powerpc/eeh: Do early EEH init only when required

The pci hotplug helper (pci_hp_add_devices()) calls
eeh_add_device_tree_early() to scan the device-tree for new PCI devices and
do the early EEH probe before the device is scanned. This early probe is a
no-op in a lot of cases because:

a) The early init is only required to satisfy a PAPR requirement that EEH
   be configured before we start doing config accesses. On PowerNV it is
   a no-op.

b) It's a no-op for devices that have already had their eeh_dev
   initialised.

There are four callers of pci_hp_add_devices():

1. arch/powerpc/kernel/eeh_driver.c
	Here the hotplug helper is called when re-scanning pci_devs that
	were removed during an EEH recovery pass. The EEH stat for each
	removed device (the eeh_dev) is retained across a recovery pass
	so the early init is a no-op in this case.

2. drivers/pci/hotplug/pnv_php.c
	This is also a no-op since the PowerNV hotplug driver is, suprisingly,
	PowerNV specific.

3. drivers/pci/hotplug/rpaphp_core.c
4. drivers/pci/hotplug/rpaphp_pci.c
	In these two cases new devices have been hotplugged and FW has
	provided new DT nodes for each. These are the only two cases where
	the EEH we might have new PCI device nodes in the DT so these are
	the only two cases where the early EEH probe needs to be done.

We can move the calls to eeh_add_device_tree_early() to the locations where
it's needed and remove it from the generic path. This is preparation for
making the early EEH probe pseries specific.
Reviewed-by: Sam Bobroff <sbobroff@linux.ibm.com>
Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200306073904.4737-3-oohall@gmail.com

arch/powerpc/kernel/pci-hotplug.c

@@ -112,8 +112,6 @@ void pci_hp_add_devices(struct pci_bus *bus)
 	struct pci_controller *phb;
 	struct device_node *dn = pci_bus_to_OF_node(bus);
 
-	eeh_add_device_tree_early(PCI_DN(dn));
-
 	phb = pci_bus_to_host(bus);
 
 	mode = PCI_PROBE_NORMAL;

drivers/pci/hotplug/rpaphp_core.c

@@ -493,6 +493,8 @@ static int enable_slot(struct hotplug_slot *hotplug_slot)
 		return retval;
 
 	if (state == PRESENT) {
+		eeh_add_device_tree_early(PCI_DN(slot->dn));
+
 		pci_lock_rescan_remove();
 		pci_hp_add_devices(slot->bus);
 		pci_unlock_rescan_remove();

drivers/pci/hotplug/rpaphp_pci.c

@@ -95,8 +95,10 @@ int rpaphp_enable_slot(struct slot *slot)
 			return -EINVAL;
 		}
 
-		if (list_empty(&bus->devices))
+		if (list_empty(&bus->devices)) {
+			eeh_add_device_tree_early(PCI_DN(slot->dn));
 			pci_hp_add_devices(bus);
+		}
 
 		if (!list_empty(&bus->devices)) {
 			slot->state = CONFIGURED;