USB overview

This article gives information about the Linux^® USB framework.
It explains how to activate USB interface and, based on examples, how to access it from user space.

1. Framework purpose

The USB (universal serial bus) Linux^® framework supports many types of:

• host controllers and peripheral devices
• gadget drivers and classes to be used within a peripheral

Linux can be used on the host machine. In this case various types of peripherals can be plugged in, such as:

• Mass storage (hard drive, USB stick..)
• HID (keyboard, mouse..)

Linux can also be used as a device on the peripheral side, using gadget drivers. In this case, it can act as:

• USB mass storage (e.g. to export some partitions, filesystem)
• ethernet card
• serial interface
• ...

2. System overview

USB Implementation architecture

2.1. Component description

• USB userland (User space)
  • Host-Side userland

  - libusb is a userland library that provides access to USB devices.

  - usbutils is a set of USB utilities for collecting information about the USB devices that are connected to the USB host. Note that usbutils depends on libusb.

  - One of the well-known utility is lsusb, used to display information about USB buses and the devices connected to them.

  • Gadget userland

  - libusbg is a userland library that provides routines for creating and parsing USB gadget devices using the configfs API.

  - Gadget configfs provides configuration interface available through user terminal, used to configure USB Gadget.

  • Common userland

  - sysfs provides an information interface available through the user terminal. See How to monitor with sysfs below.

  - debugfs provides a debugging interface available through the user terminal. See How to monitor with debugfs below.

• USB framework (Kernel space): composed of two parts, USB Host-Side and USB Gadget, which rely on the USB core with specific APIs to support USB host and devices controllers
  • Host-Side provides API interface to class drivers and forwards the request from class drivers to host controller driver.
  • Gadget requires a peripheral controller and the gadget driver to use it.

• USB controller drivers (Kernel space)
  • USB Host controller drivers such as STM32 USBH USB Host controllers in the USB Host-Side framework. STM32 USBH uses kernel community drivers (kernel space), based on the USB framework.

  - Enhanced Host Controller Interface (EHCI) driver and Generic platform ehci driver

  - Open Host Controller Interface (OHCI) driver and Generic platform ohci driver

  • USB OTG controller drivers such as STM32 OTG USB OTG controllers in the USB Host-Side framework when they are used either in otg or host mode, and/or in the USB Gadget framework when they are used either in otg or peripheral mode. STM32 OTG uses kernel community driver (kernel space), based on the USB framework.

  - DesignWare HS OTG Controller driver

  • USB controller drivers can rely on Generic PHY framework to manage the physical layer for USB data transmissions. STM32 USBPHYC PHY provider is a PHY driver in the Generic PHY framework:

  - STM32 USBPHYC driver

• USB hardware controllers (Hardware)

USB controllers such as STM32 USBH internal peripheral and STM32 OTG internal peripheral, using an on-chip High-Speed UTMI+ PHY (STM32 USBPHYC internal peripheral), or on-chip Full-Speed PHY for STM32 OTG internal peripheral.

• USB devices (External USB devices)
  • USB OTG specification defines two roles for USB devices: A-Device and B-Device. STM32 OTG controller, depending on the USB connector which is used, can accept both A-Device and B-Device, while STM32 USBH Host controller only manages B-Device:

  - A-Device is a power supplier acting as a USB Host (e.g. a PC)

  - B-Device is a power consumer and acts as a USB Peripheral (e.g. a USB key).

2.2. API description

See USB kernel documentation for more details on API functions.

3. Configuration

3.1. Kernel configuration

USB support, STM32 USBH driver and STM32 OTG driver are activated by default in ST deliveries. Nevertheless, if a specific configuration is required, this section indicates how the USB framework can be activated/deactivated in the kernel.

Activate USB support (CONFIG_USB=y) in the kernel configuration with the Linux Menuconfig tool: Menuconfig or how to configure kernel then select:

Device Drivers  --->
 [*] USB support  --->

Then activate USB controllers drivers.

