Commit f078634c authored by Liu Ying's avatar Liu Ying Committed by Luca Ceresoli
Browse files

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: bd57048e ("drm/bridge: use drm_for_each_bridge_in_chain_scoped()")
Signed-off-by: default avatarLiu Ying <victor.liu@nxp.com>
Reviewed-by: default avatarLuca Ceresoli <luca.ceresoli@bootlin.com>
Tested-by: default avatarLuca 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: default avatarLuca Ceresoli <luca.ceresoli@bootlin.com>
parent 87a70013

drivers/gpu/drm/drm_bridge.c

+11 −5
Original line number Diff line number Diff line
@@ -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;

}