PCI/LINK: Supply IRQ handler so level-triggered IRQs are acked

A threaded IRQ with a NULL handler does not work with level-triggered
interrupts.  request_threaded_irq() will return an error:

  genirq: Threaded irq requested with handler=NULL and !ONESHOT for irq 16
  pcie_bw_notification: probe of 0000:00:1b.0:pcie010 failed with error -22

For level interrupts we need to silence the interrupt before exiting the
IRQ handler, so just clear the PCI_EXP_LNKSTA_LBMS bit there.

Fixes: e8303bb7 ("PCI/LINK: Report degraded links via link bandwidth notification")
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Reported-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

drivers/pci/pcie/bw_notification.c

@@ -44,11 +44,10 @@ static void pcie_disable_link_bandwidth_notification(struct pci_dev *dev)
 	pcie_capability_write_word(dev, PCI_EXP_LNKCTL, lnk_ctl);
 }
 
-static irqreturn_t pcie_bw_notification_handler(int irq, void *context)
+static irqreturn_t pcie_bw_notification_irq(int irq, void *context)
 {
 	struct pcie_device *srv = context;
 	struct pci_dev *port = srv->port;
-	struct pci_dev *dev;
 	u16 link_status, events;
 	int ret;
 
@@ -58,6 +57,17 @@ static irqreturn_t pcie_bw_notification_handler(int irq, void *context)
 	if (ret != PCIBIOS_SUCCESSFUL || !events)
 		return IRQ_NONE;
 
+	pcie_capability_write_word(port, PCI_EXP_LNKSTA, events);
+	pcie_update_link_speed(port->subordinate, link_status);
+	return IRQ_WAKE_THREAD;
+}
+
+static irqreturn_t pcie_bw_notification_handler(int irq, void *context)
+{
+	struct pcie_device *srv = context;
+	struct pci_dev *port = srv->port;
+	struct pci_dev *dev;
+
 	/*
 	 * Print status from downstream devices, not this root port or
 	 * downstream switch port.
@@ -67,8 +77,6 @@ static irqreturn_t pcie_bw_notification_handler(int irq, void *context)
 		__pcie_print_link_status(dev, false);
 	up_read(&pci_bus_sem);
 
-	pcie_update_link_speed(port->subordinate, link_status);
-	pcie_capability_write_word(port, PCI_EXP_LNKSTA, events);
 	return IRQ_HANDLED;
 }
 
@@ -80,7 +88,8 @@ static int pcie_bandwidth_notification_probe(struct pcie_device *srv)
 	if (!pcie_link_bandwidth_notification_supported(srv->port))
 		return -ENODEV;
 
-	ret = request_threaded_irq(srv->irq, NULL, pcie_bw_notification_handler,
+	ret = request_threaded_irq(srv->irq, pcie_bw_notification_irq,
+				   pcie_bw_notification_handler,
 				   IRQF_SHARED, "PCIe BW notif", srv);
 	if (ret)
 		return ret;