drm/bridge: panel: forbid initializing a panel with unknown connector type

[ Upstream commit b296955b3a ]

Having an DRM_MODE_CONNECTOR_Unknown connector type is considered bad, and
drm_panel_bridge_add_typed() and derivatives are deprecated for this.

drm_panel_init() won't prevent initializing a panel with a
DRM_MODE_CONNECTOR_Unknown connector type. Luckily there are no in-tree
users doing it, so take this as an opportinuty to document a valid
connector type must be passed.

Returning an error if this rule is violated is not possible because
drm_panel_init() is a void function. Add at least a warning to make any
violations noticeable, especially to non-upstream drivers.

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20250214-drm-assorted-cleanups-v7-5-88ca5827d7af@bootlin.com
Signed-off-by: Sasha Levin <sashal@kernel.org>

drivers/gpu/drm/drm_panel.c

@@ -49,7 +49,7 @@ static LIST_HEAD(panel_list);
  * @dev: parent device of the panel
  * @funcs: panel operations
  * @connector_type: the connector type (DRM_MODE_CONNECTOR_*) corresponding to
- *	the panel interface
+ *	the panel interface (must NOT be DRM_MODE_CONNECTOR_Unknown)
  *
  * Initialize the panel structure for subsequent registration with
  * drm_panel_add().
@@ -57,6 +57,9 @@ static LIST_HEAD(panel_list);
 void drm_panel_init(struct drm_panel *panel, struct device *dev,
 		    const struct drm_panel_funcs *funcs, int connector_type)
 {
+	if (connector_type == DRM_MODE_CONNECTOR_Unknown)
+		DRM_WARN("%s: %s: a valid connector type is required!\n", __func__, dev_name(dev));
+
 	INIT_LIST_HEAD(&panel->list);
 	INIT_LIST_HEAD(&panel->followers);
 	mutex_init(&panel->follower_lock);