To activate the STM32 USBH driver, select:

Device Drivers  --->
   --- USB support
   <*>   Support for Host-side USB
   <*>   EHCI HCD (USB 2.0) support
   <*>     Generic EHCI driver for a platform device
   <*>   OHCI HCD (USB 1.1) support
   <*>     Generic OHCI driver for a platform device

To activate the STM32 OTG driver, select:

Device Drivers  --->
   --- USB support
   <*>   Support for Host-side USB
   <*>   USB Gadget Support  --->
   <*>   DesignWare USB2 DRD Core Support
           DWC2 Mode Selection (Dual Role mode)  --->

Then to activate the STM32 USBPHYC driver, select:

PHY Subsystem  --->
  -*- PHY Core
  <*> STMicroelectronics STM32 USB HS PHY Controller driver

3.2. Device tree configuration

Detailed DT configurations for STM32 USB internal peripherals:

• for STM32 USBH Host controller: USBH device tree configuration
• for STM32 OTG controller: OTG device tree configuration
• for STM32 USBPHYC PHY: USBPHYC device tree configuration

4. How to use the framework

4.1. How to list USB devices

lsusb displays information about the attached USB buses and devices.
In the example above, we have an onboard hub, a USB mouse and USB keyboard plugged into the hub.

 lsusb							/* root hubs correspond to STM32 USB controllers (USBH,  OTG) */
Bus 002 Device 005: ID 413c:2003 Dell Computer Corp. Keyboard
Bus 002 Device 004: ID 046d:c016 Logitech, Inc. Optical Wheel Mouse
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 002 Device 002: ID 0424:2514 Standard Microsystems Corp. USB 2.0 Hub
Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub

 lsusb -t 	 	 	 	 	 	/* lsusb -t shows the USB class, the driver used and the number of ports and speed of each USB devices */
/:  Bus 02.Port 1: Dev 1, Class=root_hub, Driver=ehci-platform/2p, 480M
    |__ Port 1: Dev 2, If 0, Class=Hub, Driver=hub/4p, 480M
        |__ Port 1: Dev 5, If 0, Class=Human Interface Device, Driver=usbhid, 1.5M
        |__ Port 3: Dev 4, If 0, Class=Human Interface Device, Driver=usbhid, 1.5M
/:  Bus 01.Port 1: Dev 1, Class=root_hub, Driver=dwc2/1p, 480M

To limit lsusb to the USB keyboard:

 lsusb -s 002:005 	 				/* lsusb -s [Bus]:[Device] */
Bus 002 Device 005: ID 413c:2003 Dell Computer Corp. Keyboard
 lsusb -d 413c:2003  	 			 	/* lsusb -d [ID] */
Bus 002 Device 005: ID 413c:2003 Dell Computer Corp. Keyboard

To limit lsusb to the USB keyboard and display its descriptors:

 lsusb -D /dev/bus/usb/002/005  	 		/* lsusb -D /dev/bus/usb/[Bus]/[Device] */
Device: ID 413c:2003 Dell Computer Corp. Keyboard
Device Descriptor:
 ...

4.2. How to mount a USB key (mass-storage)

 mkdir /usb
 mount /dev/sdxx /usb

4.3. How to configure USB Gadget through configfs

See USB Gadget configfs documentation for an introduction to USB gadget configfs structure and how to use it to configure Linux USB Gadget.

Here is an example to configure USB Gadget through configfs to use the OTG as a USB Ethernet Gadget with Remote NDIS (RNDIS). See: stm32_usbotg_eth_config.sh .

