The correct sequence for bridge initialization is:

 1. devm_drm_bridge_alloc()
 2. drm_bridge_add()
 3. drm_bridge_attach()

For bridges missing either 1 or 2 there are warnings in place already,
presenting an explanatory error message.

Bridges missing both 1 and 2 would still face a poorly understandable
message, as reported in a recent regression report [0]:

  WARNING: [...] at [...]/lib/refcount.c:25 drm_bridge_attach+0x2c/0x1dc
  ...
  Call trace:
  ...
   drm_bridge_attach
  ...

Add a new warning to ensure an understandable message is logged in such
cases. Use the same message and warning message already in place in
drm_bridge_add().

[0] https://lore.kernel.org/all/hlf4wdopapxnh4rekl5s3kvoi6egaga3lrjfbx6r223ar3txri@3ik53xw5idyh/

Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>

---

This patch was added in v2 after having received regression report [0].
---
 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 6dba601a056bb3cff8b8dd5b1ec46299235b2d85..8f355df883d8ac8de9d361ec302f4ccbf3bca0d6 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 (!bridge->container)
+		DRM_WARN("DRM bridge corrupted or not allocated by devm_drm_bridge_alloc()\n");
+
 	if (list_empty(&bridge->list))
 		DRM_WARN("Missing drm_bridge_add() before attach\n");
 

-- 
2.51.0