linux-kernel - [PATCH 3/7] drm/bridge: lock the encoder bridge chain mutex during insertion

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Message-Id: <20250926-drm-bridge-alloc-encoder-chain-mutex-v1-3-23b62c47356a@bootlin.com>
Date: Fri, 26 Sep 2025 17:59:44 +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 3/7] drm/bridge: lock the encoder bridge chain mutex during
 insertion

drm_bridge_attach() modifies the encoder bridge chain, so take a mutex
around such operations to allow users of the chain to protect themselves
from chain modifications while iterating.

This change does not apply to drm_bridge_detach() because:
 * only the drm_encoder.c calls it, not bridge drivers (unlike
   drm_bridge_attach())
 * the only drm_bridge_detach() caller is drm_encoder_cleanup() which
   already locks the mutex for the entire cleanup loop, thus additionally
   locking it here would deadlock
 * drm_bridge_detach() is recursively calling itself along the chain, so
   care would be needed to avoid deadlocks
Add a comment to clarify that is intended.

Signed-off-by: Luca Ceresoli <luca.ceresoli@...tlin.com>
---
 drivers/gpu/drm/drm_bridge.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
index 630b5e6594e0affad9ba48791207c7b403da5db8..90e467cf91a134342c80d2f958b928472aaf0d8b 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -453,10 +453,12 @@ int drm_bridge_attach(struct drm_encoder *encoder, struct drm_bridge *bridge,
 	bridge->dev = encoder->dev;
 	bridge->encoder = encoder;
 
+	drm_encoder_chain_lock(encoder);
 	if (previous)
 		list_add(&bridge->chain_node, &previous->chain_node);
 	else
 		list_add(&bridge->chain_node, &encoder->bridge_chain);
+	drm_encoder_chain_unlock(encoder);
 
 	if (bridge->funcs->attach) {
 		ret = bridge->funcs->attach(bridge, encoder, flags);
@@ -487,7 +489,9 @@ int drm_bridge_attach(struct drm_encoder *encoder, struct drm_bridge *bridge,
 err_reset_bridge:
 	bridge->dev = NULL;
 	bridge->encoder = NULL;
+	drm_encoder_chain_lock(encoder);
 	list_del(&bridge->chain_node);
+	drm_encoder_chain_unlock(encoder);
 
 	if (ret != -EPROBE_DEFER)
 		DRM_ERROR("failed to attach bridge %pOF to encoder %s: %d\n",
@@ -503,6 +507,11 @@ int drm_bridge_attach(struct drm_encoder *encoder, struct drm_bridge *bridge,
 }
 EXPORT_SYMBOL(drm_bridge_attach);
 
+/*
+ * Invoked by the encoder during encoder cleanup in drm_encoder_cleanup(),
+ * so should generally *not* be called by driver code. As opposed to
+ * drm_bridge_attach(), does not lock the encoder chain mutex.
+ */
 void drm_bridge_detach(struct drm_bridge *bridge)
 {
 	if (WARN_ON(!bridge))

-- 
2.51.0