iio: dac: ad7293: add adc reference configuration

Add support for configurating the ADC reference (internal/external).

According to the datasheet, the external reference is enabled by
default.

Signed-off-by: Antoniu Miclaus <[e-mail]>
Reviewed-by: Nuno Sá <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/dac/ad7293.c

···
 #define AD7293_REG_DATA_RAW_MSK			GENMASK(15, 4)
 #define AD7293_REG_VINX_RANGE_GET_CH_MSK(x, ch)	(((x) >> (ch)) & 0x1)
 #define AD7293_REG_VINX_RANGE_SET_CH_MSK(x, ch)	(((x) & 0x1) << (ch))
+#define AD7293_GENERAL_ADC_REF_MSK			BIT(7)
 #define AD7293_CHIP_ID				0x18
 
 enum ad7293_ch_type {
···
 	/* Protect against concurrent accesses to the device, page selection and data content */
 	struct mutex lock;
 	struct gpio_desc *gpio_reset;
+	bool vrefin_en;
 	u8 page_select;
 	u8 data[3] __aligned(IIO_DMA_MINALIGN);
 };
···
 	if (ret)
 		return dev_err_probe(&spi->dev, ret, "failed to enable VDRIVE\n");
 
+	ret = devm_regulator_get_enable_optional(&spi->dev, "vrefin");
+	if (ret < 0 && ret != -ENODEV)
+		return dev_err_probe(&spi->dev, ret, "failed to enable VREFIN\n");
+
+	st->vrefin_en = ret != -ENODEV;
+
 	st->gpio_reset = devm_gpiod_get_optional(&st->spi->dev, "reset",
 						 GPIOD_OUT_HIGH);
 	if (IS_ERR(st->gpio_reset))
···
 		dev_err(&spi->dev, "Invalid Chip ID.\n");
 		return -EINVAL;
 	}
+
+	if (!st->vrefin_en)
+		return __ad7293_spi_update_bits(st, AD7293_REG_GENERAL,
+						AD7293_GENERAL_ADC_REF_MSK,
+						AD7293_GENERAL_ADC_REF_MSK);
 
 	return 0;
 }