[PATCH v2] drm/bridge: anx7625: Drop device lock before drm_helper_hpd_irq_event()

[Chen-Yu Tsai]

The device lock is used to serialize the low level power sequencing
operations. Since drm_helper_hpd_irq_event() could end up calling
.atomic_enable, which also calls power sequencing functions through
runtime PM, this results in a real deadlock. This was observed on an
MT8192-based Chromebook's external display (with appropriate patches [1]
and DT changes applied).

Move the drm_helper_hpd_irq_event() call outside of the lock range. The
lock only needs to be held so that the device status can be read back.
This is the bare minimum change to avoid the deadlock. The lock could
be dropped completely and have pm_runtime_get_if_in_use() increase the
reference count, but this is not the same as pm_runtime_suspended().

Dropping the lock completely also causes the internal display of the
same device to not function correctly if the internal bridge's
interrupt line is added in the device tree. Both the internal and
external display of said device each use one anx7625 bridge.

[1] https://lore.kernel.org/dri-devel/20230112042104.4107253-1-treapking@chromium.org/

Fixes: 60487584a79a ("drm/bridge: anx7625: refactor power control to use runtime PM framework")
Signed-off-by: Chen-Yu Tsai <wenst at chromium.org>
---
Changes since v1:
- restore early return if event < 0

 drivers/gpu/drm/bridge/analogix/anx7625.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/bridge/analogix/anx7625.c b/drivers/gpu/drm/bridge/analogix/anx7625.c
index 866d018f4bb1..e93eba89d5ee 100644
--- a/drivers/gpu/drm/bridge/analogix/anx7625.c
+++ b/drivers/gpu/drm/bridge/analogix/anx7625.c
@@ -1593,18 +1593,20 @@ static void anx7625_work_func(struct work_struct *work)
 
 	mutex_lock(&ctx->lock);
 
-	if (pm_runtime_suspended(&ctx->client->dev))
-		goto unlock;
+	if (pm_runtime_suspended(&ctx->client->dev)) {
+		mutex_unlock(&ctx->lock);
+		return;
+	}
 
 	event = anx7625_hpd_change_detect(ctx);
+
+	mutex_unlock(&ctx->lock);
+
 	if (event < 0)
-		goto unlock;
+		return;
 
 	if (ctx->bridge_attached)
 		drm_helper_hpd_irq_event(ctx->bridge.dev);
-
-unlock:
-	mutex_unlock(&ctx->lock);
 }
 
 static irqreturn_t anx7625_intr_hpd_isr(int irq, void *data)
-- 
2.41.0.255.g8b1d071c50-goog