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Handle I/O requests in a USB host controller driver

  • Artikel

Best practices for a host controller driver for handling I/O requests sent by UCX.

UCX keeps track of all the endpoints that have been created by the host controller driver for devices on the USB bus. Any data transfer requests sent by the hub driver, or by another driver that is higher up in the USB device stack, is first handled by UCX. UCX is responsible for forwarding the framework request object to the correct endpoint queue. The USB Request Block (URB) contained in the request may specify an endpoint handle. If an endpoint handle is specified, UCX checks for the corresponding endpoint among the endpoints present for the device. If the specified endpoint handle is present, the request is forwarded to the endpoint's queue. If the specified endpoint handle is not found, the request is failed. If no handle is specified, then the request is for the default endpoint, and UCX forwards the request to the host controller driver's default endpoint queue for that device.

To ensure compatibility with existing USB drivers, the host controller must comply with the following requirements when completing URB request:

  • WdfRequestComplete must be called at DISPATCH_LEVEL.
  • If the URB was delivered to its framework queue and the driver began processing it synchronously on the calling driver's thread or DPC, the request should not also be completed synchronously. The request must be completed on a separate DPC, which can be scheduled with a call to WdfDpcEnqueue.
  • Similar to the preceding requirement, upon receiving EVT_WDF_IO_QUEUE_IO_CANCELED_ON_QUEUE or EVT_WDF_REQUEST_CANCEL, the host controller driver must complete the URB request on a separate DPC from the calling thread or DPC. By default, WDF completes canceled requests on the queue synchronously. That behavior might cause issues for URB requests. For this reason, the driver must provide an EvtIoCanceledOnQueue callback for its URB queues.

The framework request object for an IOCTL_INTERNAL_USB_SUBMIT_URB contains an URB located at Parameters.Others.Arg1 of the request. When the request is completed, the URB status must be set to either USBD_STATUS_SUCCESS, or to a failure status that indicates the nature of the failure. The failure status values are defined in the usb.h header file.