PCI / PCIe: Introduce command line option to disable ARI

There are PCIe devices on the market that report ARI support but
then fail to initialize correctly when ARI is actually used. This
leads to situations in which kernels 2.6.34 and newer fail to handle
systems where the previous kernels worked without any apparent
problems. Unfortunately, it is currently unknown how many such
devices are there.

For this reason, introduce a new kernel command line option,
pci=noari, allowing users to disable PCIe ARI altogether if they
see problems with PCIe device initialization.

Signed-off-by: Rafael J. Wysocki <[e-mail]>
Signed-off-by: Jesse Barnes <[e-mail]>

Documentation/kernel-parameters.txt

···
 				off: Turn realloc off
 				on: Turn realloc on
 		realloc		same as realloc=on
+		noari		do not use PCIe ARI.
 
 	pcie_aspm=	[PCIE] Forcibly enable or disable PCIe Active State Power
 			Management.

drivers/pci/pci.c

···
  */
 unsigned int pcibios_max_latency = 255;
 
+/* If set, the PCIe ARI capability will not be used. */
+static bool pcie_ari_disabled;
+
 /**
  * pci_bus_max_busnr - returns maximum PCI bus number of given bus' children
  * @bus: pointer to PCI bus structure to search
···
 	u16 flags, ctrl;
 	struct pci_dev *bridge;
 
-	if (!pci_is_pcie(dev) || dev->devfn)
+	if (pcie_ari_disabled || !pci_is_pcie(dev) || dev->devfn)
 		return;
 
 	pos = pci_find_ext_capability(dev, PCI_EXT_CAP_ID_ARI);
···
 				pci_realloc_get_opt("on");
 			} else if (!strcmp(str, "nodomains")) {
 				pci_no_domains();
+			} else if (!strncmp(str, "noari", 5)) {
+				pcie_ari_disabled = true;
 			} else if (!strncmp(str, "cbiosize=", 9)) {
 				pci_cardbus_io_size = memparse(str + 9, &str);
 			} else if (!strncmp(str, "cbmemsize=", 10)) {