USB Drivers

1
USB Drivers

• Ted Baker ? Andy Wang
• COP 5641 / CIS 4930

2
USB Device Basics

• Universal Serial Bus (USB) connects between a
  computer and peripheral devices
• Created to replace various slow buses (parallel,
  serial, and keyboard connections)
• Can support up to 480Mb/s
• A tree built out of point-to-point links
• Links are four-wire cables (ground, power, and
  two signal wires)

3
USB Device Basics

• A USB device can never start sending data without
  first being asked by the host controller
• Single-master implementation
• Host polls various devices
• A device can request a fixed bandwidth (for audio
  and video I/O)

4
USB Device Basics

• USB protocol defines a set of standards that any
  device can follow
• No need to write a driver for a device that is in
  a predefined class and follows that standard,
• Predefined classes storage devices, keyboards,
  mice, joysticks, network devices, and modems
• No defined standard for video devices and
  USB-to-serial devices
• A driver is needed for every device

5
USB Device Basics

• Linux supports two types of USB drivers
• Drivers on a host system
• Control the USB devices that are plugged into it
• Drivers on a device (USB gadget drivers)
• Control how that single device looks to the host
  computer as a USB device

6
USB Device Basics
7
USB Device Basics
USB drivers bind to interfaces, not the device
8
USB Overview

• A USB device has one or more configurations
• E.g., power and bandwidth requirements
• A configuration has one or more interfaces
• E.g., audio data, knobs for speakers
• An interface has one or more settings
• Different quality of services
• E.g., different frame sizes for digital cameras
• Also zero or more endpoints
• E.g., bulk, interrupt endpoints.

9
Endpoints

• The most basic form of USB communication is
  through an endpoint
• Carries data in one direction
• From the host to device (OUT endpoint)
• From the device to the host (IN endpoint)

10
Endpoints

• Four endpoint types
• CONTROL
• Used for configuring the device, retrieving
  information and status about the device, or
  sending commands to the device
• Every device has a control endpoint called
  endpoint 0
• Used by USB core to configure the device at
  insertion time
• Transfers are guaranteed with reserved bandwidth

11
Endpoints

• INTERRUPT
• Transfer small amounts of data at a fixed rate
• For USB keyboards and mice
• Also used to control the device
• Not for large transfers
• Guaranteed reserved bandwidth

12
Endpoints

• BULK
• Transfer large amounts of data
• No data loss
• Not time guaranteed
• A BULK packet might be split up across multiple
  transfers
• Used for printers, storage, and network devices

13
Endpoints

• ISOCHRONOUS
• Transfer large amount of data
• No guarantees (potential data loss)
• For real-time data collections, A/V devices
• Control and bulk endpoints are used for
  asynchronous data transfers
• Interrupt and isochronous endpoints are periodic
  with reserved bandwidth

14
Endpoints

• Endpoint information is in struct
  usb_endpoint_descriptor
• embedded in struct usb_host_endpoint
• Some important fields
• bEndpointAddress (8-bit)
• Use USB_DIR_OUT and USB_DIR_IN bit masks to
  determine the direction of data flow

15
Endpoints

• bmAttributes
• Type of the endpoint
• USB_ENDPOINT_XFERTYPE_MASK to determine if the
  endpoint is of type USB_ENDPOINT_XFER_ISOC,
  USB_ENDPOINT_XFER_BULK, or USB_ENDPOINT_XFER_INT
• wMaxPacketSize
• Maximum bytes that an endpoint can handle
• Larger transfers will be split into multiple
  transfers

16
Endpoints

• bInterval
• For interrupt endpoints, this value specifies the
  milliseconds between interrupt requests for the
  endpoint

17
Interfaces

• USB endpoints are bundled into interfaces
• A interface handles only one type of logical
  connection (e.g., a mouse)
• Some devices have multiple interfaces
• E.g., a speaker
• One interface for buttons and one for audio
  stream
• USB interface may have alternate settings
• E.g., different settings to reserve different
  amounts of bandwidth for the device

18
Interfaces

• Described via struct usb_interface
• Passed from USB core to USB drivers
• Some important fields
• struct usb_host_interface altsetting
• An array of settings for this interface
• unsigned num_altsetting
• Number of alternative settings

19
Interfaces

• struct usb_host_interface cur_altsetting
• A pointer into the altsetting array, denoting the
  current setting
• int minor
• Minor number assigned by the USB core to the
  interface
• Valid after a successful call to usb_register_dev

