x86/PCI: Add "pci=big_root_window" option for AMD 64-bit windows

Only try to enable a 64-bit window on AMD CPUs when "pci=big_root_window"
is specified.

This taints the kernel because the new 64-bit window uses address space we
don't know anything about, and it may contain unreported devices or memory
that would conflict with the window.

The pci_amd_enable_64bit_bar() quirk that enables the window is specific to
AMD CPUs.  The generic solution would be to have the firmware enable the
window and describe it in the host bridge's _CRS method, or at least
describe it in the _PRS method so the OS would have the option of enabling
it.

Signed-off-by: Christian König <christian.koenig@amd.com>
[bhelgaas: changelog, extend doc, mention taint in dmesg]
Signed-off-by: Bjorn Helgaas <helgaas@kernel.org>

Documentation/admin-guide/kernel-parameters.txt

@@ -3094,6 +3094,12 @@
 		pcie_scan_all	Scan all possible PCIe devices.  Otherwise we
 				only look for one device below a PCIe downstream
 				port.
+		big_root_window	Try to add a big 64bit memory window to the PCIe
+				root complex on AMD CPUs. Some GFX hardware
+				can resize a BAR to allow access to all VRAM.
+				Adding the window is slightly risky (it may
+				conflict with unreported devices), so this
+				taints the kernel.
 
 	pcie_aspm=	[PCIE] Forcibly enable or disable PCIe Active State Power
 			Management.

arch/x86/include/asm/pci_x86.h

@@ -38,6 +38,7 @@ do {						\
 #define PCI_NOASSIGN_ROMS	0x80000
 #define PCI_ROOT_NO_CRS		0x100000
 #define PCI_NOASSIGN_BARS	0x200000
+#define PCI_BIG_ROOT_WINDOW	0x400000
 
 extern unsigned int pci_probe;
 extern unsigned long pirq_table_addr;

arch/x86/pci/common.c

@@ -594,6 +594,11 @@ char *__init pcibios_setup(char *str)
 	} else if (!strcmp(str, "nocrs")) {
 		pci_probe |= PCI_ROOT_NO_CRS;
 		return NULL;
+#ifdef CONFIG_PHYS_ADDR_T_64BIT
+	} else if (!strcmp(str, "big_root_window")) {
+		pci_probe |= PCI_BIG_ROOT_WINDOW;
+		return NULL;
+#endif
 	} else if (!strcmp(str, "earlydump")) {
 		pci_early_dump_regs = 1;
 		return NULL;

arch/x86/pci/fixup.c

@@ -667,6 +667,9 @@ static void pci_amd_enable_64bit_bar(struct pci_dev *dev)
 	struct resource *res, *conflict;
 	struct pci_dev *other;
 
+	if (!(pci_probe & PCI_BIG_ROOT_WINDOW))
+		return;
+
 	/* Check that we are the only device of that type */
 	other = pci_get_device(dev->vendor, dev->device, NULL);
 	if (other != dev ||
@@ -714,7 +717,9 @@ static void pci_amd_enable_64bit_bar(struct pci_dev *dev)
 		res->start = conflict->end + 1;
 	}
 
-	dev_info(&dev->dev, "adding root bus resource %pR\n", res);
+	dev_info(&dev->dev, "adding root bus resource %pR (tainting kernel)\n",
+		 res);
+	add_taint(TAINT_FIRMWARE_WORKAROUND, LOCKDEP_STILL_OK);
 
 	base = ((res->start >> 8) & AMD_141b_MMIO_BASE_MMIOBASE_MASK) |
 		AMD_141b_MMIO_BASE_RE_MASK | AMD_141b_MMIO_BASE_WE_MASK;