libnvdimm: Export the target_node attribute for regions and namespaces

Aneesh points out that some platforms may have "local" attached
persistent memory and "remote" persistent memory that map to the same
"online" node, or persistent memory devices with different performance
properties. In this case 'numa_node' is identical for the two instances,
but 'target_node' is differentiated so platform firmware can communicate
distinct performance properties per range. Expose 'target_node' by
default to allow for disambiguation of devices that share the same
numa_map_to_online_node() result.
Reported-by: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Link: https://lore.kernel.org/r/157401274500.43284.2369509941678577768.stgit@dwillia2-desk3.amr.corp.intel.comSigned-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/nvdimm/bus.c

@@ -685,17 +685,46 @@ static ssize_t numa_node_show(struct device *dev,
 }
 static DEVICE_ATTR_RO(numa_node);
 
+static int nvdimm_dev_to_target_node(struct device *dev)
+{
+	struct device *parent = dev->parent;
+	struct nd_region *nd_region = NULL;
+
+	if (is_nd_region(dev))
+		nd_region = to_nd_region(dev);
+	else if (parent && is_nd_region(parent))
+		nd_region = to_nd_region(parent);
+
+	if (!nd_region)
+		return NUMA_NO_NODE;
+	return nd_region->target_node;
+}
+
+static ssize_t target_node_show(struct device *dev,
+		struct device_attribute *attr, char *buf)
+{
+	return sprintf(buf, "%d\n", nvdimm_dev_to_target_node(dev));
+}
+static DEVICE_ATTR_RO(target_node);
+
 static struct attribute *nd_numa_attributes[] = {
 	&dev_attr_numa_node.attr,
+	&dev_attr_target_node.attr,
 	NULL,
 };
 
 static umode_t nd_numa_attr_visible(struct kobject *kobj, struct attribute *a,
 		int n)
 {
+	struct device *dev = container_of(kobj, typeof(*dev), kobj);
+
 	if (!IS_ENABLED(CONFIG_NUMA))
 		return 0;
 
+	if (a == &dev_attr_target_node.attr &&
+			nvdimm_dev_to_target_node(dev) == NUMA_NO_NODE)
+		return 0;
+
 	return a->mode;
 }