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 return points inside the loops, so the change
is trivial and readable.

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

---

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 2a55ac5697e0b4faa21f01728bbe287a95cd99a6..840ea4ef4bb27ea035cda784f8ec39cd768ed704 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;
 
+	drm_encoder_chain_lock(encoder);
 	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;
 	}
+	drm_encoder_chain_unlock(encoder);
 }
 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;
 
+	drm_encoder_chain_lock(encoder);
 	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;
 	}
+	drm_encoder_chain_unlock(encoder);
 }
 EXPORT_SYMBOL(drm_atomic_bridge_chain_pre_enable);
 

-- 
2.51.0