Some of DP link compliant test expects to return fail-safe mode
if prefer detailed timing mode can not be supported by mainlink's
lane and rate after link training. Therefore add fail-safe mode
into connector mode list as backup mode. This patch fixes test
case 4.2.2.1.

Changes in v2:
-- add Fixes text string

Fixes: 4b85d405cfe9 ( "drm/msm/dp: reduce link rate if failed at link training 
1")
Signed-off-by: Kuogee Hsieh <quic_khs...@quicinc.com>
---
 drivers/gpu/drm/msm/dp/dp_panel.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/gpu/drm/msm/dp/dp_panel.c 
b/drivers/gpu/drm/msm/dp/dp_panel.c
index 71db10c..f141872 100644
--- a/drivers/gpu/drm/msm/dp/dp_panel.c
+++ b/drivers/gpu/drm/msm/dp/dp_panel.c
@@ -212,6 +212,11 @@ int dp_panel_read_sink_caps(struct dp_panel *dp_panel,
                if (drm_add_modes_noedid(connector, 640, 480))
                        drm_set_preferred_mode(connector, 640, 480);
                mutex_unlock(&connector->dev->mode_config.mutex);
+       } else {
+               /* always add fail-safe mode as backup mode */
+               mutex_lock(&connector->dev->mode_config.mutex);
+               drm_add_modes_noedid(connector, 640, 480);
+               mutex_unlock(&connector->dev->mode_config.mutex);
        }
 
        if (panel->aux_cfg_update_done) {
-- 
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