PCI: pciehp: Fix infinite loop in IRQ handler upon power fault

The Power Fault Detected bit in the Slot Status register differs from
all other hotplug events in that it is sticky:  It can only be cleared
after turning off slot power.  Per PCIe r5.0, sec. 6.7.1.8:

  If a power controller detects a main power fault on the hot-plug slot,
  it must automatically set its internal main power fault latch [...].
  The main power fault latch is cleared when software turns off power to
  the hot-plug slot.

The stickiness used to cause interrupt storms and infinite loops which
were fixed in 2009 by commits 5651c48 ("PCI pciehp: fix power fault
interrupt storm problem") and 99f0169 ("PCI: pciehp: enable
software notification on empty slots").

Unfortunately in 2020 the infinite loop issue was inadvertently
reintroduced by commit 8edf533 ("PCI: pciehp: Fix MSI interrupt
race"):  The hardirq handler pciehp_isr() clears the PFD bit until
pciehp's power_fault_detected flag is set.  That happens in the IRQ
thread pciehp_ist(), which never learns of the event because the hardirq
handler is stuck in an infinite loop.  Fix by setting the
power_fault_detected flag already in the hardirq handler.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=214989
Link: https://lore.kernel.org/linux-pci/DM8PR11MB5702255A6A92F735D90A4446868B9@DM8PR11MB5702.namprd11.prod.outlook.com
Fixes: 8edf533 ("PCI: pciehp: Fix MSI interrupt race")
Link: https://lore.kernel.org/r/66eaeef31d4997ceea357ad93259f290ededecfd.1637187226.git.lukas@wunner.de
Reported-by: Joseph Bao <joseph.bao@intel.com>
Tested-by: Joseph Bao <joseph.bao@intel.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: stable@vger.kernel.org # v4.19+
Cc: Stuart Hayes <stuart.w.hayes@gmail.com>

Author: l1k

drivers/pci/hotplug/pciehp_hpc.c

@@ -642,6 +642,8 @@ static irqreturn_t pciehp_isr(int irq, void *dev_id)
 	 */
 	if (ctrl->power_fault_detected)
 		status &= ~PCI_EXP_SLTSTA_PFD;
+	else if (status & PCI_EXP_SLTSTA_PFD)
+		ctrl->power_fault_detected = true;
 
 	events |= status;
 	if (!events) {
@@ -651,7 +653,7 @@ static irqreturn_t pciehp_isr(int irq, void *dev_id)
 	}
 
 	if (status) {
-		pcie_capability_write_word(pdev, PCI_EXP_SLTSTA, events);
+		pcie_capability_write_word(pdev, PCI_EXP_SLTSTA, status);
 
 		/*
 		 * In MSI mode, all event bits must be zero before the port
@@ -725,8 +727,7 @@ static irqreturn_t pciehp_ist(int irq, void *dev_id)
 	}
 
 	/* Check Power Fault Detected */
-	if ((events & PCI_EXP_SLTSTA_PFD) && !ctrl->power_fault_detected) {
-		ctrl->power_fault_detected = 1;
+	if (events & PCI_EXP_SLTSTA_PFD) {
 		ctrl_err(ctrl, "Slot(%s): Power fault\n", slot_name(ctrl));
 		pciehp_set_indicators(ctrl, PCI_EXP_SLTCTL_PWR_IND_OFF,
 				      PCI_EXP_SLTCTL_ATTN_IND_ON);