libdm: add dm_stats_get_{current_}area_offset()

Add a method to retrieve the offset of an area within the
containing region (rather than the offset within the containing
device returned by dm_stats_get_area_start()).

Although users of the library can calculate this themselves it is
better to provide this through a method call to avoid users making
assumptions about the structure of regions and areas.

libdm/.exported_symbols.DM_1_02_105

@@ -1 +1,3 @@
 dm_report_is_empty
+dm_stats_get_area_offset
+dm_stats_get_current_area_offset

libdm/libdevmapper.h

@@ -705,19 +705,27 @@ uint64_t dm_stats_get_region_area_len(const struct dm_stats *dms,
 				      uint64_t *area_len, uint64_t region_id);
 
 /*
- * Area properties: start and length.
+ * Area properties: start, offset and length.
  *
  * The area length is always equal to the area length of the region
  * that contains it and is obtained from dm_stats_get_region_area_len().
  *
- * The start offset of an area is a function of the area_id and the
+ * The start of an area is a function of the area_id and the containing
- * containing region's start and area length.
+ * region's start and area length: it gives the absolute offset into the
+ * containing device of the beginning of the area.
+ *
+ * The offset expresses the area's relative offset into the current
+ * region. I.e. the area start minus the start offset of the containing
+ * region.
  *
  * All values are returned in units of 512b sectors.
  */
 uint64_t dm_stats_get_area_start(const struct dm_stats *dms, uint64_t *start,
 				 uint64_t region_id, uint64_t area_id);
 
+uint64_t dm_stats_get_area_offset(const struct dm_stats *dms, uint64_t *offset,
+				  uint64_t region_id, uint64_t area_id);
+
 /*
  * Retrieve program_id and aux_data for a specific region. Only valid
  * following a call to dm_stats_list(). The returned pointer does not
@@ -876,6 +884,9 @@ uint64_t dm_stats_get_current_region_area_len(const struct dm_stats *dms,
 uint64_t dm_stats_get_current_area_start(const struct dm_stats *dms,
 					 uint64_t *start);
 
+uint64_t dm_stats_get_current_area_offset(const struct dm_stats *dms,
+					  uint64_t *offset);
+
 uint64_t dm_stats_get_current_area_len(const struct dm_stats *dms,
 				       uint64_t *start);
 

libdm/libdm-stats.c

@@ -1335,6 +1335,14 @@ uint64_t dm_stats_get_area_start(const struct dm_stats *dms, uint64_t *start,
 	return 1;
 }
 
+uint64_t dm_stats_get_area_offset(const struct dm_stats *dms, uint64_t *offset,
+				  uint64_t region_id, uint64_t area_id)
+{
+	if (!dms || !dms->regions)
+		return_0;
+	*offset = dms->regions[region_id].step * area_id;
+}
+
 uint64_t dm_stats_get_current_area_start(const struct dm_stats *dms,
 					 uint64_t *start)
 {
@@ -1342,6 +1350,13 @@ uint64_t dm_stats_get_current_area_start(const struct dm_stats *dms,
 				       dms->cur_region, dms->cur_area);
 }
 
+uint64_t dm_stats_get_current_area_offset(const struct dm_stats *dms,
+					  uint64_t *offset)
+{
+	return dm_stats_get_area_offset(dms, offset,
+				       dms->cur_region, dms->cur_area);
+}
+
 uint64_t dm_stats_get_current_area_len(const struct dm_stats *dms,
 				       uint64_t *len)
 {