regmap: Don't generate gather writes for single register raw writes

Since it is quite common for single register raw or async writes to be
generated by rbtree cache syncs or firmware downloads and essentially all
hardware will be faster with only a single transfer optimise this case by
copying single values into the internal scratch buffer before sending.

Signed-off-by: Mark Brown <broonie@linaro.org>

drivers/base/regmap/regmap.c

@@ -1116,6 +1116,16 @@ int _regmap_raw_write(struct regmap *map, unsigned int reg,
 
 	u8[0] |= map->write_flag_mask;
 
+	/*
+	 * Essentially all I/O mechanisms will be faster with a single
+	 * buffer to write.  Since register syncs often generate raw
+	 * writes of single registers optimise that case.
+	 */
+	if (val != work_val && val_len == map->format.val_bytes) {
+		memcpy(work_val, val, map->format.val_bytes);
+		val = work_val;
+	}
+
 	if (async && map->bus->async_write) {
 		struct regmap_async *async;