PCI: pciehp: Fix race condition handling surprise link down

A surprise link down may retrain very quickly causing the same slot
generate a link up event before handling the link down event completes.

Since the link is active, the power off work queued from the first link
down will cause a second down event when power is disabled. However, the
link up event sets the slot state to POWERON_STATE before the event to
handle this is enqueued, making the second down event believe it needs to
do something.

This creates constant link up and down event cycle.

To prevent this it is better to handle each event at the time in order it
occurred, so change the driver to use ordered workqueue instead.

A normal device hotplug triggers two events (presense detect and link up)
that are already handled properly in the driver but we currently log an
error if we find an existing device in the slot. Since this is not an error
change the log level to be debug instead to avoid scaring users.

This is based on the original work by Ashok Raj.

Link: https://patchwork.kernel.org/patch/9469023Suggested-by: NBjorn Helgaas <bhelgaas@google.com>
Signed-off-by: NMika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>

drivers/pci/hotplug/pciehp_ctrl.c

@@ -113,10 +113,11 @@ static int board_added(struct slot *p_slot)
 
 	retval = pciehp_configure_device(p_slot);
 	if (retval) {
-		ctrl_err(ctrl, "Cannot add device at %04x:%02x:00\n",
-			 pci_domain_nr(parent), parent->number);
-		if (retval != -EEXIST)
+		if (retval != -EEXIST) {
+			ctrl_err(ctrl, "Cannot add device at %04x:%02x:00\n",
+				 pci_domain_nr(parent), parent->number);
 			goto err_exit;
+		}
 	}
 
 	pciehp_green_led_on(p_slot);

drivers/pci/hotplug/pciehp_hpc.c

@@ -791,7 +791,7 @@ static int pcie_init_slot(struct controller *ctrl)
 	if (!slot)
 		return -ENOMEM;
 
-	slot->wq = alloc_workqueue("pciehp-%u", 0, 0, PSN(ctrl));
+	slot->wq = alloc_ordered_workqueue("pciehp-%u", 0, PSN(ctrl));
 	if (!slot->wq)
 		goto abort;
 

drivers/pci/hotplug/pciehp_pci.c

@@ -46,7 +46,11 @@ int pciehp_configure_device(struct slot *p_slot)
 
 	dev = pci_get_slot(parent, PCI_DEVFN(0, 0));
 	if (dev) {
-		ctrl_err(ctrl, "Device %s already exists at %04x:%02x:00, cannot hot-add\n",
+		/*
+		 * The device is already there. Either configured by the
+		 * boot firmware or a previous hotplug event.
+		 */
+		ctrl_dbg(ctrl, "Device %s already exists at %04x:%02x:00, skipping hot-add\n",
 			 pci_name(dev), pci_domain_nr(parent), parent->number);
 		pci_dev_put(dev);
 		ret = -EEXIST;