regmap: Allow empty read/write_flag_mask

All zero read and write masks in the regmap config are used to signal no
special mask is needed and the bus defaults are used. In some devices
all zero read/write masks are the special mask and bus defaults should
not be used. To signal this a new variable is added.

For example SPI often sets bit 7 in address to signal to the device a
read is requested. On TI AFE44xx parts with SPI interfaces no bit
needs to be set as registers are either read or write only and the
operation can be determined from the address only. For this case both
masks must be zero to not effect the address.

Signed-off-by: Andrew F. Davis <[e-mail]>
Signed-off-by: Mark Brown <[e-mail]>

drivers/base/regmap/regmap.c

···
 	INIT_LIST_HEAD(&map->async_free);
 	init_waitqueue_head(&map->async_waitq);
 
-	if (config->read_flag_mask || config->write_flag_mask) {
+	if (config->read_flag_mask ||
+	    config->write_flag_mask ||
+	    config->zero_flag_mask) {
 		map->read_flag_mask = config->read_flag_mask;
 		map->write_flag_mask = config->write_flag_mask;
 	} else if (bus) {

include/linux/regmap.h

···
  *                  a read.
  * @write_flag_mask: Mask to be set in the top bytes of the register when doing
  *                   a write. If both read_flag_mask and write_flag_mask are
- *                   empty the regmap_bus default masks are used.
+ *                   empty and zero_flag_mask is not set the regmap_bus default
+ *                   masks are used.
+ * @zero_flag_mask: If set, read_flag_mask and write_flag_mask are used even
+ *                   if they are both empty.
  * @use_single_rw: If set, converts the bulk read and write operations into
  *		    a series of single read and write operations. This is useful
  *		    for device that does not support bulk read and write.
···
 
 	unsigned long read_flag_mask;
 	unsigned long write_flag_mask;
+	bool zero_flag_mask;
 
 	bool use_single_rw;
 	bool can_multi_write;