Grabbing an Interrupt for a Brain-dead PCI Device

[Geert Uytterhoeven]

On Tue, 7 Mar 2000, Michel Lanners wrote:
> If this is in a PowerMac (at least those previous to UMA), then there
> is a fixed IRQ per physical slot. All 4 PCI interrupt lines are OR'ed
> together per slot.
>
> However, there are other IRQ routing schemes as well... the above being
> the easiest one, because the IRQ value can be hardcoded based on the
> slot, which you can determine by software.

The most popular routing scheme looks like this:

    #INTA(slot_nr) -> pci_int(slot_nr % 4)
    #INTB(slot_nr) -> pci_int((slot_nr+1) % 4)
    #INTC(slot_nr) -> pci_int((slot_nr+2) % 4)
    #INTD(slot_nr) -> pci_int((slot_nr+3) % 4)

where

    pci_int(0) = #INTW
    pci_int(1) = #INTX
    pci_int(2) = #INTY
    pci_int(3) = #INTZ

Hence if all boards use one interrupt (the first interrupt on a board _must_ be
#INTA), they're evenly spread across the available interrupt sources.

Gr{oetje,eeting}s,

						Geert

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Geert Uytterhoeven ------------- Sony Software Development Center Europe (SDCE)
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