Need advices about PCI IRQ reference count tracking

[Jiang Liu <jiang.liu@linux.intel.com>]

Hi Bjorn,
	When enabling ACPI based IOAPIC hotplug, we encountered an
issue with PCI IRQ reference count tracking and need your advices.
	In order to hot-remove an IOAPIC, we need to track and keep
balance of IOAPIC pin usage count. But acpi_pci_irq_enable() may
be called twice for a PCI device,
1) pci_acpi_init() if pci_routeirq is true
2) pci_enable_device()

So in function acpi_pci_irq_enable(), we need a way to track whether
an IOAPIC IRQ has been assigned to the PCI device. Previously we check
"if (dev->irq > 0)" for that, but that's wrong because dev->irq may
be set to non-zero in pci_read_irq() if BIOS has already assigned
a legacy IRQ for the device. So is it OK to add a flag into pci_dev
to track this information?

Another possible workaround is to disable IOAPIC hotplug when
pci_routeirq is set, but that sounds not the best solution.

Regards!
Gerry