5. How to trace and debug the framework

5.1. How to monitor

5.1.1. How to monitor with debugfs

Please refer to the USB devices chapter to decode the output.

 cat /sys/kernel/debug/usb/devices

T:  Bus=01 Lev=00 Prnt=00 Port=00 Cnt=00 Dev#=  1 Spd=480  MxCh= 1
B:  Alloc=  0/800 us ( 0%), #Int=  0, #Iso=  0
D:  Ver= 2.00 Cls=09(hub  ) Sub=00 Prot=01 MxPS=64 #Cfgs=  1
P:  Vendor=1d6b ProdID=0002 Rev= 4.14
S:  Manufacturer=Linux 4.14.0 dwc2_hsotg
S:  Product=DWC OTG Controller
S:  SerialNumber=49000000.usb-otg
C:* #Ifs= 1 Cfg#= 1 Atr=e0 MxPwr=  0mA
I:* If#= 0 Alt= 0 #EPs= 1 Cls=09(hub  ) Sub=00 Prot=00 Driver=hub
E:  Ad=81(I) Atr=03(Int.) MxPS=   4 Ivl=256ms

T:  Bus=01 Lev=01 Prnt=01 Port=00 Cnt=01 Dev#=  4 Spd=480  MxCh= 0
D:  Ver= 2.00 Cls=00(>ifc ) Sub=00 Prot=00 MxPS=64 #Cfgs=  1
P:  Vendor=05e3 ProdID=0723 Rev=94.54
S:  Manufacturer=Generic 
S:  Product=USB Storage
C:* #Ifs= 1 Cfg#= 1 Atr=80 MxPwr=500mA
I:* If#= 0 Alt= 0 #EPs= 2 Cls=08(stor.) Sub=06 Prot=50 Driver=usb-storage
E:  Ad=81(I) Atr=02(Bulk) MxPS= 512 Ivl=0ms
E:  Ad=02(O) Atr=02(Bulk) MxPS= 512 Ivl=0ms

T:  Bus=02 Lev=00 Prnt=00 Port=00 Cnt=00 Dev#=  1 Spd=480  MxCh= 2
B:  Alloc=  0/800 us ( 0%), #Int=  2, #Iso=  0
D:  Ver= 2.00 Cls=09(hub  ) Sub=00 Prot=00 MxPS=64 #Cfgs=  1
P:  Vendor=1d6b ProdID=0002 Rev= 4.14
S:  Manufacturer=Linux 4.14.0 ehci_hcd
S:  Product=EHCI Host Controller
S:  SerialNumber=5800d000.usbh-ehci
C:* #Ifs= 1 Cfg#= 1 Atr=e0 MxPwr=  0mA
I:* If#= 0 Alt= 0 #EPs= 1 Cls=09(hub  ) Sub=00 Prot=00 Driver=hub
E:  Ad=81(I) Atr=03(Int.) MxPS=   4 Ivl=256ms

T:  Bus=02 Lev=01 Prnt=01 Port=00 Cnt=01 Dev#=  2 Spd=480  MxCh= 4
D:  Ver= 2.00 Cls=09(hub  ) Sub=00 Prot=02 MxPS=64 #Cfgs=  1
P:  Vendor=0424 ProdID=2514 Rev= b.b3
C:* #Ifs= 1 Cfg#= 1 Atr=e0 MxPwr=  2mA
I:  If#= 0 Alt= 0 #EPs= 1 Cls=09(hub  ) Sub=00 Prot=01 Driver=hub
E:  Ad=81(I) Atr=03(Int.) MxPS=   1 Ivl=256ms
I:* If#= 0 Alt= 1 #EPs= 1 Cls=09(hub  ) Sub=00 Prot=02 Driver=hub
E:  Ad=81(I) Atr=03(Int.) MxPS=   1 Ivl=256ms

T:  Bus=02 Lev=02 Prnt=02 Port=03 Cnt=01 Dev#=  5 Spd=1.5  MxCh= 0
D:  Ver= 1.10 Cls=00(>ifc ) Sub=00 Prot=00 MxPS= 8 #Cfgs=  1
P:  Vendor=413c ProdID=2003 Rev= 1.00
S:  Manufacturer=Dell
S:  Product=Dell USB Keyboard
C:* #Ifs= 1 Cfg#= 1 Atr=a0 MxPwr= 70mA
I:* If#= 0 Alt= 0 #EPs= 1 Cls=03(HID  ) Sub=01 Prot=01 Driver=usbhid
E:  Ad=81(I) Atr=03(Int.) MxPS=   8 Ivl=24ms

5.1.2. How to monitor with sysfs

5.1.2.1. USB buses monitoring with sysfs

Please refer to What are the sysfs structures for Linux USB?.

 ls /sys/bus/usb/devices/
1-0:1.0  1-1  1-1:1.0  2-0:1.0  2-1  2-1.4  2-1.4:1.0  2-1:1.0  usb1  usb2

The names that begin with usb refer to USB controllers.

