When an ISR is running what happens to the interrupts on that particular IRQ line.would they be lost or stored so it can be processed at later point

Question

When an Interrupt service routine is being handled that particular IRQ line is disabled,then what happens when a device registered on the same IRQ line raises an interrupt.? Is that interrupt lost or stored so it can be processed at later point.

kindly someone explain.

Thanks in advance.

Answer 1

In general, the interrupt is lost. That is, unless the device driver can deduce that a missed interrupt occurred, like by regularly inspecting device registers related to interrupt status.

Many, if not most, device drivers do not do that. It is almost always better to handle the interrupt expeditiously and return from interrupt so the next interrupt can be handled sooner.

A reasonable goal is to limit the code path ISR logic to less than a dozen—preferably even less—lines of simple source code. This is easily achieved by servicing whatever needs servicing: usually a few transfers from/to device registers, marking a blocked task on that i/o to be ready, and returning. Of course, the rest of the driver (non ISR portions) may have to do a little more work to accomplish such ISR efficiency, but that is good driver design IMHO.

I have discussed with many device driver engineers who claim that having the ISR do more work on the spot (and not deferred to thread-based processing) can help improve latency and system performance. I remain unconvinced that assertion is ever true.

Answer 2

Check out my answer here: On x86, when the OS disables interrupts, do they vanish, or do they queue and 'wait' for interrupts to come back on?

The interrupts on that particular IRQ line are lost. So, the ISR routine should execute as quickly as possible so that such a sceanrio doesn't arise. That's why we moved to the top-half, bottom-half approach (tasklets, workqueues) and now to Threaded IRQs.