[PATCH v3 4/4] drm/panel: find_panel_by_fwnode() return a counted reference

[Albert Esteve <aesteve@redhat.com>]

find_panel_by_fwnode() is the fwnode-based counterpart to
of_drm_find_panel(), used internally by drm_panel_add_follower().
Like of_drm_find_panel(), it returned an unrefcounted pointer,
leaving a window where the panel could be freed between the lookup
and first use.

drm_panel_add_follower() worked around the missing panel kref by
calling get_device() on the panel's underlying struct device.
However, get_device() only prevents the device kobject from being freed.
It does not prevent the panel's kzalloc()'d container memory from being
released when the kref reaches zero.

Apply the same fix: call drm_panel_get() under panel_lock before
returning. Since find_panel_by_fwnode() now transfers a counted
reference to drm_panel_add_follower(), drm_panel_remove_follower()
must balance it with a matching drm_panel_put().

Acked-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Albert Esteve <aesteve@redhat.com>
---
 drivers/gpu/drm/drm_panel.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_panel.c b/drivers/gpu/drm/drm_panel.c
index 72cf86253c0cd..a00ae98ed0956 100644
--- a/drivers/gpu/drm/drm_panel.c
+++ b/drivers/gpu/drm/drm_panel.c
@@ -542,7 +542,13 @@ int of_drm_get_panel_orientation(const struct device_node *np,
 EXPORT_SYMBOL(of_drm_get_panel_orientation);
 #endif
 
-/* Find panel by fwnode. This should be identical to of_drm_find_panel(). */
+/*
+ * Find panel by fwnode, returning a counted reference.
+ *
+ * Behaves identically to of_drm_find_panel(). On success the returned
+ * pointer has been passed through drm_panel_get(); the caller must call
+ * drm_panel_put() when done with it.
+ */
 static struct drm_panel *find_panel_by_fwnode(const struct fwnode_handle *fwnode)
 {
 	struct drm_panel *panel;
@@ -554,6 +560,7 @@ static struct drm_panel *find_panel_by_fwnode(const struct fwnode_handle *fwnode
 
 	list_for_each_entry(panel, &panel_list, list) {
 		if (dev_fwnode(panel->dev) == fwnode) {
+			drm_panel_get(panel);
 			mutex_unlock(&panel_lock);
 			return panel;
 		}
@@ -690,6 +697,7 @@ void drm_panel_remove_follower(struct drm_panel_follower *follower)
 	mutex_unlock(&panel->follower_lock);
 
 	put_device(panel->dev);
+	drm_panel_put(panel);
 }
 EXPORT_SYMBOL(drm_panel_remove_follower);
 

-- 
2.55.0