power/supply: ingenic-battery: Don't change scale if there's only one

The ADC in the JZ4740 can work either in high-precision mode with a 2.5V
range, or in low-precision mode with a 7.5V range. The code in place in
this driver will select the proper scale according to the maximum
voltage of the battery.

The JZ4770 however only has one mode, with a 6.6V range. If only one
scale is available, there's no need to change it (and nothing to change
it to), and trying to do so will fail with -EINVAL.

Fixes: fb24ccfb ("power: supply: add Ingenic JZ47xx battery driver.")
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Acked-by: Artur Rojek <contact@artur-rojek.eu>
Cc: stable@vger.kernel.org
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>

drivers/power/supply/ingenic-battery.c

@@ -100,10 +100,17 @@ static int ingenic_battery_set_scale(struct ingenic_battery *bat)
 		return -EINVAL;
 	}
 
-	return iio_write_channel_attribute(bat->channel,
-					   scale_raw[best_idx],
-					   scale_raw[best_idx + 1],
-					   IIO_CHAN_INFO_SCALE);
+	/* Only set scale if there is more than one (fractional) entry */
+	if (scale_len > 2) {
+		ret = iio_write_channel_attribute(bat->channel,
+						  scale_raw[best_idx],
+						  scale_raw[best_idx + 1],
+						  IIO_CHAN_INFO_SCALE);
+		if (ret)
+			return ret;
+	}
+
+	return 0;
 }
 
 static enum power_supply_property ingenic_battery_properties[] = {