regcache: Warn if regcache_sync() is called in cache_only mode

Calling regcache_sync() while cache_only is enabled is invalid API
usage, since writes are intentionally kept in the cache and cannot
be synchronized to hardware.
Document that callers must disable cache_only before calling
regcache_sync(), and reject incorrect usage with a WARN_ON() and
-EINVAL.

Signed-off-by: bui duc phuc <phucduc.bui@gmail.com>
Link: https://patch.msgid.link/20260720033238.52479-1-phucduc.bui@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>

Author: BuiDucPhuc

drivers/base/regmap/regcache.c

@@ -397,6 +397,10 @@ static int rbtree_all(const void *key, const struct rb_node *node)
  * volatile.  In general drivers can choose not to use the provided
  * syncing functionality if they so require.
  *
+ * This pushes cached changes made while cache_only (e.g. suspend) down
+ * to hardware. The caller must disable cache_only before calling this
+ * function.
+ *
  * Return a negative value on failure, 0 on success.
  */
 int regcache_sync(struct regmap *map)
@@ -413,6 +417,12 @@ int regcache_sync(struct regmap *map)
 	BUG_ON(!map->cache_ops);
 
 	map->lock(map->lock_arg);
+
+	if (WARN_ON(map->cache_only)) {
+		map->unlock(map->lock_arg);
+		return -EINVAL;
+	}
+
 	/* Remember the initial bypass state */
 	bypass = map->cache_bypass;
 	dev_dbg(map->dev, "Syncing %s cache\n",