iio: adc: ad7124: Refuse invalid input specifiers

The ad7124-4 has 8 analog inputs; the input select values 8 to 15 are
reserved and not to be used. These are fine for ad7124-8. For both
ad7124-4 and ad7124-8 values bigger than 15 are internal channels that
might appear as inputs in the channels specified in the device
description according to the description of commit f1794fd ("iio:
adc: ad7124: Remove input number limitation"), values bigger than 31
don't fit into the respective register bit field and the driver masked
them to smaller values.

Check for these invalid input specifiers and fail to probe if one is
found.

Fixes: f1794fd ("iio: adc: ad7124: Remove input number limitation")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://patch.msgid.link/80e3bd629d2b755ab5e061c8731dafa57d08698a.1733504533.git.u.kleine-koenig@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Origin: next-20241216~37^2~24, commit:9ecad7f404b57d8a544c3211cf87a6284e49f4fd

Author: ukleinek

drivers/iio/adc/ad7124.c

@@ -814,6 +814,19 @@ static int ad7124_check_chip_id(struct ad7124_state *st)
 	return 0;
 }
 
+/*
+ * Input specifiers 8 - 15 are explicitly reserved for ad7124-4
+ * while they are fine for ad7124-8. Values above 31 don't fit
+ * into the register field and so are invalid for sure.
+ */
+static bool ad7124_valid_input_select(unsigned int ain, const struct ad7124_chip_info *info)
+{
+	if (ain >= info->num_inputs && ain < 16)
+		return false;
+
+	return ain <= FIELD_MAX(AD7124_CHANNEL_AINM_MSK);
+}
+
 static int ad7124_of_parse_channel_config(struct iio_dev *indio_dev,
 					  struct device_node *np)
 {
@@ -872,6 +885,11 @@ static int ad7124_of_parse_channel_config(struct iio_dev *indio_dev,
 		if (ret)
 			goto err;
 
+		if (!ad7124_valid_input_select(ain[0], st->chip_info) ||
+		    !ad7124_valid_input_select(ain[1], st->chip_info))
+			return dev_err_probe(indio_dev->dev.parent, -EINVAL,
+					     "diff-channels property of %pfwP contains invalid data\n", child);
+
 		st->channels[channel].nr = channel;
 		st->channels[channel].ain = AD7124_CHANNEL_AINP(ain[0]) |
 						  AD7124_CHANNEL_AINM(ain[1]);