uuid: hoist uuid_is_null() helper from libnvdimm

Hoist the libnvdimm helper as an inline helper to linux/uuid.h
using an auxiliary const variable uuid_null in lib/uuid.c.

[hch: also add the guid variant.  Both do the same but I'd like
to keep casts to a minimum]

The common helper uses the new abstract type uuid_t * instead of
u8 *.
Suggested-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
[hch: added guid_is_null]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

drivers/nvdimm/btt_devs.c

@@ -222,13 +222,6 @@ struct device *nd_btt_create(struct nd_region *nd_region)
 	return dev;
 }
 
-static bool uuid_is_null(u8 *uuid)
-{
-	static const u8 null_uuid[16];
-
-	return (memcmp(uuid, null_uuid, 16) == 0);
-}
-
 /**
  * nd_btt_arena_is_valid - check if the metadata layout is valid
  * @nd_btt:	device with BTT geometry and backing device info
@@ -249,7 +242,7 @@ bool nd_btt_arena_is_valid(struct nd_btt *nd_btt, struct btt_sb *super)
 	if (memcmp(super->signature, BTT_SIG, BTT_SIG_LEN) != 0)
 		return false;
 
-	if (!uuid_is_null(super->parent_uuid))
+	if (!guid_is_null((guid_t *)&super->parent_uuid))
 		if (memcmp(super->parent_uuid, parent_uuid, 16) != 0)
 			return false;
 

include/linux/uuid.h

@@ -35,6 +35,9 @@ typedef struct {
  */
 #define	UUID_STRING_LEN		36
 
+extern const guid_t guid_null;
+extern const uuid_t uuid_null;
+
 static inline bool guid_equal(const guid_t *u1, const guid_t *u2)
 {
 	return memcmp(u1, u2, sizeof(guid_t)) == 0;
@@ -45,6 +48,11 @@ static inline void guid_copy(guid_t *dst, const guid_t *src)
 	memcpy(dst, src, sizeof(guid_t));
 }
 
+static inline bool guid_is_null(guid_t *guid)
+{
+	return guid_equal(guid, &guid_null);
+}
+
 static inline bool uuid_equal(const uuid_t *u1, const uuid_t *u2)
 {
 	return memcmp(u1, u2, sizeof(uuid_t)) == 0;
@@ -55,6 +63,11 @@ static inline void uuid_copy(uuid_t *dst, const uuid_t *src)
 	memcpy(dst, src, sizeof(uuid_t));
 }
 
+static inline bool uuid_is_null(uuid_t *uuid)
+{
+	return uuid_equal(uuid, &uuid_null);
+}
+
 void generate_random_uuid(unsigned char uuid[16]);
 
 extern void guid_gen(guid_t *u);

lib/uuid.c

@@ -21,6 +21,11 @@
 #include <linux/uuid.h>
 #include <linux/random.h>
 
+const guid_t guid_null;
+EXPORT_SYMBOL(guid_null);
+const uuid_t uuid_null;
+EXPORT_SYMBOL(uuid_null);
+
 const u8 guid_index[16] = {3,2,1,0,5,4,7,6,8,9,10,11,12,13,14,15};
 const u8 uuid_index[16] = {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15};