irqchip/gic-v3-its: Correctly honor the RID remapping

Since 6adb35ff43a16 ("irqchip/gic-v3-its: Provide MSI parent for
PCI/MSI[-X]"), the primary domain a PCI device allocates its interrupts
from is the one that is directly attached to the device itself.

By virtue of being a PCI device, it has no OF node.

This domain is (through more layer than it is worth describing)
passed to its_pci_msi_prepare(), which tries to compute the
full RID that is presented to the ITS by the device. This is ultimately
done by calling pci_msi_domain_get_msi_rid(), passing both the
domain and the PCI device as arguments.

The baked-in assumption is that either the domain that is passed
to pci_msi_domain_get_msi_rid() describes an interrupt controller
with either an OF node or an entry in an ACPI IORT table.
In this case, it is *neither*. This domain is does not represent
anything firmware-based, but just an allocation unit for the device.

As a result, it fails to provide the full RID (which requires inspecting
the msi-map/msi-mask properties in the DT), and stick to the BDF, which
isn't very useful.

Tragedy follows with a litany of devices that randomly die as they fail to
see any MSI (because the RID is wrong) or fail to get an allocation
(because they try to steal LPIs from their neighbour's pool).

This will happen on any system where a single ITS is shared by multiple
root ports and end-points with overlapping BDF numbers, and has the
topology described in the device-tree.  Simpler DT topologies will luckily
work, and so will ACPI-based systems.

Solve it by pointing pci_msi_domain_get_msi_rid() at the *parent* domain,
which is the ITS, resulting in a correct mapping and a restored happiness
in my personal zoo.

Fixes: 6adb35ff43a16 ("irqchip/gic-v3-its: Provide MSI parent for PCI/MSI[-X]")
Reported-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Johan Hovold <johan+linaro@kernel.org>
Link: https://lore.kernel.org/r/20240717195937.2240400-1-maz@kernel.org

drivers/irqchip/irq-gic-v3-its-msi-parent.c

@@ -65,7 +65,7 @@ static int its_pci_msi_prepare(struct irq_domain *domain, struct device *dev,
 	}
 
 	/* ITS specific DeviceID, as the core ITS ignores dev. */
-	info->scratchpad[0].ul = pci_msi_domain_get_msi_rid(domain, pdev);
+	info->scratchpad[0].ul = pci_msi_domain_get_msi_rid(domain->parent, pdev);
 
 	/*
 	 * @domain->msi_domain_info->hwsize contains the size of the