PCI: Expose ari_enabled in sysfs

Some multifunction PCI devices with more than 8 functions use "alternative
routing-ID interpretation" (ARI), which means the 8-bit device/function
number field will be interpreted as 8 bits specifying the function number
(the device number is 0 implicitly), rather than the upper 5 bits
specifying the device number and the lower 3 bits specifying the function
number. The kernel can enable and use this.

Expose in a sysfs attribute whether the kernel has enabled ARI, so that a
program in userspace won't have to parse PCI devices and PCI configuration
space to figure out if it is enabled. This will allow better predictable
network naming using PCI function numbers without using PCI bus or device
numbers, which is desirable because bus and device numbers can change with
system configuration but function numbers will not.
Signed-off-by: Stuart Hayes <stuart.w.hayes@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

drivers/pci/pci-sysfs.c

@@ -278,6 +278,16 @@ static ssize_t subordinate_bus_number_show(struct device *dev,
 }
 static DEVICE_ATTR_RO(subordinate_bus_number);
 
+static ssize_t ari_enabled_show(struct device *dev,
+				struct device_attribute *attr,
+				char *buf)
+{
+	struct pci_dev *pci_dev = to_pci_dev(dev);
+
+	return sprintf(buf, "%u\n", pci_ari_enabled(pci_dev->bus));
+}
+static DEVICE_ATTR_RO(ari_enabled);
+
 static ssize_t modalias_show(struct device *dev, struct device_attribute *attr,
 			     char *buf)
 {
@@ -786,6 +796,7 @@ static struct attribute *pci_dev_attrs[] = {
 	&dev_attr_devspec.attr,
 #endif
 	&dev_attr_driver_override.attr,
+	&dev_attr_ari_enabled.attr,
 	NULL,
 };