dm: adc: uclass: get reference regulator once

device_get_supply_regulator() only needs to be called once.
But each time there's call to adc_vxx_value() for instance, it calls
adc_vxx_platdata_update() -> device_get_supply_regulator().

So, move device_get_supply_regulator() to pre_probe() routine.

This also allows vdd_supply/vss_supply to be provided directly from
uc_pdata, e.g dt-binding variant like stm32-adc provide its own
'vref-supply'.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

drivers/adc/adc-uclass.c

@@ -264,10 +264,8 @@ static int adc_vdd_platdata_update(struct udevice *dev)
 	 * will bind before its supply regulator device, then the below 'get'
 	 * will return an error.
 	 */
-	ret = device_get_supply_regulator(dev, "vdd-supply",
-					  &uc_pdata->vdd_supply);
-	if (ret)
-		return ret;
+	if (!uc_pdata->vdd_supply)
+		return 0;
 
 	ret = regulator_get_value(uc_pdata->vdd_supply);
 	if (ret < 0)
@@ -283,10 +281,8 @@ static int adc_vss_platdata_update(struct udevice *dev)
 	struct adc_uclass_platdata *uc_pdata = dev_get_uclass_platdata(dev);
 	int ret;
 
-	ret = device_get_supply_regulator(dev, "vss-supply",
-					  &uc_pdata->vss_supply);
-	if (ret)
-		return ret;
+	if (!uc_pdata->vss_supply)
+		return 0;
 
 	ret = regulator_get_value(uc_pdata->vss_supply);
 	if (ret < 0)
@@ -302,14 +298,11 @@ int adc_vdd_value(struct udevice *dev, int *uV)
 	struct adc_uclass_platdata *uc_pdata = dev_get_uclass_platdata(dev);
 	int ret, value_sign = uc_pdata->vdd_polarity_negative ? -1 : 1;
 
-	if (!uc_pdata->vdd_supply)
-		goto nodev;
-
 	/* Update the regulator Value. */
 	ret = adc_vdd_platdata_update(dev);
 	if (ret)
 		return ret;
-nodev:
+
 	if (uc_pdata->vdd_microvolts == -ENODATA)
 		return -ENODATA;
 
@@ -323,14 +316,11 @@ int adc_vss_value(struct udevice *dev, int *uV)
 	struct adc_uclass_platdata *uc_pdata = dev_get_uclass_platdata(dev);
 	int ret, value_sign = uc_pdata->vss_polarity_negative ? -1 : 1;
 
-	if (!uc_pdata->vss_supply)
-		goto nodev;
-
 	/* Update the regulator Value. */
 	ret = adc_vss_platdata_update(dev);
 	if (ret)
 		return ret;
-nodev:
+
 	if (uc_pdata->vss_microvolts == -ENODATA)
 		return -ENODATA;
 
@@ -348,7 +338,12 @@ static int adc_vdd_platdata_set(struct udevice *dev)
 	prop = "vdd-polarity-negative";
 	uc_pdata->vdd_polarity_negative = dev_read_bool(dev, prop);
 
-	ret = adc_vdd_platdata_update(dev);
+	/* Optionally get regulators */
+	ret = device_get_supply_regulator(dev, "vdd-supply",
+					  &uc_pdata->vdd_supply);
+	if (!ret)
+		return adc_vdd_platdata_update(dev);
+
 	if (ret != -ENOENT)
 		return ret;
 
@@ -368,7 +363,11 @@ static int adc_vss_platdata_set(struct udevice *dev)
 	prop = "vss-polarity-negative";
 	uc_pdata->vss_polarity_negative = dev_read_bool(dev, prop);
 
-	ret = adc_vss_platdata_update(dev);
+	ret = device_get_supply_regulator(dev, "vss-supply",
+					  &uc_pdata->vss_supply);
+	if (!ret)
+		return adc_vss_platdata_update(dev);
+
 	if (ret != -ENOENT)
 		return ret;