DRM bridges must be added before they are attached. Add a warning to catch
violations.

The warning is based on the bridge not being part of any list, so it will
trigger if the bridge is being attached without ever having been added.

It won't catch cases of bridges attached after having been added and then
removed, because in that case the bridge will be in
bridge_lingering_list. However such a case is both more demanding to detect
and less likely to happen, so it can be left unchecked, at least for now.

Link: https://lore.kernel.org/all/20250709-sophisticated-loon-of-rain-6ccdd8@houat/
Reviewed-by: Raphael Gallais-Pou <rgallaispou@gmail.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
---
 drivers/gpu/drm/drm_bridge.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
index 1246a52f8767b52c5f10139aa897824b3c2f28da..6dba601a056bb3cff8b8dd5b1ec46299235b2d85 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -441,6 +441,9 @@ int drm_bridge_attach(struct drm_encoder *encoder, struct drm_bridge *bridge,
 	if (!encoder || !bridge)
 		return -EINVAL;
 
+	if (list_empty(&bridge->list))
+		DRM_WARN("Missing drm_bridge_add() before attach\n");
+
 	drm_bridge_get(bridge);
 
 	if (previous && (!previous->dev || previous->encoder != encoder)) {

-- 
2.51.0