The device naming scheme is the following:

• bus-port.port.port... (1-1, 2-1, or 2-1.4 in the example above)

The interfaces are indicated by suffixes in the following form:

• :config.interface (1-1:1.0, 2-1:1.0, 2-1.4:1.0 in the example above)

Each interface corresponds to an entry in sysfs and can have its own driver.

5.1.2.2. USB Gadget monitoring with sysfs

Once the USB Gadget is configured, USB Device Controller sysfs is populated. See Documentation/ABI/stable/sysfs-class-udc for a description of each file.

 ls /sys/class/udc/49000000.usb-otg/
a_alt_hnp_support  device           is_selfpowered  srp
a_hnp_support      function         maximum_speed   state
b_hnp_enable       is_a_peripheral  power           subsystem
current_speed      is_otg           soft_connect    uevent

5.2. How to trace

5.2.1. How to trace with usbmon

usbmon collects traces of the input/output on the USB bus.
It relies on a kernel part and on a user part, and reports the requests made by USB device drivers to the Host controller drivers.
Activate USBMON support (CONFIG_USB_MON=y) in the kernel configuration with Linux Menuconfig tool: Menuconfig or how to configure kernel.
A usbmon entry is created in debugfs. It includes several files.
The file names consist of a number (the USB bus - 0 relates to all buses) and a letter (s, u or t). The s file contains a generic event overview. The t (deprecated) and u files will stream trace data.
To gather debug data, either use the master file 0u (to capture data from all devices) or find out the bus to which your device is connected and use the corresponding bus file. For example, if the device is connected to bus 1:

 cat /sys/kernel/debug/usb/usbmon>1u > bus1data.log

To stop the capture, just type (CTRL+C) to kill the command. You can then analyze the log with vUSBAnalyzer graphical tool on your Linux host.

5.2.2. How to trace using a protocol analyzer

A USB protocol analyzer is a USB traffic sniffer that decodes USB descriptors and displays bus states and packets sent. Refer to you USB protocol analyzer user manual.

5.3. How to debug

5.3.1. Activating USB framework debug messages

A detailed dynamic trace is available in How to use the kernel dynamic debug

  echo  "file usb* +p" > /sys/kernel/debug/dynamic_debug/control

This command enables all the traces related to the USB core and drivers at runtime.
A finer selection can be made by choosing only the files to trace.

Information

Reminder: loglevel needs to be increased to 8 either by using boot arguments or by sending the dmesg -n 8 command from the console

5.3.2. EHCI/OHCI driver debugfs entry

EHCI/OHCI drivers export a debugfs entry when CONFIG_DYNAMIC_DEBUG is enabled.

 ls /sys/kernel/debug/usb/ohci/5800c000.usbh-ohci/
async  periodic  registers
 ls /sys/kernel/debug/usb/ehci/5800d000.usbh-ehci/
async  bandwidth  periodic  registers

• async dumps a snapshot of the async schedule.
• bandwith dumps the bandwidth allocation
• periodic dumps a snapshot of the periodic schedule.
• registers dumps the USB controller registers

5.3.3. DWC2 driver debug messages and debugfs entry

To get the verbose messages from the DWC2 driver used by STM32 OTG, activate "Enable Debugging Messages" in the Linux kernel via the menuconfig Menuconfig or how to configure kernel.

