Write a function controller client driver


  • Describes the expected behavior of the function controller client driver.

Applies to

  • Windows 10
  • A driver developer writing a controller driver for a USB device

Important APIs

Describes the various tasks that a function controller client driver performs while interacting with USB function controller extension (UFX). UFX and the client driver communicate with each other by using export methods and event callback functions. Export methods (named UfxDeviceXxx or UfxEndpointXxx) are exported by UFX and invoked by the client driver. Callback functions (named EVT_UFX_Xxx)are implemented in the client driver and invoked by UFX.

UFX calls all client driver's callback functions asynchronously, and one callback at a time per object. For example, there is a USB device object and three endpoint objects. At most four callback functions (one for the device and one for each endpoint) may be called at a time. For each callback method, UFX waits until the client driver calls UfxDeviceEventComplete to indicate that the driver has completed the request. The only other export method that UFX listens for while waiting for these exports is UfxDeviceNotifyHardwareFailure. Many client callback functions are optional. Required functions are as follows:


  1. The function controller client driver starts the initialization process when Windows Driver Foundation (WDF) invokes the client driver's implementation of the EVT_WDF_DRIVER_DEVICE_ADD callback. In that implementation, the client driver is expected to call UfxFdoInit and then create the device object by calling WdfDeviceCreate.
  2. The client driver calls UfxDeviceCreate to create the USB device object and retrieve the UFXDEVICE handle.
  3. The client driver calls UfxDeviceNotifyHardwareReady to indicate to UFX that it can now invoke client driver's callback functions.
  4. UFX performs initialization tasks such as:

Class driver notification

In order to be notified of setup packets and the status of the bus, a class driver should send an IOCTL_INTERNAL_USBFN_ACTIVATE_USB_BUS requests. UFX queues these requests into class driver-specific notification queues. On receiving a notification about a bus event from the client driver, UFX pops from each appropriate queue and completes the request. To prevent class drivers from missing notifications, UFX keeps a fixed-size queue of notifications for the class driver.

Device attach and detach events

UFX assumes that the device is detached until it the function controller client driver calls UfxDeviceNotifyAttach.

After that call, UFX sets the device state to Powered as defined in the USB specification. To notify the client driver about state change, UFX invokes the client driver's EVT_UFX_DEVICE_USB_STATE_CHANGE implementation.

UFX notifies the charger driver (Cad.sys) to assist with charging the device. UFX also notifies the class drivers by completing IOCTL_INTERNAL_USBFN_BUS_EVENT_NOTIFICATION requests sent previously by class drivers.

The client driver must call UfxDeviceNotifyDetach when the bus is detached. The client must only call detach once after each call to UfxDeviceNotifyAttach. After the UfxDeviceNotifyDetach call, UFX calls EVT_UFX_DEVICE_HOST_DISCONNECT (if this is not an interface change). UFX then proceeds with all clean up tasks such as purging all endpoint queues and starting the default endpoint queue. UFX calls EVT_UFX_DEVICE_USB_STATE_CHANGE and notify the class drivers by completing IOCTL_INTERNAL_USBFN_BUS_EVENT_NOTIFICATION requests.

Hardware failure

If a hardware error occurs, the client driver is expected to call UfxDeviceNotifyHardwareFailure. In response, UFX will tear down the device stack and might try to recover from this situation by calling client driver's EVT_UFX_DEVICE_CONTROLLER_RESET. The client should reset the controller to its initial state. If another hardware failure occurs, the client should call UfxDeviceNotifyHardwareFailure again. On the second call, UFX will record its state and bug-check.

Port detection

Port detection is performed by UFX. It calls the function controller client driver's EVT_UFX_DEVICE_PORT_DETECT callback function to determine the type of port to which the device is attached. The client responds by calling UfxDevicePortDetectComplete or UfxDevicePortDetectCompleteEx with one of the port types defined in USBFN_PORT_TYPE.

If the client cannot determine the type of port, the client should report UsbfnUnknownPort. If the port is unknown or a downstream port, then UFX calls the client driver's EVT_UFX_DEVICE_HOST_CONNECT function. UFX listens to the bus for some time. If the port is unknown, but there is traffic, such as a setup packet, then UFX will assume UsbfnStandardDownstreamPort. Otherwise, UFX assigns the port to be UsbfnInvalidDedicatedChargingPort. After a port type has been determined, UFX notifies Cad.sys and calls the client driver's EVT_UFX_DEVICE_PORT_CHANGE function. In the function the client driver is expect ed to change the hardware state to match the UFX port type.

