[RFC PATCH] drm/msm/dp: move link_ready out of HPD event thread

[Abhinav Kumar]

There are cases where the userspace might still send another
frame after the HPD disconnect causing a modeset cycle after
a disconnect. This messes the internal state machine of MSM DP driver
and can lead to a crash as there can be an imbalance between
bridge_disable() and bridge_enable().

This was also previously reported on [1] for which [2] was posted
and helped resolve the issue by rejecting commits if the DP is not
in connected state.

The change resolved the bug but there can also be another race condition.
If hpd_event_thread does not pick up the EV_USER_NOTIFICATION and process it
link_ready will also not be set to false allowing the frame to sneak in.

Lets move setting link_ready outside of hpd_event_thread() processing to
eliminate a window of race condition.

[1] : https://gitlab.freedesktop.org/drm/msm/-/issues/17
[2] : https://lore.kernel.org/all/1664408211-25314-1-git-send-email-quic_khsieh@quicinc.com/

Signed-off-by: Abhinav Kumar <quic_abhinavk at quicinc.com>
---
 drivers/gpu/drm/msm/dp/dp_display.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/msm/dp/dp_display.c b/drivers/gpu/drm/msm/dp/dp_display.c
index 068d44eeaa07..e00092904ccc 100644
--- a/drivers/gpu/drm/msm/dp/dp_display.c
+++ b/drivers/gpu/drm/msm/dp/dp_display.c
@@ -345,8 +345,6 @@ static int dp_display_send_hpd_notification(struct dp_display_private *dp,
 							 dp->panel->downstream_ports);
 	}
 
-	dp->dp_display.link_ready = hpd;
-
 	drm_dbg_dp(dp->drm_dev, "type=%d hpd=%d\n",
 			dp->dp_display.connector_type, hpd);
 	drm_bridge_hpd_notify(bridge, dp->dp_display.link_ready);
@@ -399,6 +397,8 @@ static int dp_display_process_hpd_high(struct dp_display_private *dp)
 		goto end;
 	}
 
+	dp->dp_display.link_ready = true;
+
 	dp_add_event(dp, EV_USER_NOTIFICATION, true, 0);
 
 end:
@@ -466,6 +466,8 @@ static int dp_display_notify_disconnect(struct device *dev)
 {
 	struct dp_display_private *dp = dev_get_dp_display_private(dev);
 
+	dp->dp_display.link_ready = false;
+
 	dp_add_event(dp, EV_USER_NOTIFICATION, false, 0);
 
 	return 0;
@@ -487,6 +489,7 @@ static int dp_display_handle_port_status_changed(struct dp_display_private *dp)
 		drm_dbg_dp(dp->drm_dev, "sink count is zero, nothing to do\n");
 		if (dp->hpd_state != ST_DISCONNECTED) {
 			dp->hpd_state = ST_DISCONNECT_PENDING;
+			dp->dp_display.link_ready = false;
 			dp_add_event(dp, EV_USER_NOTIFICATION, false, 0);
 		}
 	} else {
-- 
2.34.1