iio: core: add iio_device_set_parent() helper

By default, the device allocation will also assign a parent device to the
IIO device object. In cases where devm_iio_device_alloc() is used,
sometimes the parent device must be different than the device used to
manage the allocation.

In that case, this helper should be used to change the parent, hence the
requirement to call this between allocation & registration.

This pattern/requirement is not very common in the IIO space, and it may be
cleaned up later.
But until then, assigning the parent manually between allocation &
registration is slightly easier.

Signed-off-by: Alexandru Ardelean <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

include/linux/iio/iio.h

···
 	return indio_dev ? dev_to_iio_dev(get_device(&indio_dev->dev)) : NULL;
 }
 
+/**
+ * iio_device_set_parent() - assign parent device to the IIO device object
+ * @indio_dev: 		IIO device structure
+ * @parent:		reference to parent device object
+ *
+ * This utility must be called between IIO device allocation
+ * (via devm_iio_device_alloc()) & IIO device registration
+ * (via {devm_}iio_device_register()).
+ * By default, the device allocation will also assign a parent device to
+ * the IIO device object. In cases where devm_iio_device_alloc() is used,
+ * sometimes the parent device must be different than the device used to
+ * manage the allocation.
+ * In that case, this helper should be used to change the parent, hence the
+ * requirement to call this between allocation & registration.
+ **/
+static inline void iio_device_set_parent(struct iio_dev *indio_dev,
+					 struct device *parent)
+{
+	indio_dev->dev.parent = parent;
+}
 
 /**
  * iio_device_set_drvdata() - Set device driver data