drm/bridge: Fix refcount shown via debugfs for encoder_bridges_show()

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@nxp.com>
Reviewed-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Tested-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Link: https://patch.msgid.link/20260318-drm-misc-next-2026-03-05-fix-encoder-bridges-refcount-v3-1-147fea581279@nxp.com
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>

diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
index d6f11c68bb6a443eba16c419e56de490d30fb453..1987789b258a1edf13e8a7fae34693f732736203 100644 (file)
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -1569,11 +1569,17 @@ 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",
@@ -1607,10 +1613,10 @@ 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);
 
@@ -1625,7 +1631,7 @@ 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;
 }