intel-iommu: Avoid panic() for DRHD at address zero.

If the BIOS does something obviously stupid, like claiming that the
registers for the IOMMU are at physical address zero, then print a nasty
message and abort, rather than trying to set up the IOMMU and then later
panicking.

It's becoming more and more obvious that trusting this stuff to the BIOS
was a mistake.
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

drivers/pci/dmar.c

@@ -173,12 +173,21 @@ dmar_parse_one_drhd(struct acpi_dmar_header *header)
 	struct dmar_drhd_unit *dmaru;
 	int ret = 0;
 
+	drhd = (struct acpi_dmar_hardware_unit *)header;
+	if (!drhd->address) {
+		/* Promote an attitude of violence to a BIOS engineer today */
+		WARN(1, "Your BIOS is broken; DMAR reported at address zero!\n"
+		     "BIOS vendor: %s; Ver: %s; Product Version: %s\n",
+		     dmi_get_system_info(DMI_BIOS_VENDOR),
+		     dmi_get_system_info(DMI_BIOS_VERSION),
+		     dmi_get_system_info(DMI_PRODUCT_VERSION));
+		return -ENODEV;
+	}
 	dmaru = kzalloc(sizeof(*dmaru), GFP_KERNEL);
 	if (!dmaru)
 		return -ENOMEM;
 
 	dmaru->hdr = header;
-	drhd = (struct acpi_dmar_hardware_unit *)header;
 	dmaru->reg_base_addr = drhd->address;
 	dmaru->segment = drhd->segment;
 	dmaru->include_all = drhd->flags & 0x1; /* BIT0: INCLUDE_ALL */