powerpc/powernv: Disable native PCIe port management

On PowerNV the PCIe topology is (currently) managed by the powernv platform
code in Linux in cooperation with the platform firmware. Linux's native
PCIe port service drivers operate independently of both and this can cause
problems.

The main issue is that the portbus driver will conflict with the platform
specific hotplug driver (pnv_php) over ownership of the MSI used to notify
the host when a hotplug event occurs. The portbus driver claims this MSI on
behalf of the individual port services because the same interrupt is used
for hotplug events, PMEs (on root ports), and link bandwidth change
notifications. The portbus driver will always claim the interrupt even if
the individual port service drivers, such as pciehp, are compiled out.

The second, bigger, problem is that the hotplug port service driver
fundamentally does not work on PowerNV. The platform assumes that all
PCI devices have a corresponding arch-specific handle derived from the DT
node for the device (pci_dn) and without one the platform will not allow
a PCI device to be enabled. This problem is largely due to historical
baggage, but it can't be resolved without significant re-factoring of the
platform PCI support.

We can fix these problems in the interim by setting the
"pcie_ports_disabled" flag during platform initialisation. The flag
indicates the platform owns the PCIe ports which stops the portbus driver
from being registered.

This does have the side effect of disabling all port services drivers
that is: AER, PME, BW notifications, hotplug, and DPC. However, this is
not a huge disadvantage on PowerNV since these services are either unused
or handled through other means.

Fixes: 66725152fb9f ("PCI/hotplug: PowerPC PowerNV PCI hotplug driver")
Signed-off-by: Oliver O'Halloran <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

arch/powerpc/platforms/powernv/pci.c

···
 	if (!firmware_has_feature(FW_FEATURE_OPAL))
 		return;
 
+#ifdef CONFIG_PCIEPORTBUS
+	/*
+	 * On PowerNV PCIe devices are (currently) managed in cooperation
+	 * with firmware. This isn't *strictly* required, but there's enough
+	 * assumptions baked into both firmware and the platform code that
+	 * it's unwise to allow the portbus services to be used.
+	 *
+	 * We need to fix this eventually, but for now set this flag to disable
+	 * the portbus driver. The AER service isn't required since that AER
+	 * events are handled via EEH. The pciehp hotplug driver can't work
+	 * without kernel changes (and portbus binding breaks pnv_php). The
+	 * other services also require some thinking about how we're going
+	 * to integrate them.
+	 */
+	pcie_ports_disabled = true;
+#endif
+
 	/* Look for IODA IO-Hubs. */
 	for_each_compatible_node(np, NULL, "ibm,ioda-hub") {
 		pnv_pci_init_ioda_hub(np);