A.4.9  SCSI Miniport Driver’s HwScsiTimer Routine

A miniport driver that does not have a HwScsiInterrupt routine because it manages all HBA I/O operations by polling should have a HwScsiTimer routine. However, miniport drivers with HwScsiInterrupt routines frequently have HwScsiTimer routines as well.

While a miniport driver can call ScsiPortStallExecution to wait for a state change on the HBA, miniport drivers should never call this routine to wait for longer than one millisecond except, possibly, for an operation performed only when a miniport is initializing. ScsiPortStallExecution ties up the processor for the given interval, preventing other code in the system from doing useful work.

Instead of calling ScsiPortStallExecution with large input intervals and wasting many CPU cycles, a miniport driver should have a HwScsiTimer routine. One or more HwScsiTimer routines are particularly useful if the HBA does not generate a completion interrupt for every operation or if any commonly requested operation, such as a bus reset, takes longer than a millisecond.

After the HBA has been programmed for such an operation, the miniport driver calls ScsiPortNotification with the NotificationType RequestTimerCall, a pointer to its HBA-specific device extension containing context about the operation, its HwScsiTimer entry point, and a driver-determined interval.

ScsiPortNotification synchronizes calls to the HwScsiTimer routine with those to the HwScsiInterrupt routine so that it cannot execute concurrently while the HwScsiTimer routine is running.

HwScsiTimer is called once for each such call to ScsiPortNotification, which can be called from the HwScsiTimer routine itself. However, any call to ScsiPortNotification with the NotificationType RequestTimerCall overrides a preceding call for which the specified interval has not expired. That is, there is only one outstanding request to call a miniport’s HwScsiTimer routine at any given moment.

The interval passed in to ScsiPortNotification is in microseconds, and the minimum overhead for each call to a HwScsiTimer routine is approximately ten microseconds. An input interval of zero cancels the preceding request to call the HwScsiTimer routine, provided it has not been called or dispatched for execution on another processor in a Windows NT SMP machine.