  cancelled at any time.
• Each URB has a completion handler.
• URBs can be linked.

22
Latest developments in USB

• USB 2.0
• Low Speed ( 1.5Mbps )
• Full Speed ( 12Mbps )
• High Speed ( 480 Mbps )
• USB On-The-Go Supplement to 2.0
• Defines a Point to Point communication protocol
  for USB

23
References

• www.usb.org
• Universal Serial Bus Specification Revision 2.0
  April 27, 2000
• On-The-Go Supplement to the USB 2.0 Specification
  Revision 1.0
• USB On-The-Go Implement / Software / Protocol /
  Electrical and Mechanical Feb, 2002

24

• Thank You
• Sebikumar.kuruvilla_at_wipro.com