Linux Kernel Architecture

Chapter 14: Kernel Activities

*/

if (unlikely((desc->status &

(IRQ_PENDING | IRQ_MASKED | IRQ_DISABLED)) ==

(IRQ_PENDING | IRQ_MASKED))) {

desc->chip->unmask(irq);

desc->status &= ~IRQ_MASKED;

}

desc->status &= ~IRQ_PENDING;

action_ret = handle_IRQ_event(irq, action);

} while ((desc->status & (IRQ_PENDING | IRQ_DISABLED)) == IRQ_PENDING);

Processing the IRQ runs in a loop. Suppose we are at the point right beneath the call to

handle_IRQ_event. While the ISR handlers for the firstIRQ were running, a second IRQ could

have appeared as shown before. This is indicated byIRQ_PENDING. If the flag is set (and the IRQ has not

been disabled in the meantime), another IRQ is waiting to be processed, and the loop is started again

from the beginning.

In this case, however, the IRQ will have beenmasked. The IRQ must thus be unmasked with

chip->unmaskand theIRQ_MASKEDflag be removed. This guarantees that only one interrupt can occur

during the execution ofhandle_IRQ_event.

After removing theIRQ_PENDINGflag — technically, one IRQ is still pending right now, but it is going to

be processed immediately —handle_IRQ_eventcan also serve the second IRQ.

Level-Triggered Interrupts

Level-triggered interrupts are alittle easier to process than their edge-triggered relatives. This is

also reflected in the code flow diagram of the flow handlerhandle_level_irq, which is depicted in

Figure 14-6.

Abort processing

No ISR registered or IRQ disabled? Abort processing

Set IRQ_INPROGRESS

Remove IRQ_INPROGRESS

Irq not disabled? chip->unmask

handle_level_irq

mask_ack_irq

IRQ_INPROGRESS?

handle_IRQ_event

Figure 14-6: Code flow diagram forhandle_level_irq.

Note that level-triggered interrupts must be masked when they are processed, so the first thing that

needstobedoneistocallmask_ack_irq. This auxiliary function masks and acknowledges the IRQ by