linux-kernel - [PATCH v3 7/7] drm/bridge: prevent encoder chain changes in pre_enable/post

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Message-Id: <20251009-drm-bridge-alloc-encoder-chain-mutex-v3-7-c90ed744efec@bootlin.com>
Date: Thu, 09 Oct 2025 13:39:02 +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 v3 7/7] drm/bridge: prevent encoder chain changes in
 pre_enable/post_disable

Take the encoder chain mutex while iterating over the encoder chain in
drm_atomic_bridge_chain_pre_enable() and
drm_atomic_bridge_chain_post_disable() to ensure the lists won't change
while being inspected.

These functions have nested list_for_each_*() loops, which makes them
complicated. list_for_each_entry_from() loops could be replaced by
drm_for_each_bridge_in_chain_from(), but it would not work in a nested way
in its current implementation. Besides, there is no "_reverse" variant of
drm_for_each_bridge_in_chain_from().

Keep code simple and readable by explicitly locking around the outer
loop. Thankfully there are no break or return points inside the loops, so
the change is trivial and readable.

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

---

Changes in v3:
- Lock encoder->bridge_chain_mutex directly, no wrappers
- Improved commit message

Changes in v2:
- Improved commit message
---
 drivers/gpu/drm/drm_bridge.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
index 6de9b296c0be247f861815b7f558a80fd7938fae..a84aa68064513c654d6cc908718c79179f776ee7 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -760,6 +760,7 @@ void drm_atomic_bridge_chain_post_disable(struct drm_bridge *bridge,
 
 	encoder = bridge->encoder;
 
+	mutex_lock(&encoder->bridge_chain_mutex);
 	list_for_each_entry_from(bridge, &encoder->bridge_chain, chain_node) {
 		limit = NULL;
 
@@ -808,6 +809,7 @@ void drm_atomic_bridge_chain_post_disable(struct drm_bridge *bridge,
 			/* Jump all bridges that we have already post_disabled */
 			bridge = limit;
 	}
+	mutex_unlock(&encoder->bridge_chain_mutex);
 }
 EXPORT_SYMBOL(drm_atomic_bridge_chain_post_disable);
 
@@ -854,6 +856,7 @@ void drm_atomic_bridge_chain_pre_enable(struct drm_bridge *bridge,
 
 	encoder = bridge->encoder;
 
+	mutex_lock(&encoder->bridge_chain_mutex);
 	list_for_each_entry_reverse(iter, &encoder->bridge_chain, chain_node) {
 		if (iter->pre_enable_prev_first) {
 			next = iter;
@@ -896,6 +899,7 @@ void drm_atomic_bridge_chain_pre_enable(struct drm_bridge *bridge,
 		if (iter == bridge)
 			break;
 	}
+	mutex_unlock(&encoder->bridge_chain_mutex);
 }
 EXPORT_SYMBOL(drm_atomic_bridge_chain_pre_enable);
 

-- 
2.51.0