i2c-imc-skylake: guard branch - verify the ECAM page before trusting it

A miscomputed page would send every engine write to some other
function's config space - the exact failure mode the ECAM path exists
to remove.  Require word 0 to read back as 8086:2085 before using the
mapping; otherwise fall back to config cycles.

i2c-imc-skylake.c

@@ -276,6 +276,23 @@ static void __iomem *imc_map_ecam(struct pci_dev *pdev)
 	}
 
 	acpi_put_table(&mcfg->header);
+	if (!map)
+		return NULL;
+
+	/*
+	 * A wrong page here would make every engine write land on some other
+	 * function - the exact failure mode this path exists to remove.
+	 * Word 0 is the one register whose value is known in advance; require
+	 * it before trusting the mapping with writes.
+	 */
+	if (readl(map) != (PCU_DEVICE << 16 | PCI_VENDOR_ID_INTEL)) {
+		dev_warn(&pdev->dev,
+			 "ECAM page does not identify as %04x:%04x; not using it\n",
+			 PCI_VENDOR_ID_INTEL, PCU_DEVICE);
+		devm_iounmap(&pdev->dev, map);
+		return NULL;
+	}
+
 	return map;
 }