20
Configurations

• USB interfaces are bundled into configurations
• A USB device can have multiple configurations
• Might switch between them
• Described in struct usb_host_config
• embedded in struct usb_device

21
USB and Sysfs

• Both USB devices and its interfaces are shown in
  sysfs as individual devices
• A USB mouse device can be represented as
• /sys/devices/pci000000/00000009.0/usb2/2-1
• The interface of the USB mouse device driver is
  represented as
• /sys/devices/pci000000/00000009.0/usb2/2-1/2-1
  1.0

root_hub-hub_portconfiguration.interface
22
USB and Sysfs

• For a two-level USB connection, the device name
  is in the following format
• root_hub-hub_port-hub_portconfiguration.interf
  ace
• In the sysfs directory, all USB information is
  available
• E.g., idVendor, idProduct, bMaxPower
• bConfigurationValue can be written to change the
  active configuration

23
USB and Sysfs

• More information is available in /proc/bus/usb
  directory
• User-space programs can directly communicate with
  USB devices via the directory

24
USB Urbs (USB Request Block)

• struct urb
• Used to send and receive data between endpoints
• Asynchronous
• Defined in ltinclude/linux/usb.hgt
• Dynamically created
• Contains reference count for garbage collection
• Designed to achieve high throughput

25
USB Urbs (USB Request Block)

• A typical lifecycle of a urb
• A USB device driver creates a urb
• Assigns it to a specific endpoint of a device
• Submits it to the USB core
• The USB core submits the urb to specific USB host
  controller driver
• The USB host controller driver processes the urb
  and transfers it to the device
• Notifies the USB device driver when the urb is
  done

26
USB Urbs (USB Request Block)

• A urb can be cancelled by the driver or the USB
  core if the device is removed from the system

27
struct urb

• Important fields
• / destination USB device /
• / must be initialized by the USB driver before
  the urb can be sent to the USB core /
• struct usb_device dev
• / end point type information /
• / set to the return value from one of the usb
  send and receive pipe functions /
• / must be initialized by the USB driver before
  the urb can be sent to the USB core /
• unsigned int pipe

28
struct urb

• / assigned to one of the transfer flags /
• unsigned int transfer_flags
• void transfer_buffer / points to a kmalloced
  buffer /
• dma_addr_t transfer_dma / buffer for DMA
  transfers /
• / buffer length for either the transfer_buffer
  or the transfer_dma variable, 0 if neither
  buffers are used /
• int transfer_buffer_length
• / pointer to a setup packet for a control urb /
• / transferred before the data in the transfer
  buffer /
• unsigned char setup_packet
• / DMA buffer for the setup packet for a control
  urb /
• dma_addr_t setup_dma

29
struct urb

• / pointer to the completion handler called by
  USB core /
• usb_complete_t complete
• / pointer to a data blob that can be set by the
  USB driver /
• void context
• / actual length of data sent/received by the urb
  /
• int actual_length
• / accessed in the completion handler /
• / see status values /
• int status
• / the initial frame number for isochronous
  transfers /
• int start_frame

30
struct urb

• / polling interval for the urb /
• / valid only for interrupt or isochronous urbs
  /
• / for slow devices, the unit is in frames or
  milliseconds /
• / for other devices, the unit is in 1/8
  milliseconds /
• int interval
• / the number of isochronous transfer buffers
  handled by this urb /
• / must be set by the USB driver before the urb
  is sent to the USB core /
• int number_of_packets
• / number of isochronous transfers with errors /
• int error_count

31
struct urb

• / allows a single urb to define a number of
  isochronous transfers at once /
• struct usb_iso_packet_descriptor
  iso_frame_desc0
• struct usb_iso_packet_descriptor
• unsigned int offset / byte into the transfer
  buffer /
• unsigned int length / length of the transfer
  buffer /
• / length of data received into the transfer
  buffer /
• unsigned int actual_length
• unsigned int status / see status values /

32
USB send and receive pipe functions

• / specifies a control OUT endpoint for the
  specified USB device with the specified endpoint
  number /
• unsigned int usb_sndctrlpipe(struct usb_device
  dev,
• unsigned int
  endpoint)
• / specifies a control IN endpoint for the
  specified USB device with the specified endpoint
  number /
• unsigned int usb_rcvctrlpipe(struct usb_device
  dev,
• unsigned int
  endpoint)
• / specifies a bulk OUT endpoint for the
  specified USB device with the specified endpoint
  number /
• unsigned int usb_sndbulkpipe(struct usb_device
  dev,
• unsigned int
  endpoint)

