dax: Document struct dev_dax_range

The device DAX structure is being enhanced to track additional DCD
information.  Specifically the range tuple needs additional parameters.
The current range tuple is not fully documented and is large enough to
warrant its own definition.

Separate the struct dax_dev_range definition and document it prior to
adding information for DC.

Suggested-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Link: https://patch.msgid.link/20241107-dcd-type2-upstream-v7-3-56a84e66bc36@intel.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>

Author: weiny2

drivers/dax/dax-private.h

@@ -40,12 +40,30 @@ struct dax_region {
 	struct device *youngest;
 };
 
+/**
+ * struct dax_mapping - device to display mapping range attributes
+ * @dev: device representing this range
+ * @range_id: index within dev_dax ranges array
+ * @id: ida of this mapping
+ */
 struct dax_mapping {
 	struct device dev;
 	int range_id;
 	int id;
 };
 
+/**
+ * struct dev_dax_range - tuple represenging a range of memory used by dev_dax
+ * @pgoff: page offset
+ * @range: resource-span
+ * @mapping: reference to the dax_mapping for this range
+ */
+struct dev_dax_range {
+	unsigned long pgoff;
+	struct range range;
+	struct dax_mapping *mapping;
+};
+
 /**
  * struct dev_dax - instance data for a subdivision of a dax region, and
  * data while the device is activated in the driver.
@@ -58,7 +76,7 @@ struct dax_mapping {
  * @dev - device core
  * @pgmap - pgmap for memmap setup / lifetime (driver owned)
  * @nr_range: size of @ranges
- * @ranges: resource-span + pgoff tuples for the instance
+ * @ranges: range tuples of memory used
  */
 struct dev_dax {
 	struct dax_region *region;
@@ -72,11 +90,7 @@ struct dev_dax {
 	struct dev_pagemap *pgmap;
 	bool memmap_on_memory;
 	int nr_range;
-	struct dev_dax_range {
-		unsigned long pgoff;
-		struct range range;
-		struct dax_mapping *mapping;
-	} *ranges;
+	struct dev_dax_range *ranges;
 };
 
 /*