[PATCH 2/2] device property: add description of fwnode cases

[Bard Liao <yung-chuan.liao@xxxxxxxxxxxxxxx>]

There are only four valid fwnode cases which are
- primary --> secondary --> -ENODEV
- primary --> NULL
- secondary --> -ENODEV
- NULL

dev->fwnode should be converted between the 4 cases above no matter
how/when set_primary_fwnode() and set_secondary_fwnode() are called.
Describe it in the code so people will keep it in mind.

Signed-off-by: Bard Liao <yung-chuan.liao@xxxxxxxxxxxxxxx>
---
 drivers/base/core.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/base/core.c b/drivers/base/core.c
index 51b9545a050b..17eb14607074 100644
--- a/drivers/base/core.c
+++ b/drivers/base/core.c
@@ -4414,6 +4414,12 @@ static inline bool fwnode_is_primary(struct fwnode_handle *fwnode)
  *
  * Set the device's firmware node pointer to @fwnode, but if a secondary
  * firmware node of the device is present, preserve it.
+ *
+ * Valid fwnode cases are:
+ *  - primary --> secondary --> -ENODEV
+ *  - primary --> NULL
+ *  - secondary --> -ENODEV
+ *  - NULL
  */
 void set_primary_fwnode(struct device *dev, struct fwnode_handle *fwnode)
 {
@@ -4432,6 +4438,7 @@ void set_primary_fwnode(struct device *dev, struct fwnode_handle *fwnode)
 	} else {
 		if (fwnode_is_primary(fn)) {
 			dev->fwnode = fn->secondary;
+			/* Set fn->secondary = NULL to keep fn as a primary fwnode */
 			if (!(parent && fn == parent->fwnode))
 				fn->secondary = NULL;
 		} else {
-- 
2.17.1