33
USB send and receive pipe functions

• / specifies a bulk IN endpoint for the specified
  USB device with the specified endpoint number /
• unsigned int usb_rcvbulkpipe(struct usb_device
  dev,
• unsigned int
  endpoint)
• / specifies a interrupt OUT endpoint for the
  specified USB device with the specified endpoint
  number /
• unsigned int usb_sndintpipe(struct usb_device
  dev,
• unsigned int
  endpoint)
• / specifies a interrupt IN endpoint for the
  specified USB device with the specified endpoint
  number /
• unsigned int usb_rcvintpipe(struct usb_device
  dev,
• unsigned int
  endpoint)

34
USB send and receive pipe functions

• / specifies a isochronous OUT endpoint for the
  specified USB device with the specified endpoint
  number /
• unsigned int usb_sndisocpipe(struct usb_device
  dev,
• unsigned int
  endpoint)
• / specifies a isochronous IN endpoint for the
  specified USB device with the specified endpoint
  number /
• unsigned int usb_rcvisocpipe(struct usb_device
  dev,
• unsigned int
  endpoint)

35
Transfer flags

• URB_SHORT_NOT_OK
• Partial read should be treated as an error by the
  USB core
• URB_ISO_ASAP
• If the driver wants the isochronous urb to be
  scheduled as soon as bandwidth allows
• Set the start_frame variable

36
Transfer flags

• URB_NO_TRANSFER_DMA_MAP
• Set when the urb contains a DMA buffer to be
  transferred
• Tells the USB core to use the buffer pointed by
  the transfer_dma pointer, not the transfer_buffer
  pointer

37
Transfer flags

• URB_NO_SETUP_DMA_MAP
• Used for control urbs with DMA buffer already set
  up
• Tells the USB core to use the buffer pointed by
  the setup_dma pointer instead of the setup_packet
  pointer
• URB_ASYNC_UNLINK
• Tells usb_unlink_urb() to return immediate and
  unlink the urb in the background

38
Transfer flags

• URB_NO_FSBR
• For UHCI USB host controller
• Generally not used
• URB_ZERO_PACKET
• Tells a bulk out urb finishes by sending an empty
  packet when the data is aligned to an endpoint
  packet boundary

39
Transfer flags

• URB_NO_INTERRUPT
• Indicates that the HW may not generate an
  interrupt when the urb is finished
• Used when queuing multiple urbs to the same
  endpoint
• Used by USB core to perform DMA transfers

40
Status Values

• 0
• The urb transfer was successful
• For isochronous urbs, only indicates whether the
  urb has been unlinked
• Detailed status in iso_frame_desc
• -ENOENT
• Urb stopped by usb_kill_urb
• -ECONNRESET
• Urb was unlinked by usb_unlink_urb
• transfer_flags set to URB_ASYNC_UNLINK

41
Status Values

• -EINPROGRESS
• Urb still being processed by the USB host
  controller
• A bug if seen at the driver level
• -EPROTO (a HW problem)
• A bitstuff error happened during the transfer
• No response packet was received
• -EILSEQ (a HW problem)
• CRC mismatch

42
Status Values

• -EPIPE
• The endpoint is now stalled
• If not a control endpoint, can clear this error
  with usb_clear_halt
• -ECOMM
• Data received faster than it could be written to
  system memory
• -ENOSR
• Data cannot be retrieved from the system memory
  during the transfer fast enough to keep up with
  the requested USB data rate

43
Status Values

• -EOVERFLOW (a HW problem)
• When the endpoint receives more data than the
  specified max
• -EREMOTEIO
• Full amount of data was not received
• Occurs when the URB_SHORT_NOT_OK is set
• -ENODEV
• The USB device is gone from the system

44
Status Values

• -EXDEV
• Only for a isochronous urb
• Transfer was partially completed
• -EINVAL
• ISO madness, if this happens Log off and go
  home
• Incorrect function parameter

45
Status Values

• -ESHUTDOWN
• Host controller driver has been disabled or
  disconnected
• Urb was submitted after the device was removed
• Configuration change while the urb was submitted

