Messages in this thread | | | Date | Sat, 3 Mar 2001 10:00:53 -0800 (PST) | From | Linus Torvalds <> | Subject | Re: Question about IRQ_PENDING/IRQ_REPLAY |
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On Sat, 3 Mar 2001, Benjamin Herrenschmidt wrote: > > Especially, my question is about the code in enable_irq() which checks > for IRQ_PENDING, and then > "replays" the interrupt by asking the APIC to issue it again. > > I don't have a simple way on PPC to cause the interrupt to happen again, > as you can imagine this is rather controller-specific. However, looking > at the code closely, I couldn't figure out a case where having > IRQ_PENDING in enable_irq() makes sense.
It only makes sense for broken irq controllers. And most aren't. You can likely ignore it - and note how even on an x86, most of the irq controller code _does_ ignore it.
> How can IRQ_PENDING happen to be set on an IRQ_DISABLED interrupt, and > why would that matter (why should we take this interrupt) ?
We have to take the interrupt - we can't just drop it on the floor. And some stupid interrupts controllers _will_ ignore it.
In particular, if an edge-triggered interrupt comes in on an x86 IO-APIC while that interrupt is disabled, enabling the interrupt will have caused that irq to get dropped. And if it gets dropped, it will never ever happen again: the interrupt line is now active, and there will never be another edge again.
> I'd be glad if you could take the time to enlighten me about this as I'm > trying to make the PPC code as close as the i386, according to your > comment stating that it would be generic in 2.5, and I don't like having > code I don't fully understand ;)
You likely don't have this problem at all. Most sane interrupt controllers are level-triggered, and won't show the problem. And others (like the i8259) will see a disabled->enabled transition as an edge if the interrupt is active (ie they have the edge-detection logic _after_ the disable logic), and again won't have this problem.
Linus
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