PCI: Enable PCIe Extended Tags if supported

Every PCIe device can generate 5-bit transaction Tags, which allow up to 32
concurrent requests.  Some devices can generate 8-bit Extended Tags, which
allow up to 256 concurrent requests.

Per the ECN mentioned below, all PCIe Receivers are expected to support
Extended Tags, so devices are allowed (but not required) to enable them by
default.

If a device supports Extended Tags but does not enable them by default,
enable them.  This allows the device to have up to 256 outstanding
transactions at a time, which may improve performance.

[bhelgaas: changelog, check for PCIe device]
Link: https://pcisig.com/sites/default/files/specification_documents/ECN_Extended_Tag_Enable_Default_05Sept2008_final.pdf
Signed-off-by: Sinan Kaya <okaya@codeaurora.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

drivers/pci/probe.c

@@ -1661,12 +1661,30 @@ static void program_hpp_type2(struct pci_dev *dev, struct hpp_type2 *hpp)
 	 */
 }
 
+static void pci_configure_extended_tags(struct pci_dev *dev)
+{
+	u32 dev_cap;
+	int ret;
+
+	if (!pci_is_pcie(dev))
+		return;
+
+	ret = pcie_capability_read_dword(dev, PCI_EXP_DEVCAP, &dev_cap);
+	if (ret)
+		return;
+
+	if (dev_cap & PCI_EXP_DEVCAP_EXT_TAG)
+		pcie_capability_set_word(dev, PCI_EXP_DEVCTL,
+					 PCI_EXP_DEVCTL_EXT_TAG);
+}
+
 static void pci_configure_device(struct pci_dev *dev)
 {
 	struct hotplug_params hpp;
 	int ret;
 
 	pci_configure_mps(dev);
+	pci_configure_extended_tags(dev);
 
 	memset(&hpp, 0, sizeof(hpp));
 	ret = pci_get_hp_params(dev, &hpp);