46
Creating and Destroying Urbs

• All urbs need to be created dynamically
• Or the reference count would not work
• To create a urb, call
• struct urb usb_alloc_urb(int iso_packets,
• gfp_t mem_flags)
• Returns a pointer to the urb or NULL on failure
• iso_packets number of isochronous packets this
  urb should contain
• mem_flags same as kmalloc flags
• To destroy a urb, call
• void usb_free_urb(struct urb urb)

47
Interrupt urbs

• To initialize an interrupt urb, call
• void
• usb_fill_int_urb(struct urb urb, struct
  usb_device dev,
• unsigned int pipe, void
  transfer_buffer,
• int buffer_length, usb_complete_t
  complete,
• void context, int interval)
• urb a pointer to the urb to be initialized
• dev The destination USB device
• pipe the destination endpoint of this urb

48
Interrupt urbs

• transfer_buffer a pointer to a kmalloced buffer
• buffer_length the length of the transfer buffer
• complete pointer to the completion handler
• context pointer to the blob, retrieved by the
  completion handler function
• interval scheduling interval for this urb

49
Bulk urbs

• To initialize an bulk urb, call
• void
• usb_fill_bulk_urb(struct urb urb, struct
  usb_device dev,
• unsigned int pipe, void
  transfer_buffer,
• int buffer_length, usb_complete_t
  complete,
• void context)
• Similar to interrupt urb initialization
• No interval parameter

50
Control urbs

• To initialize a control urb, call
• void
• usb_fill_control_urb(struct urb urb, struct
  usb_device dev,
• unsigned int pipe,
• unsigned char setup_packet,
• void transfer_buffer, int
  buffer_length,
• usb_complete_t complete,
  void context)
• Similar to bulk urb initialization
• setup_packet points to the setup packet data
• Does not set the transfer_flags

51
Isochronous urbs
New location for 2.6.25

• Have no initialization functions
• Need to be initialized by hand
• / from /drivers/media/video/usbvideo/konicawc.c
  /
• urb-gtdev dev
• urb-gtcontext uvd
• urb-gtpipe usb_rcvisocpipe(dev, uvd-gtvideo_endp
  - 1)
• urb-gtinterval 1
• urb-gttransfer_flags URB_ISO_ASAP
• urb-gttransfer_buffer cam-gtsts_bufi
• urb-gtcomplete konicawc_isoc_irq
• urb-gtnumber_of_packets FRAMES_PER_DESC
• urb-gttransfer_buffer_length FRAMES_PER_DESC
• for (j0 j lt FRAMES_PER_DESC j)
• urb-gtiso_frame_descj.offset j
• urb-gtiso_frame_descj.length 1

52
Submitting Urbs

• To send a urb to the USB core, call
• int usb_submit_urb(struct urb urb, gfp_t
  mem_flags)
• urb a pointer to the urb
• mem_flags same as kmalloc flags
• Should not access a submitted urb until the
  complete function is called

53
Completing Urbs The Completion Callback Handler

• Called exactly once when the urb is completed
• When this function is called, the USB core is
  finished with the urb, and control is returned to
  the device driver

54
Completing Urbs The Completion Callback Handler

• The completion handler is called under three
  conditions
• The urb is successfully sent to the device and
  acknowledged
• An error has occurred
• Check the status variable
• The urb was unlinked (the submission was
  cancelled) when a device is removed from the
  system

55
Canceling Urbs

• To stop a submitted urb, call
• int usb_kill_urb(struct urb urb)
• Used when the device is disconnected from the
  system
• int usb_unlink_urb(struct urb urb)
• Tells the USB core to stop an urb
• Returns before the urb is fully stopped
• Useful while in an interrupt handler
• Requires setting the URB_ASYNC_UNLINK

56
Writing a USB Driver

• Similar to a pci_driver
• Driver registers its driver object with the USB
  subsystem
• Later uses vendor and device identifiers to tell
  if its hardware has been installed

57
What Devices Does the Driver Support?

• struct usb_device_id lists supported types of USB
  devices
• Important fields
• __u16 match_flags
• Determines which fields in the structure the
  device should be matched against
• Check include/linux/mod_devicetable.h
• __u16 idVendor
• __u16 idProduct

