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SubjectRe: [Patch V4 12/42] x86, ioapic: kill static variable nr_irqs_gsi
On Tue, 10 Jun 2014, Jiang Liu wrote:

> Hi Thomas,
> This piece of code is inherited from current IOAPIC driver
> and I think it's a workaround for some weird platforms.
> For normal platforms with both 8259A and IOAPIC controllers,
> legacy ISA IRQs should be connected to both 8259A and IOAPIC pins
> (ignore timer and cascade IRQs for simplicity). According to comments
> in current kernel, there are some platforms on which:
> 1) some ISA IRQs are only connected to 8259A controllers.
> 2) the corresponding IOAPIC pins are connected to some non-ISA IRQs.
> For such platforms, IRQ0-15 are used for ISA IRQs and another
> 16 IRQs just above gsi_top are reserved for IOAPIC pins 0-15 which
> are connected to non-ISA IRQs.
> I have no real experience with such a platform, but just
> guessing possible cases according to kernel comments and "Multiple
> Processor Specification". Please look at these two pictures for quick
> reference.

Duh. I completely forgot about the 82489 mess.

We probably want a comment somewhere why we have this gsi + legacy



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