drm/panel: Add refcount support

Allocate panel via reference counting. Add _get() and _put() helper
functions to ensure panel allocations are refcounted. Avoid use after
free by ensuring panel pointer is valid and can be usable till the last
reference is put.

Reviewed-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Anusha Srivatsa <asrivats@redhat.com>
Link: https://lore.kernel.org/r/20250331-b4-panel-refcounting-v4-2-dad50c60c6c9@redhat.com
Signed-off-by: Maxime Ripard <mripard@kernel.org>

drivers/gpu/drm/drm_panel.c

@@ -355,24 +355,86 @@ struct drm_panel *of_drm_find_panel(const struct device_node *np)
 }
 EXPORT_SYMBOL(of_drm_find_panel);
 
+static void __drm_panel_free(struct kref *kref)
+{
+	struct drm_panel *panel = container_of(kref, struct drm_panel, refcount);
+
+	kfree(panel->container);
+}
+
+/**
+ * drm_panel_get - Acquire a panel reference
+ * @panel: DRM panel
+ *
+ * This function increments the panel's refcount.
+ * Returns:
+ * Pointer to @panel
+ */
+struct drm_panel *drm_panel_get(struct drm_panel *panel)
+{
+	if (!panel)
+		return panel;
+
+	kref_get(&panel->refcount);
+
+	return panel;
+}
+EXPORT_SYMBOL(drm_panel_get);
+
+/**
+ * drm_panel_put - Release a panel reference
+ * @panel: DRM panel
+ *
+ * This function decrements the panel's reference count and frees the
+ * object if the reference count drops to zero.
+ */
+void drm_panel_put(struct drm_panel *panel)
+{
+	if (panel)
+		kref_put(&panel->refcount, __drm_panel_free);
+}
+EXPORT_SYMBOL(drm_panel_put);
+
+/**
+ * drm_panel_put_void - wrapper to drm_panel_put() taking a void pointer
+ *
+ * @data: pointer to @struct drm_panel, cast to a void pointer
+ *
+ * Wrapper of drm_panel_put() to be used when a function taking a void
+ * pointer is needed, for example as a devm action.
+ */
+static void drm_panel_put_void(void *data)
+{
+	struct drm_panel *panel = (struct drm_panel *)data;
+
+	drm_panel_put(panel);
+}
+
 void *__devm_drm_panel_alloc(struct device *dev, size_t size, size_t offset,
 			     const struct drm_panel_funcs *funcs,
 			     int connector_type)
 {
 	void *container;
 	struct drm_panel *panel;
+	int err;
 
 	if (!funcs) {
 		dev_warn(dev, "Missing funcs pointer\n");
 		return ERR_PTR(-EINVAL);
 	}
 
-	container = devm_kzalloc(dev, size, GFP_KERNEL);
+	container = kzalloc(size, GFP_KERNEL);
 	if (!container)
 		return ERR_PTR(-ENOMEM);
 
 	panel = container + offset;
+	panel->container = container;
 	panel->funcs = funcs;
+	kref_init(&panel->refcount);
+
+	err = devm_add_action_or_reset(dev, drm_panel_put_void, panel);
+	if (err)
+		return ERR_PTR(err);
 
 	drm_panel_init(panel, dev, funcs, connector_type);
 

include/drm/drm_panel.h

@@ -28,6 +28,7 @@
 #include <linux/errno.h>
 #include <linux/list.h>
 #include <linux/mutex.h>
+#include <linux/kref.h>
 
 struct backlight_device;
 struct dentry;
@@ -266,6 +267,17 @@ struct drm_panel {
 	 * If true then the panel has been enabled.
 	 */
 	bool enabled;
+
+	/**
+	 * @container: Pointer to the private driver struct embedding this
+	 * @struct drm_panel.
+	 */
+	void *container;
+
+	/**
+	 * @refcount: reference count of users referencing this panel.
+	 */
+	struct kref refcount;
 };
 
 void *__devm_drm_panel_alloc(struct device *dev, size_t size, size_t offset,
@@ -282,6 +294,10 @@ void *__devm_drm_panel_alloc(struct device *dev, size_t size, size_t offset,
  * @connector_type: the connector type (DRM_MODE_CONNECTOR_*) corresponding to
  * the panel interface
  *
+ * The reference count of the returned panel is initialized to 1. This
+ * reference will be automatically dropped via devm (by calling
+ * drm_panel_put()) when @dev is removed.
+ *
  * Returns:
  * Pointer to container structure embedding the panel, ERR_PTR on failure.
  */
@@ -294,6 +310,9 @@ void drm_panel_init(struct drm_panel *panel, struct device *dev,
 		    const struct drm_panel_funcs *funcs,
 		    int connector_type);
 
+struct drm_panel *drm_panel_get(struct drm_panel *panel);
+void drm_panel_put(struct drm_panel *panel);
+
 void drm_panel_add(struct drm_panel *panel);
 void drm_panel_remove(struct drm_panel *panel);