58
What Devices Does the Driver Support?

• __u16 bcdDevice_lo
• __u16 bcdDevice_hi
• Define low and high ends of the range of the
  vendor-assigned product version number
• Expressed in binary-coded decimal (BCD)
• __u8 bDeviceClass
• __u8 bDeviceSubClass
• __u8 bDeviceProtocol
• Define the class, subclass, and protocol of the
  device

59
What Devices Does the Driver Support?

• __u8 bInterfaceClass
• __u8 bInterfaceSubClass
• __u8 bInterfaceProtocol
• Class, subclass, and protocol of the individual
  interface
• kernel_ulong_t driver_info
• Used to differentiate different devices in the
  probe callback function

60
What Devices Does the Driver Support?

• To initialize usb_device_id, use the following
  macros
• USB_DEVICE(vendor, product)
• Creates a usb_device_id that can be used to match
  only the specified vendor and product IDs
• USB_DEVICE_VER(vendor, product, lo, hi)
• Creates a usb_device_id that can be used to match
  only the specified vendor and product IDs within
  a version range
• USB_DEVICE_INFO(class, subclass, protocol)
• Creates a usb_device_id that can be used to match
  a specific class of USB devices

61
What Devices Does the Driver Support?

• USB_INTERFACE_INFO(class, subclass, protocol)
• Creates a usb_device_id that can be used to match
  a specific class of USB interfaces
• Example
• / table of devices that work with this driver /
• static struct usb_device_id skel_table
• USB_DEVICE(USB_SKEL_VENDOR_ID,
  USB_SKEL_PRODUCT_ID) ,
• / Terminating entry /
• / allow user-space tools to figure out what
  devices this driver can control /
• MODULE_DEVICE_TABLE(usb, skel_table)

62
Registering a USB Driver

• The main structure for a USB driver is struct
  usb_driver
• Important fields
• struct module owner
• Set to THIS_MODULE to track the reference count
  of the module owning this driver
• const char name
• Points to a unique driver name

63
Registering a USB Driver

• const struct usb_device_id id_table
• Pointer to the list of supported USB devices
• If you want your driver always be called for
  every USB device, create an entry that sets only
  the driver_info field
• static struct usb_device_id usb_ids
• .driver_info 42,

64
Registering a USB Driver

• int (probe) (struct usb_interface intf,
• const struct usb_device_id id)
• Called when the USB core thinks it has a struct
  usb_interface that this driver can handle
• The USB driver should initialize the usb
  interface and return 0, or return a negative
  error number on failure
• void (disconnect) (struct usb_interface intf)
• Called when the usb_interface has been removed
  from the system, or when the driver is being
  unloaded

65
Registering a USB Driver

• To create a struct usb_driver, only five fields
  need to be initialized
• static struct usb_driver skel_driver
• .owner THIS_MODULE,
• .name "skeleton",
• .id_table skel_table,
• .probe skel_probe,
• .disconnect skel_disconnect,

66
Registering a USB Driver

• To register a USB driver call usb_register_driver
• Example
• static int __init usb_skel_init(void)
• int result
• / register this driver with the USB subsystem
  /
• result usb_register(skel_driver)
• if (result)
• err("usb_register failed. Error number d",
  result)
• return result

67
Registering a USB Driver

• To unload a USB driver call usb_deregister
• Example
• static void __exit usb_skel_exit(void)
• / deregister this driver with the USB
  subsystem /
• / invokes disconnect() within usb_deregister()
  /
• usb_deregister(skel_driver)

68
Probe and Disconnect in Detail

• Called in the context of the USB hub kernel
  thread
• Sleep is allowed
• However, should do most of the work when the
  device is opened by a user
• USB core handles addition and removal of USB
  devices in a single thread
• A slow device driver can slow down USB device
  detection

69
Probe and Disconnect in Detail

• Probe function should
• Initialize local structures that it might use to
  manage the USB device
• Save any information that it needs to the local
  structure
• Detect endpoint address and buffer sizes
• Example usb/usb-skeleton.c

70
Probe and Disconnect in Detail

• static int skel_prob(struct usb_interface
  interface,
• const struct usb_device_id
  id)
• struct usb_skel dev NULL
• struct usb_host_interface iface_desc
• struct usb_endpoint_descriptor endpoint
• size_t buffer_size
• int i
• int retval -ENOMEM
• dev kmalloc(sizeof(struct usb_skel),
  GFP_KERNEL)
• if (dev NULL)
• err(Out of memory)
• goto error
• memset(dev, 0x00, sizeof(dev))
• kref_init(dev-gtkref)

