nvdimm, btt: add a size attribute for BTTs

To be consistent with other namespaces, expose a 'size' attribute for
BTT devices also.

Cc: Dan Williams <dan.j.williams@intel.com>
Reported-by: Linda Knippers <linda.knippers@hpe.com>
Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/nvdimm/btt.c

@@ -1269,6 +1269,7 @@ static int btt_blk_init(struct btt *btt)
 		}
 	}
 	set_capacity(btt->btt_disk, btt->nlba * btt->sector_size >> 9);
+	btt->nd_btt->size = btt->nlba * (u64)btt->sector_size;
 	revalidate_disk(btt->btt_disk);
 
 	return 0;

drivers/nvdimm/btt_devs.c

@@ -140,10 +140,30 @@ static ssize_t namespace_store(struct device *dev,
 }
 static DEVICE_ATTR_RW(namespace);
 
+static ssize_t size_show(struct device *dev,
+		struct device_attribute *attr, char *buf)
+{
+	struct nd_btt *nd_btt = to_nd_btt(dev);
+	ssize_t rc;
+
+	device_lock(dev);
+	if (dev->driver)
+		rc = sprintf(buf, "%llu\n", nd_btt->size);
+	else {
+		/* no size to convey if the btt instance is disabled */
+		rc = -ENXIO;
+	}
+	device_unlock(dev);
+
+	return rc;
+}
+static DEVICE_ATTR_RO(size);
+
 static struct attribute *nd_btt_attributes[] = {
 	&dev_attr_sector_size.attr,
 	&dev_attr_namespace.attr,
 	&dev_attr_uuid.attr,
+	&dev_attr_size.attr,
 	NULL,
 };
 

drivers/nvdimm/nd.h

@@ -143,6 +143,7 @@ struct nd_btt {
 	struct nd_namespace_common *ndns;
 	struct btt *btt;
 	unsigned long lbasize;
+	u64 size;
 	u8 *uuid;
 	int id;
 };