mm/memory_hotplug: add mhp_online_type_to_str() and export string helpers

Add mhp_online_type_to_str() as the inverse of mhp_online_type_from_str(),
and export both so a driver can render and parse the memory online type
through its own sysfs interface.

Link: https://lore.kernel.org/20260712154505.3564379-3-gourry@gourry.net
Signed-off-by: Gregory Price <gourry@gourry.net>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Dan Williams <djbw@kernel.org>
Cc: Alison Schofield <alison.schofield@intel.com>
Cc: Danilo Krummrich <dakr@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Pankaj Gupta <pankaj.gupta@amd.com>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vishal Verma <vishal.l.verma@intel.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

drivers/base/memory.c

@@ -46,6 +46,15 @@ int mhp_online_type_from_str(const char *str)
 	}
 	return -EINVAL;
 }
+EXPORT_SYMBOL_GPL(mhp_online_type_from_str);
+
+const char *mhp_online_type_to_str(int online_type)
+{
+	if (online_type < 0 || online_type >= (int)ARRAY_SIZE(online_type_to_str))
+		return NULL;
+	return online_type_to_str[online_type];
+}
+EXPORT_SYMBOL_GPL(mhp_online_type_to_str);
 
 #define to_memory_block(dev) container_of(dev, struct memory_block, dev)
 

include/linux/memory_hotplug.h

@@ -127,6 +127,7 @@ extern int arch_add_memory(int nid, u64 start, u64 size,
 extern u64 max_mem_size;
 
 extern int mhp_online_type_from_str(const char *str);
+const char *mhp_online_type_to_str(int online_type);
 
 /* If movable_node boot option specified */
 extern bool movable_node_enabled;