71
Probe and Disconnect in Detail

• dev-gtudev usb_get_dev(interface_to_usbdev(inte
  rface))
• dev-gtinterface interface
• / set up the endpoint information /
• / use only the first bulk-in and bulk-out
  endpoints /
• iface_desc interface-gtcur_altsetting
• for (i 0 i lt iface_desc-gtdesc.bNumEndpoints
  i)
• endpoint iface_desc-gtendpointi.desc
• if (!dev-gtbulk_in_endpointAddr
• (endpoint-gtbEndpointAddress USB_DIR_IN)
  
• ((endpoint-gtbmAttributes
  USB_ENDPOINT_XFERTYPE_MASK)
• USB_ENDPOINT_XFER_BULK))
• / we found a bulk in endpoint /

72
Probe and Disconnect in Detail

• / we found a bulk in endpoint /
• buffer_size endpoint-gtwMaxPacketSize
• dev-gtbulk_in_size buffer_size
• dev-gtbulk_in_endpointAddr
  endpoint-gtbEndpointAddress
• dev-gtbulk_in_buffer kmalloc(buffer_size,
  GFP_KERNEL)
• if (!dev-gtbulk_in_buffer)
• err("Could not allocate bulk_in_buffer")
• goto error
• if (!dev-gtbulk_out_endpointAddr
• !(endpoint-gtbEndpointAddress
  USB_DIR_IN)
• ((endpoint-gtbmAttributes
  USB_ENDPOINT_XFERTYPE_MASK)
• USB_ENDPOINT_XFER_BULK))
• / we found a bulk out endpoint /

73
Probe and Disconnect in Detail

• / we found a bulk out endpoint /
• dev-gtbulk_out_endpointAddr
  endpoint-gtbEndpointAddress
• if (!(dev-gtbulk_in_endpointAddr
• dev-gtbulk_out_endpointAddr))
• err("Could not find both bulk-in and bulk-out
  endpoints")
• goto error
• / save our data pointer in this interface
  device /
• usb_set_intfdata(interface, dev)

Use usb_get_intfdata(interface) to retrieve dev
74
Probe and Disconnect in Detail

• / we can register the device now, as it is
  ready /
• retval usb_register_dev(interface,
  skel_class)
• if (retval)
• err(Not able to get a minor for this
  device.)
• usb_set_intfdata(interface, NULL)
• goto error
• return 0
• error
• if (dev)
• kref_put(dev-gtkref, skel_delete)
• return retval

Need to call usb_register_dev() instead of
usb_register() for input, tty, video USB devices
that are not associated with predefined types
Call usb_put_dev(), free bulk_in_buffer and dev
75
Probe and Disconnect in Detail

• The definition of skel_class
• static struct usb_class_driver skel_class
• .name "usb/skeld",
• .fops skel_fops,
• .mode S_IFCHRS_IRUSRS_IWUSRS_IRGRPS_IWGRP
  S_IROTH,
• .minor_base USB_SKEL_MINOR_BASE,
• The definition of skel_fops
• static struct file_operations skel_fops
• .owner THIS_MODULE,
• .read skel_read,
• .write skel_write,
• .open skel_open,
• .release skel_release,

The start of the assigned minor range for this
driver
76
Probe and Disconnect in Detail

• The disconnect function
• static void skel_disconnect(struct usb_interface
  interface)
• struct usb_skel dev
• int minor interface-gtminor
• / prevent skel_open() from racing
  skel_disconnect() /
• lock_kernel()
• dev usb_get_intfdata(interface)
• usb_set_intfdata(interface, NULL)
• / give back our minor /
• usb_deregister_dev(interface, skel_class)
• unlock_kernel()
• / decrement our usage count /
• kref_put(dev-gtkref, skel_delete)

The big kernel lock
All in-transit urbs are canceled by the USB core
before disconnect() is called no need to call
usb_kill_urb()
77
Submitting and Controlling a Urb

• static ssize_t skel_write(struct file file,
• const char __user
  user_buffer,
• size_t count, loff_t
  ppos)
• struct usb_skel dev
• int retval 0
• struct urb urb NULL
• char buf NULL
• if (count 0) goto exit
• dev (struct usb_skel )file-gtprivate_data
• / create a urb and its buffer copy the data
  to the urb /
• urb usb_alloc_urb(0, GFP_KERNEL)
• if (!urb)
• retval -ENOMEM
• goto error

