iio: adc: stm32-adc: skip adc-channels setup if none is present

If only adc differential channels are defined driver will fail with
stm32-adc: probe of 48003000.adc:adc@0 failed with error -22

Fix this by skipping the initialization if no channels are defined.

This applies only to the legacy way of initializing adc channels.

Fixes: d7705f35 ("iio: adc: stm32-adc: convert to device properties")
Signed-off-by: Sean Nyekjaer <sean@geanix.com>
Reviewed-by: Olivier Moysan <olivier.moysan@foss.st.com>
Link: https://lore.kernel.org/r/20230503162029.3654093-2-sean@geanix.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/stm32-adc.c

@@ -2036,6 +2036,7 @@ static int stm32_adc_legacy_chan_init(struct iio_dev *indio_dev,
 	struct stm32_adc_diff_channel diff[STM32_ADC_CH_MAX];
 	struct device *dev = &indio_dev->dev;
 	u32 num_diff = adc->num_diff;
+	int num_se = nchans - num_diff;
 	int size = num_diff * sizeof(*diff) / sizeof(u32);
 	int scan_index = 0, ret, i, c;
 	u32 smp = 0, smps[STM32_ADC_CH_MAX], chans[STM32_ADC_CH_MAX];
@@ -2062,13 +2063,14 @@ static int stm32_adc_legacy_chan_init(struct iio_dev *indio_dev,
 			scan_index++;
 		}
 	}
-
-	ret = device_property_read_u32_array(dev, "st,adc-channels", chans,
-					     nchans);
-	if (ret)
+	if (num_se > 0) {
+		ret = device_property_read_u32_array(dev, "st,adc-channels", chans, num_se);
+		if (ret) {
+			dev_err(&indio_dev->dev, "Failed to get st,adc-channels %d\n", ret);
 			return ret;
+		}
 
-	for (c = 0; c < nchans; c++) {
+		for (c = 0; c < num_se; c++) {
 			if (chans[c] >= adc_info->max_channels) {
 				dev_err(&indio_dev->dev, "Invalid channel %d\n",
 					chans[c]);
@@ -2078,7 +2080,8 @@ static int stm32_adc_legacy_chan_init(struct iio_dev *indio_dev,
 			/* Channel can't be configured both as single-ended & diff */
 			for (i = 0; i < num_diff; i++) {
 				if (chans[c] == diff[i].vinp) {
-				dev_err(&indio_dev->dev, "channel %d misconfigured\n",	chans[c]);
+					dev_err(&indio_dev->dev, "channel %d misconfigured\n",
+						chans[c]);
 					return -EINVAL;
 				}
 			}
@@ -2086,6 +2089,7 @@ static int stm32_adc_legacy_chan_init(struct iio_dev *indio_dev,
 						chans[c], 0, scan_index, false);
 			scan_index++;
 		}
+	}
 
 	if (adc->nsmps > 0) {
 		ret = device_property_read_u32_array(dev, "st,min-sample-time-nsecs",
@@ -2305,7 +2309,7 @@ static int stm32_adc_chan_fw_init(struct iio_dev *indio_dev, bool timestamping)
 
 	if (legacy)
 		ret = stm32_adc_legacy_chan_init(indio_dev, adc, channels,
-						 num_channels);
+						 timestamping ? num_channels - 1 : num_channels);
 	else
 		ret = stm32_adc_generic_chan_init(indio_dev, adc, channels);
 	if (ret < 0)