Endpoint creation

UFX creates the default endpoint (endpoint 0) by calling the client driver's EVT_UFX_DEVICE_DEFAULT_ENDPOINT_ADD so that it can handle setup packets sent by the host. UFX creates other endpoints by calling EVT_UFX_DEVICE_ENDPOINT_ADD. UFX only creates endpoints after the client driver calls UfxDeviceNotifyHardwareReady. In these callback functions, the client driver is expected to call UfxEndpointCreate to the endpoint object and obtain its UFXENDPOINT handle. UFX sets the parent to the class driver PDO associated with the interface to which the endpoint belongs. Parent of the default endpoint is the USB device object. An endpoint contains two framework queue objects: a transfer queue, and a command Queue, both of which can only be accessed when the device is in the Configured state (with the exception of Endpoint 0, which can be accessed after UFX calls EVT_UFX_DEVICE_HOST_CONNECT).

Device enumeration

The client driver should not allow connections to a host before UFX calls the driver's EVT_UFX_DEVICE_HOST_CONNECT. Device enumeration begins when the client driver calls UfxDeviceNotifyReset. In the Default state, UFX handles standard setup packets.


UFX purges all endpoint queues and sends an IOCTL_INTERNAL_USBFN_DESCRIPTOR_UPDATE request to the client driver to update the wMaxPacketSize of endpoint 0. UFX starts the default endpoint's queue and sets the state to Default.


UFX calls the client driver's EVT_UFX_DEVICE_USB_STATE_CHANGE function. It also notifies class drivers of the state. After UFX receives the SET_ADDRESS standard setup packet, UFX sets the state to Addressed.


UFX calls the client driver's EVT_UFX_DEVICE_ADDRESSED function to indicate to the client which address it should use. - If the address is 0, UFX sets the state back to Default and calls EVT_UFX_DEVICE_USB_STATE_CHANGE and notifies class drivers. On receiving the SET_CONFIGURATION standard setup packet, UFX sets the state to Configured.


If the selected configuration is 0, UFX purges the interface endpoints and sets the state to Addressed. UFX sends an IOCTL_INTERNAL_USBFN_DESCRIPTOR_UPDATE request to the client driver to update the wMaxPacketSize of the interface endpoints. UFX makes sure all interface endpoint queues have finished purging and start interface endpoint queues. If the port type is not UsbfnStandardDownstreamPort or UsbfnChargingDownstreamPort, UFX change the port type to UsbfnStandardDownstreamPort and informs Cad.sys; the client driver by calling EVT_UFX_DEVICE_PORT_CHANGE and EVT_UFX_DEVICE_USB_STATE_CHANGE to update the state; the class drivers of the configured state.

Standard control transfers

UFX can handle control transfers on the default endpoint at any time after it calls EVT_UFX_DEVICE_DEFAULT_ENDPOINT_ADD, in which the client driver creates the default endpoint using. All control transfers begin with an 8-byte setup packet. To send a setup packet to the host, the client driver should call UfxEndpointNotifySetup. Standard control transfers are completed by UFX. If there is data associated with the control transfer, UFX reads from and writes to the default control endpoint as appropriate.

Non-Standard control transfers

If UFX cannot handle a control transfer, the transfer is forwarded to the appropriate class driver by completing an IOCTL_INTERNAL_USBFN_BUS_EVENT_NOTIFICATION request. Control transfers can occur on any endpoint which is defined as a control endpoint in the endpoint descriptor. Control transfers on endpoints other than the default control endpoint are always non-standard control transfers. If the control endpoint is the default control endpoint, UFX will notify a class drivers of setup packets which are marked as class requests for that class driver. If the control endpoint belongs to an interface, then UFX notifies the class driver associated with that interface. If necessary, class drivers are expected to read from and write to the control endpoint.

Data transfers

Data transfers are initiated by class drivers by sending IOCTL_INTERNAL_USBFN_TRANSFER_IN, IOCTL_INTERNAL_USBFN_TRANSFER_IN_APPEND_ZERO_PKT, or IOCTL_INTERNAL_USBFN_TRANSFER_OUT requests. After validating each of those requests, UFX forwards it to the appropriate endpoint queue to be handled by the client driver. The client driver is expected to perform additional validation. The client driver receives transfer requests on endpoint queues. The client driver can retrieve as many requests from this queue as it needs to maximize bus utilization. The client driver should complete successful requests with STATUS_SUCCESS. The driver should make a best-effort attempt to cancel requests, and complete cancelled requests with STATUS_CANCELLED if cancelled. If invalid parameters are passed, the client driver completes the request with STATUS_INVALID_PARAMETER.

