Add a helper function to add string property updates to an OF changeset.
This is similar to of_changeset_add_prop_string(), but instead of adding
the property (and failing if it exists), it will update the property.

This shall be used later in the DT hardware prober.

Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
---
Changes since v7:
none

Changes since v6:
none

Changes since v5:
- Collected Rob's tag

Changes since v4:
- Use modern designated initializer for |prop|

Changes since v3:
- Use new __of_prop_free() helper
- Add new line before header declaration

Changes since v2:
- New patch added in v3
---
 drivers/of/dynamic.c | 44 ++++++++++++++++++++++++++++++++++++++++++++
 include/linux/of.h   |  4 ++++
 2 files changed, 48 insertions(+)

diff --git a/drivers/of/dynamic.c b/drivers/of/dynamic.c
index d45a8df61380..0aba760f7577 100644
--- a/drivers/of/dynamic.c
+++ b/drivers/of/dynamic.c
@@ -1072,3 +1072,47 @@ int of_changeset_add_prop_bool(struct of_changeset *ocs, struct device_node *np,
 	return of_changeset_add_prop_helper(ocs, np, &prop);
 }
 EXPORT_SYMBOL_GPL(of_changeset_add_prop_bool);
+
+static int of_changeset_update_prop_helper(struct of_changeset *ocs,
+					   struct device_node *np,
+					   const struct property *pp)
+{
+	struct property *new_pp;
+	int ret;
+
+	new_pp = __of_prop_dup(pp, GFP_KERNEL);
+	if (!new_pp)
+		return -ENOMEM;
+
+	ret = of_changeset_update_property(ocs, np, new_pp);
+	if (ret)
+		__of_prop_free(new_pp);
+
+	return ret;
+}
+
+/**
+ * of_changeset_update_prop_string - Add a string property update to a changeset
+ *
+ * @ocs:	changeset pointer
+ * @np:		device node pointer
+ * @prop_name:	name of the property to be updated
+ * @str:	pointer to null terminated string
+ *
+ * Create a string property to be updated and add it to a changeset.
+ *
+ * Return: 0 on success, a negative error value in case of an error.
+ */
+int of_changeset_update_prop_string(struct of_changeset *ocs,
+				    struct device_node *np,
+				    const char *prop_name, const char *str)
+{
+	struct property prop = {
+		.name = (char *)prop_name,
+		.length = strlen(str) + 1,
+		.value = (void *)str,
+	};
+
+	return of_changeset_update_prop_helper(ocs, np, &prop);
+}
+EXPORT_SYMBOL_GPL(of_changeset_update_prop_string);
diff --git a/include/linux/of.h b/include/linux/of.h
index 086a60f3b8a6..d0307e3b093d 100644
--- a/include/linux/of.h
+++ b/include/linux/of.h
@@ -1651,6 +1651,10 @@ static inline int of_changeset_add_prop_u32(struct of_changeset *ocs,
 	return of_changeset_add_prop_u32_array(ocs, np, prop_name, &val, 1);
 }
 
+int of_changeset_update_prop_string(struct of_changeset *ocs,
+				    struct device_node *np,
+				    const char *prop_name, const char *str);
+
 int of_changeset_add_prop_bool(struct of_changeset *ocs, struct device_node *np,
 			       const char *prop_name);
 
-- 
2.47.0.163.g1226f6d8fa-goog