regmap: Add better support for devices without readback support

Currently regmap requires that a reg_read callback is supplied, otherwise a
warning is emitted each time regmap_read() is called. This means a device
or bus without readback support needs to supply dummy reg_read callback.
Apart from that regmap_read() will still work fine if a cache is used.

Remove the warning and let regmap_readable() return false if not reg_read
callback is supplied. This means a device no longer has to supply a dummy
callback if it does not support readback and it also doesn't have to have a
readable_reg callback that always returns false since this is now implicit.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/base/regmap/regmap.c

@@ -93,6 +93,9 @@ bool regmap_writeable(struct regmap *map, unsigned int reg)
 
 bool regmap_readable(struct regmap *map, unsigned int reg)
 {
+	if (!map->reg_read)
+		return false;
+
 	if (map->max_register && reg > map->max_register)
 		return false;
 
@@ -2097,8 +2100,6 @@ static int _regmap_read(struct regmap *map, unsigned int reg,
 	int ret;
 	void *context = _regmap_map_get_context(map);
 
-	WARN_ON(!map->reg_read);
-
 	if (!map->cache_bypass) {
 		ret = regcache_read(map, reg, val);
 		if (ret == 0)