regmap: Check for register readability before checking cache during read

Ensure that we don't return a spurious cache hit for unreadable registers
(eg, with the flat cache which doesn't understand sparseness) by checking
for readability before we do a cache lookup.
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230613-b4-regmap-check-readability-before-cache-v1-1-b144c0b01ed9@kernel.orgSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/base/regmap/regmap.c

@@ -2897,6 +2897,9 @@ static int _regmap_read(struct regmap *map, unsigned int reg,
 	int ret;
 	void *context = _regmap_map_get_context(map);
 
+	if (!regmap_readable(map, reg))
+		return -EIO;
+
 	if (!map->cache_bypass) {
 		ret = regcache_read(map, reg, val);
 		if (ret == 0)
@@ -2906,9 +2909,6 @@ static int _regmap_read(struct regmap *map, unsigned int reg,
 	if (map->cache_only)
 		return -EBUSY;
 
-	if (!regmap_readable(map, reg))
-		return -EIO;
-
 	ret = map->reg_read(context, reg, val);
 	if (ret == 0) {
 		if (regmap_should_log(map))