drm/amd/display: lower lane count first when CR done partially fails in EQ

[why]
According to DP specs, in EQ DONE phase of link training, we
should lower lane count when at least one CR DONE bit is set to 1, while
lower link rate when all CR DONE bits are 0s. However in our code, we will
treat both cases as latter. This is not exactly correct based on the specs
expectation.

[how]
Check lane0 CR DONE bit when it is still set but CR DONE fails,
we treat it as a partial CR DONE failure in EQ DONE phase, we
will follow the same fallback flow as when ED DONE fails in EQ
DONE phase.
Reviewed-by: NGeorge Shen <George.Shen@amd.com>
Acked-by: NHamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: NWenjing Liu <wenjing.liu@amd.com>
Tested-by: NDaniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/core/dc_link_dp.c

@@ -1326,7 +1326,9 @@ static enum link_training_result perform_channel_equalization_sequence(
 
 		/* 5. check CR done*/
 		if (!dp_is_cr_done(lane_count, dpcd_lane_status))
-			return LINK_TRAINING_EQ_FAIL_CR;
+			return dpcd_lane_status[0].bits.CR_DONE_0 ?
+					LINK_TRAINING_EQ_FAIL_CR_PARTIAL :
+					LINK_TRAINING_EQ_FAIL_CR;
 
 		/* 6. check CHEQ done*/
 		if (dp_is_ch_eq_done(lane_count, dpcd_lane_status) &&
@@ -1882,6 +1884,9 @@ static void print_status_message(
 	case LINK_TRAINING_EQ_FAIL_CR:
 		lt_result = "CR failed in EQ";
 		break;
+	case LINK_TRAINING_EQ_FAIL_CR_PARTIAL:
+		lt_result = "CR failed in EQ partially";
+		break;
 	case LINK_TRAINING_EQ_FAIL_EQ:
 		lt_result = "EQ failed";
 		break;
@@ -3612,11 +3617,6 @@ static bool decide_fallback_link_setting(
 		struct dc_link_settings *cur,
 		enum link_training_result training_result)
 {
-	if (!cur)
-		return false;
-	if (!max)
-		return false;
-
 	if (dp_get_link_encoding_format(max) == DP_128b_132b_ENCODING ||
 			link->dc->debug.force_dp2_lt_fallback_method)
 		return decide_fallback_link_setting_max_bw_policy(link, max, cur,
@@ -3646,6 +3646,7 @@ static bool decide_fallback_link_setting(
 		break;
 	}
 	case LINK_TRAINING_EQ_FAIL_EQ:
+	case LINK_TRAINING_EQ_FAIL_CR_PARTIAL:
 	{
 		if (!reached_minimum_lane_count(cur->lane_count)) {
 			cur->lane_count = reduce_lane_count(cur->lane_count);

drivers/gpu/drm/amd/display/include/link_service_types.h

@@ -67,6 +67,8 @@ enum link_training_result {
 	LINK_TRAINING_CR_FAIL_LANE23,
 	/* CR DONE bit is cleared during EQ step */
 	LINK_TRAINING_EQ_FAIL_CR,
+	/* CR DONE bit is cleared but LANE0_CR_DONE is set during EQ step */
+	LINK_TRAINING_EQ_FAIL_CR_PARTIAL,
 	/* other failure during EQ step */
 	LINK_TRAINING_EQ_FAIL_EQ,
 	LINK_TRAINING_LQA_FAIL,