drm/i915/dp: Disable link retraining after the last fallback step

After a link training failure if the link parameters can't be further
reduced, there is no point in trying to retrain the link in the driver.
This avoids excessive retrain attempts after detecting a bad link, for
instance while handling MST HPD IRQs, which is likely redundant as the
link training failed already twice with the same minimum link
parameters. Userspace can still try to retrain the link with these
parameters via a modeset.

While at it make the error message more accurate.

v2: Move converting the error to a debug message to the relevant
    follow-up patch. (Ville)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240610164933.2947366-15-imre.deak@intel.com

drivers/gpu/drm/i915/display/intel_display_types.h

@@ -1771,6 +1771,7 @@ struct intel_dp {
 		int max_lane_count;
 		/* Max rate for the current link */
 		int max_rate;
+		bool retrain_disabled;
 		/* Sequential link training failures after a passing LT */
 		int seq_train_failures;
 	} link;

drivers/gpu/drm/i915/display/intel_dp.c

@@ -2966,6 +2966,7 @@ static void intel_dp_reset_link_params(struct intel_dp *intel_dp)
 {
 	intel_dp->link.max_lane_count = intel_dp_max_common_lane_count(intel_dp);
 	intel_dp->link.max_rate = intel_dp_max_common_rate(intel_dp);
+	intel_dp->link.retrain_disabled = false;
 	intel_dp->link.seq_train_failures = 0;
 }
 
@@ -5086,6 +5087,9 @@ intel_dp_needs_link_retrain(struct intel_dp *intel_dp)
 					intel_dp->lane_count))
 		return false;
 
+	if (intel_dp->link.retrain_disabled)
+		return false;
+
 	if (intel_dp->link.seq_train_failures)
 		return true;
 

drivers/gpu/drm/i915/display/intel_dp_link_training.c

@@ -1164,10 +1164,8 @@ static int intel_dp_get_link_train_fallback_values(struct intel_dp *intel_dp,
 		new_link_rate = intel_dp_max_common_rate(intel_dp);
 	}
 
-	if (new_lane_count < 0) {
-		lt_err(intel_dp, DP_PHY_DPRX, "Link Training Unsuccessful\n");
+	if (new_lane_count < 0)
 		return -1;
-	}
 
 	if (intel_dp_is_edp(intel_dp) &&
 	    !intel_dp_can_link_train_fallback_for_edp(intel_dp, new_link_rate, new_lane_count)) {
@@ -1188,7 +1186,7 @@ static int intel_dp_get_link_train_fallback_values(struct intel_dp *intel_dp,
 }
 
 /* NOTE: @state is only valid for MST links and can be %NULL for SST. */
-static void intel_dp_schedule_fallback_link_training(struct intel_atomic_state *state,
+static bool intel_dp_schedule_fallback_link_training(struct intel_atomic_state *state,
 						     struct intel_dp *intel_dp,
 						     const struct intel_crtc_state *crtc_state)
 {
@@ -1197,7 +1195,7 @@ static void intel_dp_schedule_fallback_link_training(struct intel_atomic_state *
 
 	if (!intel_digital_port_connected(&dp_to_dig_port(intel_dp)->base)) {
 		lt_dbg(intel_dp, DP_PHY_DPRX, "Link Training failed on disconnected sink.\n");
-		return;
+		return true;
 	}
 
 	if (intel_dp->hobl_active) {
@@ -1205,16 +1203,18 @@ static void intel_dp_schedule_fallback_link_training(struct intel_atomic_state *
 		       "Link Training failed with HOBL active, not enabling it from now on\n");
 		intel_dp->hobl_failed = true;
 	} else if (intel_dp_get_link_train_fallback_values(intel_dp, crtc_state)) {
-		return;
+		return false;
 	}
 
 	if (drm_WARN_ON(&i915->drm,
 			intel_crtc_has_type(crtc_state, INTEL_OUTPUT_DP_MST) &&
 			!state))
-		return;
+		return false;
 
 	/* Schedule a Hotplug Uevent to userspace to start modeset */
 	intel_dp_queue_modeset_retry_for_link(state, encoder, crtc_state);
+
+	return true;
 }
 
 /* Perform the link training on all LTTPRs and the DPRX on a link. */
@@ -1533,7 +1533,12 @@ void intel_dp_start_link_train(struct intel_atomic_state *state,
 		return;
 	}
 
-	intel_dp_schedule_fallback_link_training(state, intel_dp, crtc_state);
+	if (intel_dp_schedule_fallback_link_training(state, intel_dp, crtc_state))
+		return;
+
+	intel_dp->link.retrain_disabled = true;
+
+	lt_err(intel_dp, DP_PHY_DPRX, "Can't reduce link training parameters after failure\n");
 }
 
 void intel_dp_128b132b_sdp_crc16(struct intel_dp *intel_dp,