device-dax: Add a 'resource' attribute

device-dax based devices were missing a 'resource' attribute to indicate
the physical address range contributed by the device in question. This
information is desirable to userspace tooling that may want to use the
dax device as system-ram, and wants to selectively hotplug and online
the memory blocks associated with a given device.

Without this, the tooling would have to parse /proc/iomem for the memory
ranges contributed by dax devices, which can be a workaround, but it is
far easier to provide this information in the sysfs hierarchy.

Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/dax/bus.c

@@ -295,6 +295,22 @@ static ssize_t target_node_show(struct device *dev,
 }
 static DEVICE_ATTR_RO(target_node);
 
+static unsigned long long dev_dax_resource(struct dev_dax *dev_dax)
+{
+	struct dax_region *dax_region = dev_dax->region;
+
+	return dax_region->res.start;
+}
+
+static ssize_t resource_show(struct device *dev,
+		struct device_attribute *attr, char *buf)
+{
+	struct dev_dax *dev_dax = to_dev_dax(dev);
+
+	return sprintf(buf, "%#llx\n", dev_dax_resource(dev_dax));
+}
+static DEVICE_ATTR_RO(resource);
+
 static ssize_t modalias_show(struct device *dev, struct device_attribute *attr,
 		char *buf)
 {
@@ -313,6 +329,8 @@ static umode_t dev_dax_visible(struct kobject *kobj, struct attribute *a, int n)
 
 	if (a == &dev_attr_target_node.attr && dev_dax_target_node(dev_dax) < 0)
 		return 0;
+	if (a == &dev_attr_resource.attr)
+		return 0400;
 	return a->mode;
 }
 
@@ -320,6 +338,7 @@ static struct attribute *dev_dax_attributes[] = {
 	&dev_attr_modalias.attr,
 	&dev_attr_size.attr,
 	&dev_attr_target_node.attr,
+	&dev_attr_resource.attr,
 	NULL,
 };