iio: adc: ad7192: Correct reference voltage

The avdd and the reference voltage are two different sources but the
reference voltage was assigned according to the avdd supply.

Add vref regulator structure and set the reference voltage according to
the vref supply from the devicetree.

In case vref supply is missing, reference voltage is set according to
the avdd supply for compatibility with old devicetrees.

Fixes: b581f74 ("staging: iio: adc: ad7192: move out of staging")
Signed-off-by: Alisa-Dariana Roman <alisa.roman@analog.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20230924152149.41884-1-alisadariana@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/ad7192.c

@@ -177,6 +177,7 @@ struct ad7192_chip_info {
 struct ad7192_state {
 	const struct ad7192_chip_info	*chip_info;
 	struct regulator		*avdd;
+	struct regulator		*vref;
 	struct clk			*mclk;
 	u16				int_vref_mv;
 	u32				fclk;
@@ -1008,10 +1009,30 @@ static int ad7192_probe(struct spi_device *spi)
 	if (ret)
 		return dev_err_probe(&spi->dev, ret, "Failed to enable specified DVdd supply\n");
 
-	ret = regulator_get_voltage(st->avdd);
-	if (ret < 0) {
-		dev_err(&spi->dev, "Device tree error, reference voltage undefined\n");
-		return ret;
+	st->vref = devm_regulator_get_optional(&spi->dev, "vref");
+	if (IS_ERR(st->vref)) {
+		if (PTR_ERR(st->vref) != -ENODEV)
+			return PTR_ERR(st->vref);
+
+		ret = regulator_get_voltage(st->avdd);
+		if (ret < 0)
+			return dev_err_probe(&spi->dev, ret,
+					     "Device tree error, AVdd voltage undefined\n");
+	} else {
+		ret = regulator_enable(st->vref);
+		if (ret) {
+			dev_err(&spi->dev, "Failed to enable specified Vref supply\n");
+			return ret;
+		}
+
+		ret = devm_add_action_or_reset(&spi->dev, ad7192_reg_disable, st->vref);
+		if (ret)
+			return ret;
+
+		ret = regulator_get_voltage(st->vref);
+		if (ret < 0)
+			return dev_err_probe(&spi->dev, ret,
+					     "Device tree error, Vref voltage undefined\n");
 	}
 	st->int_vref_mv = ret / 1000;