iio: adc: qcom-spmi-vadc: Propagate fw node label to userspace

Set the read_label() callback to return a friendly name provided in DT
(firmware), in order to make in_{therm,voltage}X_label attributes show
up in sysfs for userspace to consume a channel name. This is
particularly useful for custom thermistors being attached to otherwise
generically named GPIOs, where the name is known by the board DT.

If the channel name isn't set in DT, use the datasheet_name hardcoded in
the driver instead.

Note that this doesn't fall back to fwnode_get_name() as that provides
suboptimally readable names, with an @xx address suffix from board DT.

Signed-off-by: Marijn Suijten <[e-mail]>
Reviewed-by: Dmitry Baryshkov <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/adc/qcom-spmi-vadc.c

···
  *	that is an average of multiple measurements.
  * @scale_fn_type: Represents the scaling function to convert voltage
  *	physical units desired by the client for the channel.
+ * @channel_name: Channel name used in device tree.
  */
 struct vadc_channel_prop {
 	unsigned int channel;
···
 	unsigned int hw_settle_time;
 	unsigned int avg_samples;
 	enum vadc_scale_fn_type scale_fn_type;
+	const char *channel_name;
 };
 
 /**
···
 	return -EINVAL;
 }
 
+static int vadc_read_label(struct iio_dev *indio_dev,
+			   struct iio_chan_spec const *chan, char *label)
+{
+	struct vadc_priv *vadc = iio_priv(indio_dev);
+	const char *name = vadc->chan_props[chan->address].channel_name;
+
+	return sysfs_emit(label, "%s\n", name);
+}
+
 static const struct iio_info vadc_info = {
 	.read_raw = vadc_read_raw,
+	.read_label = vadc_read_label,
 	.fwnode_xlate = vadc_fwnode_xlate,
 };
 
···
 				    struct vadc_channel_prop *prop,
 				    struct fwnode_handle *fwnode)
 {
-	const char *name = fwnode_get_name(fwnode);
+	const char *name = fwnode_get_name(fwnode), *label;
 	u32 chan, value, varr[2];
 	int ret;
 
···
 		dev_err(dev, "%s invalid channel number %d\n", name, chan);
 		return -EINVAL;
 	}
+
+	ret = fwnode_property_read_string(fwnode, "label", &label);
+	if (ret)
+		label = vadc_chans[chan].datasheet_name;
+	prop->channel_name = label;
 
 	/* the channel has DT description */
 	prop->channel = chan;