linux-kernel - [PATCH 5/7] 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-v1-5-23b62c47356a@bootlin.com>
Date: Fri, 26 Sep 2025 17:59:46 +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 5/7] drm/bridge: prevent encoder chain changes while
 iterating with list_for_each_entry_from()

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

Convert most of those loops to instead use
drm_for_each_bridge_in_chain_from(), which locks the chain.

This also simplifies code.

All the "simple" loops are converted here. The only ones not touched are
those in drm_atomic_bridge_chain_pre_enable() and
drm_atomic_bridge_chain_post_disable(), because they have nested loops
which are not well handled by drm_for_each_bridge_in_chain_from(). These
two functions are handled by a separate commit.

Signed-off-by: Luca Ceresoli <luca.ceresoli@...tlin.com>
---
 drivers/gpu/drm/drm_bridge.c | 35 ++++++++++++-----------------------
 1 file changed, 12 insertions(+), 23 deletions(-)

diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
index 90e467cf91a134342c80d2f958b928472aaf0d8b..554959147a00efa9807c245290cba8977ce88fc5 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -623,7 +623,7 @@ void drm_bridge_detach(struct drm_bridge *bridge)
 /**
  * drm_bridge_chain_mode_valid - validate the mode against all bridges in the
  *				 encoder chain.
- * @bridge: bridge control structure
+ * @first_bridge: bridge control structure
  * @info: display info against which the mode shall be validated
  * @mode: desired mode to be validated
  *
@@ -637,17 +637,14 @@ void drm_bridge_detach(struct drm_bridge *bridge)
  * MODE_OK on success, drm_mode_status Enum error code on failure
  */
 enum drm_mode_status
-drm_bridge_chain_mode_valid(struct drm_bridge *bridge,
+drm_bridge_chain_mode_valid(struct drm_bridge *first_bridge,
 			    const struct drm_display_info *info,
 			    const struct drm_display_mode *mode)
 {
-	struct drm_encoder *encoder;
-
-	if (!bridge)
+	if (!first_bridge)
 		return MODE_OK;
 
-	encoder = bridge->encoder;
-	list_for_each_entry_from(bridge, &encoder->bridge_chain, chain_node) {
+	drm_for_each_bridge_in_chain_from(first_bridge, bridge) {
 		enum drm_mode_status ret;
 
 		if (!bridge->funcs->mode_valid)
@@ -665,7 +662,7 @@ EXPORT_SYMBOL(drm_bridge_chain_mode_valid);
 /**
  * drm_bridge_chain_mode_set - set proposed mode for all bridges in the
  *			       encoder chain
- * @bridge: bridge control structure
+ * @first_bridge: bridge control structure
  * @mode: desired mode to be set for the encoder chain
  * @adjusted_mode: updated mode that works for this encoder chain
  *
@@ -674,20 +671,16 @@ EXPORT_SYMBOL(drm_bridge_chain_mode_valid);
  *
  * Note: the bridge passed should be the one closest to the encoder
  */
-void drm_bridge_chain_mode_set(struct drm_bridge *bridge,
+void drm_bridge_chain_mode_set(struct drm_bridge *first_bridge,
 			       const struct drm_display_mode *mode,
 			       const struct drm_display_mode *adjusted_mode)
 {
-	struct drm_encoder *encoder;
-
-	if (!bridge)
+	if (!first_bridge)
 		return;
 
-	encoder = bridge->encoder;
-	list_for_each_entry_from(bridge, &encoder->bridge_chain, chain_node) {
+	drm_for_each_bridge_in_chain_from(first_bridge, bridge)
 		if (bridge->funcs->mode_set)
 			bridge->funcs->mode_set(bridge, mode, adjusted_mode);
-	}
 }
 EXPORT_SYMBOL(drm_bridge_chain_mode_set);
 
@@ -911,7 +904,7 @@ EXPORT_SYMBOL(drm_atomic_bridge_chain_pre_enable);
 
 /**
  * drm_atomic_bridge_chain_enable - enables all bridges in the encoder chain
- * @bridge: bridge control structure
+ * @first_bridge: bridge control structure
  * @state: atomic state being committed
  *
  * Calls &drm_bridge_funcs.atomic_enable (falls back on
@@ -921,22 +914,18 @@ EXPORT_SYMBOL(drm_atomic_bridge_chain_pre_enable);
  *
  * Note: the bridge passed should be the one closest to the encoder
  */
-void drm_atomic_bridge_chain_enable(struct drm_bridge *bridge,
+void drm_atomic_bridge_chain_enable(struct drm_bridge *first_bridge,
 				    struct drm_atomic_state *state)
 {
-	struct drm_encoder *encoder;
-
-	if (!bridge)
+	if (!first_bridge)
 		return;
 
-	encoder = bridge->encoder;
-	list_for_each_entry_from(bridge, &encoder->bridge_chain, chain_node) {
+	drm_for_each_bridge_in_chain_from(first_bridge, bridge)
 		if (bridge->funcs->atomic_enable) {
 			bridge->funcs->atomic_enable(bridge, state);
 		} else if (bridge->funcs->enable) {
 			bridge->funcs->enable(bridge);
 		}
-	}
 }
 EXPORT_SYMBOL(drm_atomic_bridge_chain_enable);
 

-- 
2.51.0