Device Drivers  --->
   [*] USB support
   <*>   Support for Host-side USB
   <*>   USB Gadget Support  --->
   <*>   DesignWare USB2 DRD Core Support
   [*]     Enable Debugging Messages
   [*]       Enable Verbose Debugging Messages
   [ ]     Enable Missed SOF Tracking
   [*]     Enable Debugging Messages For Periodic Transfers

This can be done manually in your kernel .config file:

CONFIG_USB_SUPPORT=y
CONFIG_USB_DWC2=y
CONFIG_USB_DWC2_DEBUG=y
CONFIG_USB_DWC2_VERBOSE=y
CONFIG_USB_DWC2_DEBUG_PERIODIC=y

The debug support for DWC2 driver (CONFIG_USB_DWC2_DEBUG) compiles all the files located in Linux kernel drivers/usb/dwc2/ folder with DEBUG flag.

Information

Reminder: loglevel needs to be increased to 8 by using either boot arguments or the dmesg -n 8 command through the console

The DWC2 driver also exports a debugfs entry that contains useful information:

 ls /sys/kernel/debug/usb/49000000.usb-otg/
dr_mode  ep0  ep1in  ep1out  ep2in  ep2out  ep3in  ep3out  ep4in  ep4out  ep5in  ep5out  ep6in  ep6out  ep7in  ep7out  ep8in  ep8out  fifo  hw_params  params  regdump state  testmode

• dr_mode indicates the working mode of the USB controller. It can be "host", "peripheral" or "otg". The value is set through a device tree property.
• ep* files show the state of the given endpoint.
• fifo shows the FIFO information for the overall FIFO and all the periodic transmission FIFOs.
• hw_params shows the parameters read from USB controller registers.
• params shows the parameters used by the driver.
• regdump dumps all the USB controller registers.
• state shows the overall state of the hardware controller and some general information on the available endpoints.
• testmode shows/sets usb test mode ("test_j", "test_k", "test_se0_nak", "test_packet", "test_force_enable").

6. Source code location

The source files are located inside the Linux kernel.

• The USB framework is under drivers/usb/
• The drivers used for STM32 USBH are under drivers/usb/host/ehci-platform.c , drivers/usb/host/ehci-hcd.c and drivers/usb/host/ohci-platform.c , drivers/usb/host/ohci-hcd.c
• The driver used for STM32 OTG is under drivers/usb/dwc2/

Linux^® is a registered trademark of Linus Torvalds.

Human Interface Device (for USB, Bluetooth...)

Configuration File System (See https://en.wikipedia.org/wiki/Configfs for more details)

Application programming interface

System File System (See https://en.wikipedia.org/wiki/Sysfs for more details)

Debug File System (See https://en.wikipedia.org/wiki/Debugfs for more details)

USB On-The-Go (Capability/type of USB port, acting primarily as USB device, to also act as USB host. Also known as USB OTG.)

USB 2.0 Transceiver Macrocell Interface

Enhanced Host Controller Interface

Open Host Controller Interface

Dual-Role Device (USB standard defines host and device roles. OTG controllers support both roles and can be called Dual-Role Devices controllers.)

High Speed (MIPI^® Alliance DSI standard)

Device Tree

Linux^® is a registered trademark of Linus Torvalds.

Human Interface Device (for USB, Bluetooth...)

Configuration File System (See https://en.wikipedia.org/wiki/Configfs for more details)

Application programming interface

System File System (See https://en.wikipedia.org/wiki/Sysfs for more details)

Debug File System (See https://en.wikipedia.org/wiki/Debugfs for more details)

USB On-The-Go (Capability/type of USB port, acting primarily as USB device, to also act as USB host. Also known as USB OTG.)

USB 2.0 Transceiver Macrocell Interface

Enhanced Host Controller Interface

Open Host Controller Interface

Dual-Role Device (USB standard defines host and device roles. OTG controllers support both roles and can be called Dual-Role Devices controllers.)

High Speed (MIPI^® Alliance DSI standard)

Device Tree

Capability/type of USB port, acting primarily as USB device, to also act as USB host. Also known as USB OTG.

Mobile Industry Processor Interface, open membership organization that includes leading companies in the mobile industry that share the objective of defining and promoting open specifications for interfaces inside mobile terminals, see MIPI^® Alliance standard web site https://www.mipi.org

Display Serial Interface (MIPI^® Alliance standard)