Using IoCompletion Routines
Higher-level drivers that monitor on an IRP-specific basis how lower-level drivers carried out particular requests can have one or more IoCompletion routines. Higher-level drivers that allocate IRPs to send requests to lower drivers must have an IoCompletion routine.
A highest-level or intermediate driver's DispatchRead or DispatchWrite routine is most likely to set an IoCompletion routine for an IRP, because lower-level drivers must handle transfer requests asynchronously.
The lowest-level driver in a driver stack cannot register IoCompletion routines.
Drivers generally do not register IoCompletion routines for IRPs associated with synchronous I/O operations. For instance, a higher-level driver's DispatchDeviceControl routine can allocate an IRP using IoBuildDeviceIoControlRequest. In this case, the dispatch routine typically does not register an IoCompletion routine, because device control requests are generally handled synchronously. Instead, the driver can allocate and initialize an event object, and its DispatchDeviceControl routine can wait for an event to be initialized when it sends on driver-allocated IRPs. Usually, a higher-level driver does not register an IoCompletion routine for an IRP allocated with IoBuildSynchronousFsdRequest, for the same reason.
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