A controller interrupt does not necessarily indicate that both the controller and one of its slave devices are ready. For some controllers, an interrupt may indicate that either the controller is ready or one of its devices is ready, but not both.
Not all devices power up with interrupts disabled and then start interrupting only when told to do so.
Some devices do not provide a way to determine that the board has generated an interrupt.
Not all interrupting boards shut off interrupts when told to do so or after a bus reset.