[PATCH 3/4] PCI/PM: Do not restore a config space snapshot that is all ones

["Francisco Beltrán Millalén" <fbeltranmillalen@gmail.com>]

Belt and braces for the previous patch: even if a snapshot of all ones
is somehow recorded, never write it back.  The vendor ID of a device
that was present can never be 0xffffffff, so a snapshot whose first
dword is all ones is known to be garbage.

Signed-off-by: Francisco Beltrán Millalén <fbeltranmillalen@gmail.com>
---
diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -1833,6 +1833,17 @@
 
 static void pci_restore_config_space(struct pci_dev *pdev)
 {
+	/*
+	 * Vendor and Device ID are read-only, so a snapshot whose first
+	 * dword reads as all ones was taken from an inaccessible device and
+	 * is garbage.  Writing it back would corrupt a device that has since
+	 * become accessible again, so leave the hardware alone.
+	 */
+	if (PCI_POSSIBLE_ERROR(pdev->saved_config_space[0])) {
+		pci_warn(pdev, "not restoring config space, saved state is invalid\n");
+		return;
+	}
+
 	if (pdev->hdr_type == PCI_HEADER_TYPE_NORMAL) {
 		pci_restore_config_space_range(pdev, 10, 15, 0, false);
 		/* Restore BARs before the command register. */