Storage Filter Driver's Dispatch Routines

Like any other higher-level kernel-mode driver, a storage filter driver (SFD) must have one or more Dispatch routines to handle every IRP_MJ_XXX request for which the underlying storage driver supplies a Dispatch entry point. Depending on the nature of its device, the Dispatch entry point of an SFD might do one of the following for any given request:

Processing requests

For requests that require no special handling, the Dispatch routine of an SFD usually calls IoSkipCurrentIrpStackLocation with an input IRP and then calls IoCallDriver with pointers to the class driver's device object and the IRP. Note that an SFD seldom sets its IoCompletion routine in IRPs that require no special handling both because a call to the IoCompletion routine is unnecessary and because it degrades I/O throughput for the driver's devices. If an SFD does set an IoCompletion routine, it calls IoCopyCurrentIrpStackLocationToNext instead of IoSkipCurrentIrpStackLocation and then calls IoSetCompletionRoutine before calling IoCallDriver.

For requests that do require special handling, the SFD can do the following:

  1. Create a new IRP with IoBuildDeviceIoControlRequest, IoAllocateIrp, IoBuildSynchronousFsdRequest, or IoBuildAsynchronousFsdRequest, usually specifying an I/O stack location for itself.

  2. Check the returned IRP pointer for NULL and return STATUS_INSUFFICIENT_RESOURCES if an IRP could not be allocated.

  3. If the driver-created IRP includes an I/O stack location for the SFD, call IoSetNextIrpStackLocation to set up the IRP stack location pointer. Then, call IoGetCurrentIrpStackLocation to get a pointer to its own I/O stack location in the driver-created IRP and set up it up with state to be used by its own IoCompletion routine.

  4. Call IoGetNextIrpStackLocation to get a pointer to the next-lower driver's I/O stack location in the driver-created IRP and set it up with the major function code IRP_MJ_SCSI and an SRB (see Storage Class Drivers).

  5. Translate data to be transferred to the device into a device-specific, nonstandard format if necessary.

  6. Call IoSetCompletionRoutine if the driver allocated any memory, such as memory for an SRB, SCSI request-sense buffer, MDL, and/or IRP with a call to IoAllocateIrp or IoBuildAsynchronousFsdRequest, or if the driver must translate data transferred from the device in a device-specific, nonstandard format.

  7. Pass the driver-created IRP to (and through) the next-lower driver with IoCallDriver.

Handling SRB formats

Starting with Windows 8, an SFD filtering between the class driver and the port driver must check for the supported SRB format. Specifically, this involves detecting the SRB format and accessing the members of the structure correctly. The SRB in the IRP is either an SCSI_REQUEST_BLOCK and or an STORAGE_REQUEST_BLOCK. A filter driver can determine ahead of time which SRBs are supported by the port driver below by issuing an IOCTL_STORAGE_QUERY_PROPERTY request and specifying the StorageAdapterProperty identifier. The SrbType and AddressType values returned in the STORAGE_ADAPTER_DESCRIPTOR structure indicate the SRB format and addressing scheme used by the port driver. Any new SRBs allocated and sent by the filter driver must be of the type returned by the query.

Similarly, starting with Windows 8, SFDs supporting only SRBs of the SCSI_REQUEST_BLOCK type must check that the SrbType value returned in the STORAGE_ADAPTER_DESCRIPTOR structure is set to SRB_TYPE_SCSI_REQUEST_BLOCK. To handle the situation when SrbType is set to SRB_TYPE_STORAGE_REQUEST_BLOCK instead, the filter driver must set a completion routine for IOCTL_STORAGE_QUERY_PROPERTY when the StorageAdapterProperty identifier is set in the request sent by drivers above it. In the completion routine, the SrbType member in the STORAGE_ADAPTER_DESCRIPTOR is modified to SRB_TYPE_SCSI_REQUEST_BLOCK to correctly set the supported type.

The following is an example of a filter dispatch routine which handles both SRB formats.

NTSTATUS FilterScsiIrp(
    PDEVICE_OBJECT DeviceObject,
    PIRP Irp
    PFILTER_DEVICE_EXTENSION  deviceExtension = DeviceObject->DeviceExtension;
    PIO_STACK_LOCATION irpStack = IoGetCurrentIrpStackLocation(Irp);
    NTSTATUS status;
    ULONG srbFunction;
    ULONG srbFlags;

    // Acquire the remove lock so that device will not be removed while
    // processing this irp.

    status = IoAcquireRemoveLock(&deviceExtension->RemoveLock, Irp);

    if (!NT_SUCCESS(status)) {
        Irp->IoStatus.Status = status;
        IoCompleteRequest(Irp, IO_NO_INCREMENT);
        return status;

    srb = irpStack->Parameters.Scsi.Srb;

    if (srb->Function == SRB_FUNCTION_STORAGE_REQUEST_BLOCK) {
        srbFunction = ((PSTORAGE_REQUEST_BLOCK)srb)->SrbFunction;
        srbFlags = ((PSTORAGE_REQUEST_BLOCK)srb)->SrbFlags;
    } else {
        srbFunction = srb->Function;
        srbFlags = srb->SrbFlags;

    if (srbFunction == SRB_FUNCTION_EXECUTE_SCSI) {
            // ...

            // filter processing for SRB_FUNCTION_EXECUTE_SCSI

            // ...

                           TRUE, TRUE, TRUE);
    IoCallDriver(DeviceExtension->TargetDeviceObject, Irp);

    return STATUS_PENDING; 

Setting up requests

Like a storage class driver, an SFD might have BuildRequest or SplitTransferRequest routines to be called from the driver's Dispatch routines, or might implement the same functionality inline.

For more information about BuildRequest and SplitTransferRequest routines, see Storage Class Drivers. For more information about general requirements for Dispatch routines, see Writing Dispatch Routines.