The usefulness of /sys/kernel/debug/dri/bridges is limited as it only shows
bridges between drm_bridge_add() and drm_bridge_remove(). However
refcounted bridges can stay allocated for a long time after
drm_bridge_remove(), and a memory leak due to a missing drm_bridge_put()
would not be visible in this debugfs file.

Add removed bridges to the /sys/kernel/debug/dri/bridges output.

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

---

Changes in v8:
- updated to rename bridge_removed_list -> bridge_lingering_list

Changes in v7:
- rebased on current code which is in drm_bridge.c now
- removed if (drm_bridge_is_refcounted(bridge)), refcounting is not
  optional
- don't show bridge address
- improve commit message

This patch was added in v6.
---
 drivers/gpu/drm/drm_bridge.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
index 9491ae7c884d355be4a82fb02a43a42d17fa8e0c..fccc42017fd4df6ecfb596325df2dc4d17566f39 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -1491,17 +1491,20 @@ EXPORT_SYMBOL(devm_drm_put_bridge);
 
 static void drm_bridge_debugfs_show_bridge(struct drm_printer *p,
 					   struct drm_bridge *bridge,
-					   unsigned int idx)
+					   unsigned int idx,
+					   bool removed)
 {
 	drm_printf(p, "bridge[%u]: %ps\n", idx, bridge->funcs);
 
-	drm_printf(p, "\trefcount: %u\n", kref_read(&bridge->refcount));
+	drm_printf(p, "\trefcount: %u%s\n", kref_read(&bridge->refcount),
+		   removed ? " [removed]" : "");
 
 	drm_printf(p, "\ttype: [%d] %s\n",
 		   bridge->type,
 		   drm_get_connector_type_name(bridge->type));
 
-	if (bridge->of_node)
+	/* The OF node could be freed after drm_bridge_remove() */
+	if (bridge->of_node && !removed)
 		drm_printf(p, "\tOF: %pOFfc\n", bridge->of_node);
 
 	drm_printf(p, "\tops: [0x%x]", bridge->ops);
@@ -1527,7 +1530,10 @@ static int allbridges_show(struct seq_file *m, void *data)
 	mutex_lock(&bridge_lock);
 
 	list_for_each_entry(bridge, &bridge_list, list)
-		drm_bridge_debugfs_show_bridge(&p, bridge, idx++);
+		drm_bridge_debugfs_show_bridge(&p, bridge, idx++, false);
+
+	list_for_each_entry(bridge, &bridge_lingering_list, list)
+		drm_bridge_debugfs_show_bridge(&p, bridge, idx++, true);
 
 	mutex_unlock(&bridge_lock);
 
@@ -1542,7 +1548,7 @@ static int encoder_bridges_show(struct seq_file *m, void *data)
 	unsigned int idx = 0;
 
 	drm_for_each_bridge_in_chain_scoped(encoder, bridge)
-		drm_bridge_debugfs_show_bridge(&p, bridge, idx++);
+		drm_bridge_debugfs_show_bridge(&p, bridge, idx++, false);
 
 	return 0;
 }

-- 
2.51.0

On Fri, Sep 12, 2025 at 07:03:42PM +0200, Luca Ceresoli wrote:
> The usefulness of /sys/kernel/debug/dri/bridges is limited as it only shows
> bridges between drm_bridge_add() and drm_bridge_remove(). However
> refcounted bridges can stay allocated for a long time after
> drm_bridge_remove(), and a memory leak due to a missing drm_bridge_put()
> would not be visible in this debugfs file.
> 
> Add removed bridges to the /sys/kernel/debug/dri/bridges output.
> 
> Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
> 
> ---
> 
> Changes in v8:
> - updated to rename bridge_removed_list -> bridge_lingering_list
> 
> Changes in v7:
> - rebased on current code which is in drm_bridge.c now
> - removed if (drm_bridge_is_refcounted(bridge)), refcounting is not
>   optional
> - don't show bridge address
> - improve commit message
> 
> This patch was added in v6.
> ---
>  drivers/gpu/drm/drm_bridge.c | 16 +++++++++++-----
>  1 file changed, 11 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
> index 9491ae7c884d355be4a82fb02a43a42d17fa8e0c..fccc42017fd4df6ecfb596325df2dc4d17566f39 100644
> --- a/drivers/gpu/drm/drm_bridge.c
> +++ b/drivers/gpu/drm/drm_bridge.c
> @@ -1491,17 +1491,20 @@ EXPORT_SYMBOL(devm_drm_put_bridge);
>  
>  static void drm_bridge_debugfs_show_bridge(struct drm_printer *p,
>  					   struct drm_bridge *bridge,
> -					   unsigned int idx)
> +					   unsigned int idx,
> +					   bool removed)
>  {
>  	drm_printf(p, "bridge[%u]: %ps\n", idx, bridge->funcs);
>  
> -	drm_printf(p, "\trefcount: %u\n", kref_read(&bridge->refcount));
> +	drm_printf(p, "\trefcount: %u%s\n", kref_read(&bridge->refcount),
> +		   removed ? " [removed]" : "");

I would use lingering here too, and your commit logs haven't been
updated either.

Maxime