Initialized in skel_open()
78
Submitting and Controlling a Urb

• buf usb_buffer_alloc(dev-gtudev, count,
  GFP_KERNEL,
• urb-gttransfer_dma)
• if (!buf)
• retval -ENOMEM
• goto error
• if (copy_from_user(buf, user_buffer, count))
• retval -EFAULT
• goto error
• / initialize the urb properly /
• usb_fill_bulk_urb(urb, dev-gtudev,
• usb_sndbulkpipe(dev-gtudev,
  dev-gtbulk_out_endpointAddr),
• buf, count, skel_write_bulk_callback,
  dev)
• urb-gttransfer_flags URB_NO_TRANSFER_DMA_MAP

79
Submitting and Controlling a Urb

• / send the data out the bulk port /
• retval usb_submit_urb(urb, GFP_KERNEL)
• if (retval)
• err("s - failed submitting write urb, error
  d",
• __FUNCTION__, retval)
• goto error
• / release our reference to this urb, the USB
  core will
• eventually free it entirely /
• usb_free_urb(urb)
• exit
• return count

80
Submitting and Controlling a Urb

• error
• usb_buffer_free(dev-gtudev, count, buf,
  urb-gttransfer_dma)
• usb_free_urb(urb)
• kfree(buf)
• return retval

81
Probe and Disconnect in Detail

• The callback (complete) function
• static void skel_write_bulk_callback(struct urb
  urb,
• struct
  pt_regs regs)
• / in interrupt context /
• / sync/async unlink faults aren't errors /
• if (urb-gtstatus !(urb-gtstatus -ENOENT
• urb-gtstatus -ECONNRESET
  
• urb-gtstatus
  -ESHUTDOWN))
• dbg("s - nonzero write bulk status received
  d",
• __FUNCTION__, urb-gtstatus)
• / free up our allocated buffer /
• usb_buffer_free(urb-gtdev, urb-gttransfer_buffer_l
  ength,
• urb-gttransfer_buffer,
  urb-gttransfer_dma)

82
USB Transfers Without Urbs

• usb_bulk_msg
• Creates a USB bulk urb and sends it to the
  specified device
• Waits for it to complete before returning to the
  caller
• int usb_bulk_msg(struct usb_device usb_dev,
• unsigned int pipe, void data,
  int len,
• int actual_length, int
  timeout)
• timeout
• in jiffies
• Set to 0 to wait until the message to complete

83
USB Transfers Without Urbs

• Cannot be called from within interrupt context
• Cannot be called with a spinlock held
• Cannot be cancelled by any other function
• The disconnect call needs to wait for
  usb_bulk_msg to complete before allowing itself
  to be unloaded

84
USB Transfers Without Urbs

• Example
• static ssize_t skel_read(struct file file,
• char __user buffer,
  size_t count,
• loff_t ppos)
• struct usb_skel dev
• int retval 0
• dev (struct usb_skel )file-gtprivate_data
• / do a blocking bulk read to get data from the
  device /
• retval usb_bulk_msg(dev-gtudev,
• usb_rcvbulkpipe(dev-gtudev, dev-gtbulk_in_endpoi
  ntAddr),
• dev-gtbulk_in_buffer, min(dev-gtbulk_in_size,
  count),
• count, HZ10)

85
USB Transfers Without Urbs

• / if successful read, copy the data to user
  space /
• if (!retval)
• if (copy_to_user(buffer, dev-gtbulk_in_buffer,
  count))
• retval -EFAULT
• else
• retval count
• return retval

86
usb_control_msg

• Works just like the usb_bulk_msg
• Allows a driver to send and receive USB control
  messages
• int usb_control_msg(struct usb_device dev,
• unsigned int pipe, __u8
  request,
• __u8 requesttype, __u16
  value,
• __u16 index, void data,
  __u16 size,
• int timeout)
• Returns the number of bytes transferred on
  success, or a negative error number
• Same restrictions as usb_bulk_msg

87
Other USB Data Functions

• Cannot be called from within interrupt context or
  with a spinlock held
• int usb_get_descriptor(struct usb_device dev,
• unsigned char type,
• unsigned char index,
• void buf, int size)
• int usb_get_string(struct usb_device dev,
• unsigned short langid,
  unsigned char index,
• void buf, int size)