PCI: vmd: Fix secondary bus reset for Intel bridges

The reset was never applied in the current implementation because Intel
Bridges owned by VMD are parentless. Internally, pci_reset_bus() applies
a reset to the parent of the PCI device supplied as argument, but in this
case it failed because there wasn't a parent.

In more detail, this change allows the VMD driver to enumerate NVMe devices
in pass-through configurations when guest reboots are performed. There was
an attempted to fix this, but later we discovered that the code inside
pci_reset_bus() wasn’t triggering secondary bus resets. Therefore, we
updated the parameters passed to it, and now NVMe SSDs attached to VMD
bridges are properly enumerated in VT-d pass-through scenarios.

Link: https://lore.kernel.org/r/20221206001637.4744-1-francisco.munoz.ruiz@linux.intel.com
Fixes: 6aab562 ("PCI: vmd: Clean up domain before enumeration")
Signed-off-by: Francisco Munoz <francisco.munoz.ruiz@linux.intel.com>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Reviewed-by: Nirmal Patel <nirmal.patel@linux.intel.com>
Reviewed-by: Jonathan Derrick <jonathan.derrick@linux.dev>

Author: Francisco Munoz

drivers/pci/controller/vmd.c

@@ -719,6 +719,7 @@ static int vmd_enable_domain(struct vmd_dev *vmd, unsigned long features)
 	resource_size_t offset[2] = {0};
 	resource_size_t membar2_offset = 0x2000;
 	struct pci_bus *child;
+	struct pci_dev *dev;
 	int ret;
 
 	/*
@@ -859,8 +860,25 @@ static int vmd_enable_domain(struct vmd_dev *vmd, unsigned long features)
 
 	pci_scan_child_bus(vmd->bus);
 	vmd_domain_reset(vmd);
-	list_for_each_entry(child, &vmd->bus->children, node)
-		pci_reset_bus(child->self);
+
+	/* When Intel VMD is enabled, the OS does not discover the Root Ports
+	 * owned by Intel VMD within the MMCFG space. pci_reset_bus() applies
+	 * a reset to the parent of the PCI device supplied as argument. This
+	 * is why we pass a child device, so the reset can be triggered at
+	 * the Intel bridge level and propagated to all the children in the
+	 * hierarchy.
+	 */
+	list_for_each_entry(child, &vmd->bus->children, node) {
+		if (!list_empty(&child->devices)) {
+			dev = list_first_entry(&child->devices,
+					       struct pci_dev, bus_list);
+			if (pci_reset_bus(dev))
+				pci_warn(dev, "can't reset device: %d\n", ret);
+
+			break;
+		}
+	}
+
 	pci_assign_unassigned_bus_resources(vmd->bus);
 
 	/*