Control transfers

Control transfers begin with an 8-byte setup packet. To send a setup packet to the host, the client driver should call UfxEndpointNotifySetup. UFX notifies class drivers of non-standard control transfers by completing notification requests. Both clients and UFX use IOCTL_INTERNAL_USBFN_TRANSFER_IN, IOCTL_INTERNAL_USBFN_TRANSFER_IN_APPEND_ZERO_PKT, or IOCTL_INTERNAL_USBFN_TRANSFER_OUT to read from and write to the default control endpoint. However, an interface can define other control endpoints, which only the corresponding class driver can use. Control endpoints can be stalled in response to a setup packet. Class drivers send the IOCTL_INTERNAL_USBFN_SET_PIPE_STATE request to stall the endpoint. The hardware or the client driver is expected to immediately resume traffic on the endpoint after the stall is sent. Control endpoints can also send and receive zero-length packets (ZLP) without any prior data. The client driver and UFX can do this by using IOCTL_INTERNAL_USBFN_CONTROL_STATUS_HANDSHAKE_IN and IOCTL_INTERNAL_USBFN_CONTROL_STATUS_HANDSHAKE_OUT.

Bulk and interrupt transfers

Bulk transfers guarantee data delivery and are used to send large amounts of data. Transfers can be sent on a bulk endpoint using IOCTL_INTERNAL_USBFN_TRANSFER_IN, IOCTL_INTERNAL_USBFN_TRANSFER_IN_APPEND_ZERO_PKT, or IOCTL_INTERNAL_USBFN_TRANSFER_OUT. Bulk endpoints can be stalled similarly to control endpoints using IOCTL_INTERNAL_USBFN_SET_PIPE_STATE. The client driver is expected to send a STALL packet in response to all host requests and hold IOCTL requests. Unlike control endpoints, a stalled bulk endpoint remains stalled until the stall state is explicitly cleared.

Interrupt Transfers Interrupt transfers are like bulk transfers, but have a guaranteed latency. Interrupt transfers have the same interface as bulk transfers, but do not have streaming capabilities.

Isochronous transfers

The client driver is not expected to support isochronous transfers in this version.

Power management

The client driver owns all aspects of power management. Because callback functions are asynchronous, the client driver is expected to come back to an appropriate power state and complete the request before calling the appropriate event-complete export function, such as UfxDeviceEventComplete.

UFX is in a Working state if the device state (defined in USBFN_DEVICE_STATE) is UsbfnDeviceStateSuspended and UsbfnDeviceStateAttached, and has not reported a port type. Alternately, UFX has reported the port type (defined in USBFN_PORT_TYPE) UsbfnStandardDownstreamPort or UsbfnChargingDownstreamPort.

UFX enters and exits a Working state by calling EVT_UFX_DEVICE_USB_STATE_CHANGE or EVT_UFX_DEVICE_PORT_CHANGE implementations. The transition to or from a Working state is complete when the client driver calls UfxDeviceEventComplete.

In a Working state, UFX may call any callback. While not in the Working state, UFX only calls EVT_UFX_DEVICE_USB_STATE_CHANGE to enter a working state; EVT_UFX_DEVICE_REMOTE_WAKEUP_SIGNAL to issue a remote-wake during suspend (if supported).

Device suspend

Device suspend occurs when there is no traffic on the bus for 3 milliseconds. In this case, the client driver must inform UFX when it detects suspend and resume by calling UfxDeviceNotifySuspend and UfxDeviceNotifyResume. On receiving those calls, UFX calls EVT_UFX_DEVICE_USB_STATE_CHANGE and notifies class drivers by completing IOCTL_INTERNAL_USBFN_BUS_EVENT_NOTIFICATION requests. If remote wake is supported by the device and enabled by the host, UFX may call calls EVT_UFX_DEVICE_USB_STATE_CHANGE while suspended to issue a remote wake signal.

USB device-side drivers in Windows
Developing Windows drivers for USB function controllers
UFX objects and handles used by a USB function client driver