NTSTATUS Ctrl2capDispatchRead(
    IN PDEVICE_OBJECT DeviceObject,
    IN PIRP Irp )
{
    PDEVICE_EXTENSION   devExt;
    PIO_STACK_LOCATION  currentIrpStack;
    PIO_STACK_LOCATION  nextIrpStack;

    //
    // Gather our variables.
    //
    devExt = (PDEVICE_EXTENSION) DeviceObject->DeviceExtension;
    currentIrpStack = IoGetCurrentIrpStackLocation(Irp);
    nextIrpStack = IoGetNextIrpStackLocation(Irp);   

    //
    // Push params down for keyboard class driver.
    //就是以下这句,不知道下层驱动设备为什么要用到这层IO堆栈中的参数，请说明   
    *nextIrpStack = *currentIrpStack;

    //  
    // Set the completion callback, so we can "frob" the keyboard data.
    //            
    IoSetCompletionRoutine( Irp, Ctrl2capReadComplete,
                            DeviceObject, TRUE, TRUE, TRUE );

    //
    // Return the results of the call to the keyboard class driver.
    //
    return IoCallDriver( devExt->TopOfStack, Irp );
}

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