There is more to mapping IRQs to IO APIC pins

[Jakub Jermář]

The kernel provides two mechanisms for mapping an IRQ number to an IO APIC pin. At a time, only one mechanism is active, depending on whether the SMP was configured using the MPS code or the ACPI MADT parsing code.

It turns out that especially the MPS way is more complicated than originally believed by its author. The problem is that the IRQ is specified as a tuple (bus, bus-relative IRQ number) while the current understanding is rather one-dimensional and absolute. For legacy ISA IRQs it appears to work, but for translating e.g. PCI IRQs, we need to do something more sophisticated.