iio: Add iio_read_channel_label to inkern API

It can be convenient for other in-kernel drivers to reuse IIO channel
labels. Export the iio_read_channel_label function to allow this. The
signature is different depending on where we are calling it from, so
the meat is moved to do_iio_read_channel_label.
Signed-off-by: Sean Anderson <sean.anderson@linux.dev>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://patch.msgid.link/20240624174601.1527244-2-sean.anderson@linux.devSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/iio_core.h

@@ -34,6 +34,10 @@ void iio_device_ioctl_handler_register(struct iio_dev *indio_dev,
 				       struct iio_ioctl_handler *h);
 void iio_device_ioctl_handler_unregister(struct iio_ioctl_handler *h);
 
+ssize_t do_iio_read_channel_label(struct iio_dev *indio_dev,
+				  const struct iio_chan_spec *c,
+				  char *buf);
+
 int __iio_add_chan_devattr(const char *postfix,
 			   struct iio_chan_spec const *chan,
 			   ssize_t (*func)(struct device *dev,

drivers/iio/industrialio-core.c

@@ -727,22 +727,27 @@ ssize_t iio_format_value(char *buf, unsigned int type, int size, int *vals)
 }
 EXPORT_SYMBOL_GPL(iio_format_value);
 
-static ssize_t iio_read_channel_label(struct device *dev,
-				      struct device_attribute *attr,
+ssize_t do_iio_read_channel_label(struct iio_dev *indio_dev,
+				  const struct iio_chan_spec *c,
 				  char *buf)
 {
-	struct iio_dev *indio_dev = dev_to_iio_dev(dev);
-	struct iio_dev_attr *this_attr = to_iio_dev_attr(attr);
-
 	if (indio_dev->info->read_label)
-		return indio_dev->info->read_label(indio_dev, this_attr->c, buf);
+		return indio_dev->info->read_label(indio_dev, c, buf);
 
-	if (this_attr->c->extend_name)
-		return sysfs_emit(buf, "%s\n", this_attr->c->extend_name);
+	if (c->extend_name)
+		return sysfs_emit(buf, "%s\n", c->extend_name);
 
 	return -EINVAL;
 }
 
+static ssize_t iio_read_channel_label(struct device *dev,
+				      struct device_attribute *attr,
+				      char *buf)
+{
+	return do_iio_read_channel_label(dev_to_iio_dev(dev),
+					 to_iio_dev_attr(attr)->c, buf);
+}
+
 static ssize_t iio_read_channel_info(struct device *dev,
 				     struct device_attribute *attr,
 				     char *buf)

drivers/iio/inkern.c

@@ -1010,3 +1010,9 @@ ssize_t iio_write_channel_ext_info(struct iio_channel *chan, const char *attr,
 			       chan->channel, buf, len);
 }
 EXPORT_SYMBOL_GPL(iio_write_channel_ext_info);
+
+ssize_t iio_read_channel_label(struct iio_channel *chan, char *buf)
+{
+	return do_iio_read_channel_label(chan->indio_dev, chan->channel, buf);
+}
+EXPORT_SYMBOL_GPL(iio_read_channel_label);

include/linux/iio/consumer.h

@@ -441,4 +441,14 @@ ssize_t iio_read_channel_ext_info(struct iio_channel *chan,
 ssize_t iio_write_channel_ext_info(struct iio_channel *chan, const char *attr,
 				   const char *buf, size_t len);
 
+/**
+ * iio_read_channel_label() - read label for a given channel
+ * @chan:		The channel being queried.
+ * @buf:		Where to store the attribute value. Assumed to hold
+ *			at least PAGE_SIZE bytes.
+ *
+ * Returns the number of bytes written to buf, or an error code.
+ */
+ssize_t iio_read_channel_label(struct iio_channel *chan, char *buf);
+
 #endif