IRQS on 6 Slot Macs

Benjamin Herrenschmidt benh at
Mon Nov 3 17:54:41 EST 2003

On Mon, 2003-11-03 at 14:35, Robert E Brose II wrote:

> A couple of things right off, the rage128 appears to be the only thing on
> irq 23 however 23 doesn't show up in /proc/interrupts meaning, I suppose,
> that it's not using the interrupt. So why does the drm complain?
> How come everything from slots 3-6 says it's on the same interrupt (25)?

I don't know what's up with the DRM not liking your interrupt. I can
answer for the sharing of USB, symbios and firewire interrupts: all 3
slots share one interrupt because of bad motherboard design :)
Basicallly, what they did when designing that machine was to use a
standard powersurge design with 3 slots and replace one of them
with a PCI<->PCI bridge. Since they didn't "know" how to get more
interrupt lines out of Grand Central, they just also stuffed all
interrupt lines together for those 4 slots (I'm pretty sure GC do
have spare lines they could have used, but that would have meant
updating Open Firmware to understand the layout, I doubt the people
who designed that machine wanted to dive into that).


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