[PATCH v3] drm/bridge: Fix refcount shown via debugfs for encoder_bridges_show()

[Liu Ying]

A typical bridge refcount value is 3 after a bridge chain is formed:
- devm_drm_bridge_alloc() initializes the refcount value to be 1.
- drm_bridge_add() gets an additional reference hence 2.
- drm_bridge_attach() gets the third reference hence 3.

This typical refcount value aligns with allbridges_show()'s behaviour.
However, since encoder_bridges_show() uses
drm_for_each_bridge_in_chain_scoped() to automatically get/put the
bridge reference while iterating, a bogus reference is accidentally
got when showing the wrong typical refcount value as 4 to users via
debugfs.  Fix this by caching the refcount value returned from
kref_read() while iterating and explicitly decreasing the cached
refcount value by 1 before showing it to users.

Fixes: bd57048e4576 ("drm/bridge: use drm_for_each_bridge_in_chain_scoped()")
Signed-off-by: Liu Ying <victor.liu@xxxxxxx>
---
Changes in v3:
- Changed '--refcount' to 'refcount--'.  No functional impact.
- Link to v2: https://lore.kernel.org/r/20260318-drm-misc-next-2026-03-05-fix-encoder-bridges-refcount-v2-1-cb705bd84a31@xxxxxxx

Changes in v2:
- Improved code to manipulate cached refcount.  (Luca)
- Dropped boilerplate code.  (Luca)
- Link to v1: https://lore.kernel.org/r/20260312-drm-misc-next-2026-03-05-fix-encoder-bridges-refcount-v1-1-b9ba3d844732@xxxxxxx
---
 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 f8b0333a0a3b..96e1bd86a585 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -1570,11 +1570,17 @@ EXPORT_SYMBOL(devm_drm_put_bridge);
 static void drm_bridge_debugfs_show_bridge(struct drm_printer *p,
 					   struct drm_bridge *bridge,
 					   unsigned int idx,
-					   bool lingering)
+					   bool lingering,
+					   bool scoped)
 {
+	unsigned int refcount = kref_read(&bridge->refcount);
+
+	if (scoped)
+		refcount--;
+
 	drm_printf(p, "bridge[%u]: %ps\n", idx, bridge->funcs);
 
-	drm_printf(p, "\trefcount: %u%s\n", kref_read(&bridge->refcount),
+	drm_printf(p, "\trefcount: %u%s\n", refcount,
 		   lingering ? " [lingering]" : "");
 
 	drm_printf(p, "\ttype: [%d] %s\n",
@@ -1608,10 +1614,10 @@ 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++, false);
+		drm_bridge_debugfs_show_bridge(&p, bridge, idx++, false, false);
 
 	list_for_each_entry(bridge, &bridge_lingering_list, list)
-		drm_bridge_debugfs_show_bridge(&p, bridge, idx++, true);
+		drm_bridge_debugfs_show_bridge(&p, bridge, idx++, true, false);
 
 	mutex_unlock(&bridge_lock);
 
@@ -1626,7 +1632,7 @@ 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++, false);
+		drm_bridge_debugfs_show_bridge(&p, bridge, idx++, false, true);
 
 	return 0;
 }

---
base-commit: d2e20c8951e4bb5f4a828aed39813599980353b6
change-id: 20260312-drm-misc-next-2026-03-05-fix-encoder-bridges-refcount-8f8ee3f58339

Best regards,
-- 
Liu Ying <victor.liu@xxxxxxx>