关于IoCallDriver使用的疑惑

#pragma PAGEDCODE

NTSTATUS HelloDDKRead(IN PDEVICE_OBJECT pDevObj,

                                 IN PIRP pIrp)

{

    KdPrint(("DriverB:Enter B HelloDDKRead\n"));

    NTSTATUS ntStatus = STATUS_SUCCESS;

    UNICODE_STRING DeviceName;

    RtlInitUnicodeString( &DeviceName, L"\\Device\\MyDDKDeviceA" );

    PDEVICE_OBJECT DeviceObject = NULL;

    PFILE_OBJECT FileObject = NULL;

    //得到设备对象指针

    ntStatus = IoGetDeviceObjectPointer(&DeviceName,FILE_ALL_ACCESS,&FileObject,&DeviceObject);

    KdPrint(("DriverB:FileObject:%x\n",FileObject));

    KdPrint(("DriverB:DeviceObject:%x\n",DeviceObject));

    if (!NT_SUCCESS(ntStatus))

    {

        KdPrint(("DriverB:IoGetDeviceObjectPointer() 0x%x\n", ntStatus ));

        ntStatus = STATUS_UNSUCCESSFUL;

        // 完成IRP

        pIrp->IoStatus.Status = ntStatus;

        pIrp->IoStatus.Information = ;    // bytes xfered

        IoCompleteRequest( pIrp, IO_NO_INCREMENT );

        KdPrint(("DriverB:Leave B HelloDDKRead\n"));

        return ntStatus;

    }

    KEVENT event;

    KeInitializeEvent(&event,NotificationEvent,FALSE);

    PIRP pNewIrp = IoAllocateIrp(DeviceObject->StackSize,FALSE);

    KdPrint(("pNewIrp->UserEvent :%x\n",pNewIrp->UserEvent));

    pNewIrp->UserEvent = &event;

    IO_STATUS_BLOCK status_block;

    pNewIrp->UserIosb = &status_block;

    pNewIrp->Tail.Overlay.Thread = PsGetCurrentThread();

    //因为DriverA是BUFFER IO设备

    pNewIrp->AssociatedIrp.SystemBuffer = NULL;

     KdPrint(("DriverB:pNewIrp:%x\n",pNewIrp));

    PIO_STACK_LOCATION stack = IoGetNextIrpStackLocation(pNewIrp);

    stack->MajorFunction = IRP_MJ_READ;

    stack->MinorFunction=IRP_MN_NORMAL;//

    stack->FileObject = FileObject;

    //调用DriverA驱动

    NTSTATUS status = IoCallDriver(DeviceObject,pNewIrp);

    if (status == STATUS_PENDING) {

       status = KeWaitForSingleObject(

                            &event,

                            Executive,

                            KernelMode,

                            FALSE, // Not alertable

                            NULL);

       KdPrint(("STATUS_PENDING\n"));

    }

     ObDereferenceObject( FileObject );

    IoFreeIrp(pNewIrp);

    ntStatus = STATUS_SUCCESS;

    // 完成IRP

    pIrp->IoStatus.Status = ntStatus;

    pIrp->IoStatus.Information = ;    // bytes xfered

    IoCompleteRequest( pIrp, IO_NO_INCREMENT );

    KdPrint(("DriverB:Leave B HelloDDKRead\n"));

    return ntStatus;

}

《windows驱动开发详解》中有一个例子，使用DriverB创建一个DriverA的IRP并使用IoCallDriver发送IRP请求，我不明白为什么在设置DriverA堆栈时使用IoGetNextIrpStackLocation 获取下层堆栈并设置