staging: iio: resolver: ad2s1210: add label attribute support

The ad2s1210 resolver driver has quite a few channels, mostly for
internal signals for event support. This makes it difficult to know
which channel is which. This patch adds a label attribute to the
channels to make it easier to identify them.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20231005-ad2s1210-mainline-v4-15-ec00746840fc@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/staging/iio/resolver/ad2s1210.c

@@ -1158,6 +1158,34 @@ error_ret:
 	return ret;
 }
 
+static int ad2s1210_read_label(struct iio_dev *indio_dev,
+			       struct iio_chan_spec const *chan,
+			       char *label)
+{
+	if (chan->type == IIO_ANGL) {
+		if (chan->channel == 0)
+			return sprintf(label, "position\n");
+		if (chan->channel == 1)
+			return sprintf(label, "tracking error\n");
+	}
+	if (chan->type == IIO_ANGL_VEL)
+		return sprintf(label, "velocity\n");
+	if (chan->type == IIO_PHASE)
+		return sprintf(label, "synthetic reference\n");
+	if (chan->type == IIO_ALTVOLTAGE) {
+		if (chan->output)
+			return sprintf(label, "excitation\n");
+		if (chan->channel == 0)
+			return sprintf(label, "monitor signal\n");
+		if (chan->channel == 1)
+			return sprintf(label, "cosine\n");
+		if (chan->channel == 2)
+			return sprintf(label, "sine\n");
+	}
+
+	return -EINVAL;
+}
+
 static int ad2s1210_read_event_value(struct iio_dev *indio_dev,
 				     const struct iio_chan_spec *chan,
 				     enum iio_event_type type,
@@ -1338,6 +1366,7 @@ static const struct iio_info ad2s1210_info = {
 	.read_raw = ad2s1210_read_raw,
 	.read_avail = ad2s1210_read_avail,
 	.write_raw = ad2s1210_write_raw,
+	.read_label = ad2s1210_read_label,
 	.attrs = &ad2s1210_attribute_group,
 	.read_event_value = ad2s1210_read_event_value,
 	.write_event_value = ad2s1210_write_event_value,