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Communicating with GenericUSBFn.sys from a user-mode service

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All user-mode requests are sent to the Microsoft-provided kernel-mode driver GenericUSBFn.sys. You can create a user-mode service that communicates with GenericUSBFn.sys by sending these I/O control code (IOCTL), and GenericUSBFn.sys handles kernel-mode communication with the USB function drivers.

The IOCTLs declared in Genericusbfnioctl.h are used for communicating with GenericUSBFn.sys from a user-mode service.

The following steps describe how you can define a USB interface service that interacts with GenericUSBFn.sys to communicate with the USB function drivers:

  1. On startup, the service listens for the device interface arrival of the interface. The device interface GUID is the InterfaceGUID value that is declared in the registry under the OEM-defined subkey of HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\USBFN\Interfaces. There are two common methods for listening to device arrival:
    • Trigger start the service. For more information, see Service Trigger Events.
    • Register for device interface arrival. For more information, see the CM_Register_Notification function.
  2. After the interface arrives, the service opens a handle to the device:
    • Get the symbolic name for the device by calling the CM_Get_Device_Interface_List function. Specify the device interface GUID that is declared in the InterfaceGUID value in the registry.
    • After you have the symbolic name for the device, open a handle to the device by using CreateFile.
  3. The service issues IOCTL_GENERICUSBFN_GET_CLASS_INFO to retrieve information about the available pipes, as configured in the registry.
  4. After the service is ready to communicate, it issues IOCTL_GENERICUSBFN_ACTIVATE_USB_BUS. After all class drivers are activated, the USB function class extension can connect to the host.
  5. To receive USB notifications, the service issues IOCTL_GENERICUSBFN_BUS_EVENT_NOTIFICATION. This IOCTL completes when a new USB event has occurred. Events (USBFN_EVENT) of particular interest include:
  6. UsbfnEventReset: This is used to determine the speed of the connected USB device.
  7. UsbfnEventConfigured: The service can now issue transfer requests.
  8. UsbfnEventSetupPacket: The USB function class extension has received an interface-specific setup packet (bmRequestType.Type == BMREQUEST_CLASS). The service should reply to the setup packet by issuing a transfer request in pipe 0, followed by a handshake request (IOCTL_GENERICUSBFN_CONTROL_STATUS_HANDSHAKE_OUT) in the opposite direction on pipe 0.
  9. After the UsbfnEventConfigured event is received, the service can begin issuing transfer requests using IOCTL_GENERICUSBFN_TRANSFER_IN, IOCTL_GENERICUSBFN_TRANSFER_IN_APPEND_ZERO_PKT, and IOCTL_GENERICUSBFN_TRANSFER_OUT.