x86/PCI: Move and shrink AMD 64-bit window to avoid conflict

Avoid problems with BIOS implementations which don't report all used
resources to the OS by only allocating a 256GB window directly below the
hardware limit (from the BKDG, sec 2.4.6).

Fixes a silent reboot loop reported by Aaro Koskinen <[e-mail]>
on an AMD-based MSI MS-7699/760GA-P43(FX) system. This was apparently
caused by RAM or other unreported hardware that conflicted with the new
window.

Link: https://support.amd.com/TechDocs/49125_15h_Models_30h-3Fh_BKDG.pdf
Link: https://lkml.kernel.org/r/[e-mail]
Fixes: fa564ad96366 ("x86/PCI: Enable a 64bit BAR on AMD Family 15h (Models 00-1f, 30-3f, 60-7f)")
Reported-by: Aaro Koskinen <[e-mail]>
Signed-off-by: Christian König <[e-mail]>
[bhelgaas: changelog, comment, Fixes:]
Signed-off-by: Bjorn Helgaas <[e-mail]>

arch/x86/pci/fixup.c

···
  */
 static void pci_amd_enable_64bit_bar(struct pci_dev *dev)
 {
-	unsigned i;
 	u32 base, limit, high;
-	struct resource *res, *conflict;
 	struct pci_dev *other;
+	struct resource *res;
+	unsigned i;
+	int r;
 
 	if (!(pci_probe & PCI_BIG_ROOT_WINDOW))
 		return;
···
 	if (!res)
 		return;
 
+	/*
+	 * Allocate a 256GB window directly below the 0xfd00000000 hardware
+	 * limit (see AMD Family 15h Models 30h-3Fh BKDG, sec 2.4.6).
+	 */
 	res->name = "PCI Bus 0000:00";
 	res->flags = IORESOURCE_PREFETCH | IORESOURCE_MEM |
 		IORESOURCE_MEM_64 | IORESOURCE_WINDOW;
-	res->start = 0x100000000ull;
+	res->start = 0xbd00000000ull;
 	res->end = 0xfd00000000ull - 1;
 
-	/* Just grab the free area behind system memory for this */
-	while ((conflict = request_resource_conflict(&iomem_resource, res))) {
-		if (conflict->end >= res->end) {
-			kfree(res);
-			return;
-		}
-		res->start = conflict->end + 1;
+	r = request_resource(&iomem_resource, res);
+	if (r) {
+		kfree(res);
+		return;
 	}
 
 	dev_info(&dev->dev, "adding root bus resource %pR (tainting kernel)\n",