linux-kernel - [PATCH 6/7] drm/bridge: prevent encoder chain changes while iterating with list_for_each_entry

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Message-Id: <20250926-drm-bridge-alloc-encoder-chain-mutex-v1-6-23b62c47356a@bootlin.com>
Date: Fri, 26 Sep 2025 17:59:47 +0200
From: Luca Ceresoli <luca.ceresoli@...tlin.com>
To: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>, 
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>, 
 Andrzej Hajda <andrzej.hajda@...el.com>, 
 Neil Armstrong <neil.armstrong@...aro.org>, Robert Foss <rfoss@...nel.org>, 
 Laurent Pinchart <Laurent.pinchart@...asonboard.com>, 
 Jonas Karlman <jonas@...boo.se>, Jernej Skrabec <jernej.skrabec@...il.com>
Cc: Hui Pu <Hui.Pu@...ealthcare.com>, 
 Thomas Petazzoni <thomas.petazzoni@...tlin.com>, 
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org, 
 Luca Ceresoli <luca.ceresoli@...tlin.com>
Subject: [PATCH 6/7] drm/bridge: prevent encoder chain changes while
 iterating with list_for_each_entry_reverse()

These loops in drm_bridge.c iterate over the encoder chain using
list_for_each_entry_reverse), which does not prevent changes to the bridge
chain while iterating over it.

Take the encoder chain mutex while iterating to avoid chain changes while
iterating.

All the "simple" loops are converted. drm_atomic_bridge_chain_pre_enable()
and drm_atomic_bridge_chain_post_disable() are handled by a separate
commit.

Signed-off-by: Luca Ceresoli <luca.ceresoli@...tlin.com>
---
 drivers/gpu/drm/drm_bridge.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
index 554959147a00efa9807c245290cba8977ce88fc5..66c0a80db8426ffb360248895cfe2a11d6007ed7 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -706,6 +706,7 @@ void drm_atomic_bridge_chain_disable(struct drm_bridge *bridge,
 		return;
 
 	encoder = bridge->encoder;
+	drm_encoder_chain_lock(encoder);
 	list_for_each_entry_reverse(iter, &encoder->bridge_chain, chain_node) {
 		if (iter->funcs->atomic_disable) {
 			iter->funcs->atomic_disable(iter, state);
@@ -716,6 +717,7 @@ void drm_atomic_bridge_chain_disable(struct drm_bridge *bridge,
 		if (iter == bridge)
 			break;
 	}
+	drm_encoder_chain_unlock(encoder);
 }
 EXPORT_SYMBOL(drm_atomic_bridge_chain_disable);
 
@@ -1220,6 +1222,7 @@ int drm_atomic_bridge_chain_check(struct drm_bridge *bridge,
 		return ret;
 
 	encoder = bridge->encoder;
+	drm_encoder_chain_lock(encoder);
 	list_for_each_entry_reverse(iter, &encoder->bridge_chain, chain_node) {
 		int ret;
 
@@ -1234,12 +1237,15 @@ int drm_atomic_bridge_chain_check(struct drm_bridge *bridge,
 						      crtc_state->state);
 
 		ret = drm_atomic_bridge_check(iter, crtc_state, conn_state);
-		if (ret)
+		if (ret) {
+			drm_encoder_chain_unlock(encoder);
 			return ret;
+		}
 
 		if (iter == bridge)
 			break;
 	}
+	drm_encoder_chain_unlock(encoder);
 
 	return 0;
 }

-- 
2.51.0