[PATCH v2 2/2] drm/i915/dp: Handle LT fallback modes when sink/lttpr changes supported config

[Khaled Almahallawy]

During LT fallback, sometimes the sink/LTTPR reports a different lane
count than what it reported previously. This can cause
intel_dp->link.num_configs to have a different value than before,
meaning that the previous link_rate/lane_config no longer exists, which
results in the LT fallback failing prematurely and blank screen.

This issue was observed on an MTL RVP using an Asus DP2.1 monitor:

[  226.207376] AUX USBC2/DDI TC2/PHY TC2: 0xf0000 AUX -> (ret=  8) 20 1e 80 aa 04 00 01 03
[  226.222334] AUX USBC2/DDI TC2/PHY TC2: 0x00000 AUX -> (ret= 15) 12 14 c4 81 01 00 03 c0 02 00 06 00 00 00 82
[  226.236312] intel_dp_link_config_init 711 num_common_rates=5 num_common_lanes_configs=3 num_configs=15

[  227.305515] AUX USBC2/DDI TC2/PHY TC2: 0xf0000 AUX -> (ret=  8) 20 1e 62 aa 01 00 01 00
[  227.308100] AUX USBC2/DDI TC2/PHY TC2: 0x00000 AUX -> (ret= 15) 12 14 c4 81 01 00 03 c0 02 00 06 00 00 00 82
[  227.315679] intel_dp_link_config_init 711 num_common_rates=5 num_common_lanes_configs=1 num_configs=5

Fix this by choosing the lowest config (link_rate/lane_count).

Cc: Imre Deak <imre.deak at intel.com>
Cc: Jani Nikula <jani.nikula at intel.com>
Signed-off-by: Charlton Lin <charlton.lin at intel.com>
Signed-off-by: Khaled Almahallawy <khaled.almahallawy at intel.com>
---
 drivers/gpu/drm/i915/display/intel_dp_link_training.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/gpu/drm/i915/display/intel_dp_link_training.c b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
index 26db4c49deec..7d2f1e540699 100644
--- a/drivers/gpu/drm/i915/display/intel_dp_link_training.c
+++ b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
@@ -1199,6 +1199,15 @@ static bool reduce_link_params_in_bw_order(struct intel_dp *intel_dp,
 	int i;
 
 	i = intel_dp_link_config_index(intel_dp, crtc_state->port_clock, crtc_state->lane_count);
+
+	if (i < 0) {
+		/* Old config is not located. Skip to the lowest*/
+		intel_dp_link_config_get(intel_dp, 0, &link_rate, &lane_count);
+		*new_link_rate = link_rate;
+		*new_lane_count = lane_count;
+		return true;
+	}
+
 	for (i--; i >= 0; i--) {
 		intel_dp_link_config_get(intel_dp, i, &link_rate, &lane_count);
 
-- 
2.43.0