regmap: Let users check if a register is cached

The HDA driver has a use case for checking if a register is cached which
it bodges in awkwardly and unclearly. Provide an API which allows it to
directly do what it's trying to do.

Reviewed-by: Takashi Iwai <[e-mail]>
Signed-off-by: Mark Brown <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

drivers/base/regmap/regcache.c

···
 }
 EXPORT_SYMBOL_GPL(regcache_cache_bypass);
 
+/**
+ * regcache_reg_cached - Check if a register is cached
+ *
+ * @map: map to check
+ * @reg: register to check
+ *
+ * Reports if a register is cached.
+ */
+bool regcache_reg_cached(struct regmap *map, unsigned int reg)
+{
+	unsigned int val;
+	int ret;
+
+	map->lock(map->lock_arg);
+
+	ret = regcache_read(map, reg, &val);
+
+	map->unlock(map->lock_arg);
+
+	return ret == 0;
+}
+EXPORT_SYMBOL_GPL(regcache_reg_cached);
+
 void regcache_set_val(struct regmap *map, void *base, unsigned int idx,
 		      unsigned int val)
 {

include/linux/regmap.h

···
 void regcache_cache_only(struct regmap *map, bool enable);
 void regcache_cache_bypass(struct regmap *map, bool enable);
 void regcache_mark_dirty(struct regmap *map);
+bool regcache_reg_cached(struct regmap *map, unsigned int reg);
 
 bool regmap_check_range_table(struct regmap *map, unsigned int reg,
 			      const struct regmap_access_table *table);