USB Host-Side Drivers

• include/nuttx/usb/usbhost.h. All structures and APIs needed to work with USB host-side drivers are provided in this header file.

• struct usbhost_driver_s and struct usbhost_connection_s. Each USB host controller driver must implement an instance of struct usbhost_driver_s and struct usbhost_connection_s: struct usbhost_driver_s provides the interface between the USB host driver and the USB class driver; struct usbhost_connection_s provides the interface between the USB host driver and platform-specific connection management and device enumeration logic. These structures are defined in include/nuttx/usb/usbhost.h.

  Examples: arch/arm/src/lpc17xx_40xx/lpc17_40_usbhost.c, arch/arm/src/stm32/stm32_otgfshost.c, arch/arm/src/sama5/sam_ohci.c, and arch/arm/src/sama5/sam_ehci.c.

• struct usbhost_class_s. Each USB host class driver must implement an instance of struct usbhost_class_s. This structure is also defined in include/nuttx/usb/usbhost.h.

  Examples: drivers/usbhost/usbhost_storage.c

• USB Host Class Driver Registry. The NuttX USB host infrastructure includes a registry. During its initialization, each USB host class driver must call the interface, usbhost_registerclass() in order add its interface to the registry. Later, when a USB device is connected, the USB host controller will look up the USB host class driver that is needed to support the connected device in this registry.

  Examples: drivers/usbhost/usbhost_registry.c, drivers/usbhost/usbhost_registerclass.c, and drivers/usbhost/usbhost_findclass.c,

• Detection and Enumeration of Connected Devices. Each USB host device controller supports two methods that are used to detect and enumeration newly connected devices (and also detect disconnected devices):

  • int (*wait)(FAR struct usbhost_connection_s *drvr, FAR const bool *connected);

    Wait for a device to be connected or disconnected.

  • int (*enumerate)(FAR struct usbhost_connection_s *drvr, int rhpndx);

    Enumerate the device connected to a root hub port. As part of this enumeration process, the driver will (1) get the device’s configuration descriptor, (2) extract the class ID info from the configuration descriptor, (3) call usbhost_findclass() to find the class that supports this device, (4) call the create() method on the struct usbhost_registry_s interface to get a class instance, and finally (5) call the connect() method of the struct usbhost_class_s interface. After that, the class is in charge of the sequence of operations.

• Binding USB Host-Side Drivers. USB host-side controller drivers are not normally directly accessed by user code, but are usually bound to another, higher level USB host class driver. The class driver exports the standard NuttX device interface so that the connected USB device can be accessed just as with other, similar, on-board devices. For example, the USB host mass storage class driver (drivers/usbhost/usbhost_storage.c) will register a standard, NuttX block driver interface (like /dev/sda) that can be used to mount a file system just as with any other other block driver instance. In general, the binding sequence is:

  1. Each USB host class driver includes an initialization entry point that is called from the application at initialization time. This driver calls usbhost_registerclass() during this initialization in order to makes itself available in the event the device that it supports is connected.

     Examples: The function usbhost_msc_initialize() in the file drivers/usbhost/usbhost_storage.c

  2. Each application must include a waiter thread thread that (1) calls the USB host controller driver’s wait() to detect the connection of a device, and then (2) call the USB host controller driver’s enumerate method to bind the registered USB host class driver to the USB host controller driver.

     Examples: The function nsh_waiter() in the file boards/arm/lpc17xx_40xx/olimex-lpc1766stk/src/lpc17_40_appinit.c.

  3. As part of its operation during the binding operation, the USB host class driver will register an instances of a standard NuttX driver under the /dev directory. To repeat the above example, the USB host mass storage class driver (drivers/usbhost/usbhost_storage.c) will register a standard, NuttX block driver interface (like /dev/sda) that can be used to mount a file system just as with any other other block driver instance.

     Examples: See the call to register_blockdriver() in the function usbhost_initvolume() in the file drivers/usbhost/usbhost_storage.c.