drm/i915/dp: Fix LTTPR vswing/pre-emp setting in non-transparent mode

The DP PHY vswing/pre-emphasis level programming the driver does is
related to the DPTX -> first LTTPR link segment only. Accordingly it
should be only programmed when link training the first LTTPR and kept
as-is when training subsequent LTTPRs and the DPRX. For these latter
PHYs the vs/pe levels will be set in response to writing the
DP_TRAINING_LANEx_SET_PHY_REPEATERy DPCD registers (by an upstream LTTPR
TX PHY snooping this write access of its downstream LTTPR/DPRX RX PHY).
The above is also described in DP Standard v2.0 under 3.6.6.1.

While at it simplify and add the LTTPR that is link trained to the debug
message in intel_dp_set_signal_levels().

Fixes: b30edfd8 ("drm/i915: Switch to LTTPR non-transparent mode link training")
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201229172201.4155327-2-imre.deak@intel.com

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

@@ -6114,7 +6114,7 @@ static void intel_dp_process_phy_request(struct intel_dp *intel_dp,
 
 	intel_dp_autotest_phy_ddi_disable(intel_dp, crtc_state);
 
-	intel_dp_set_signal_levels(intel_dp, crtc_state);
+	intel_dp_set_signal_levels(intel_dp, crtc_state, DP_PHY_DPRX);
 
 	intel_dp_phy_pattern_update(intel_dp, crtc_state);
 

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

@@ -335,21 +335,24 @@ intel_dp_set_link_train(struct intel_dp *intel_dp,
 }
 
 void intel_dp_set_signal_levels(struct intel_dp *intel_dp,
-				const struct intel_crtc_state *crtc_state)
+				const struct intel_crtc_state *crtc_state,
+				enum drm_dp_phy dp_phy)
 {
 	struct drm_i915_private *dev_priv = dp_to_i915(intel_dp);
 	u8 train_set = intel_dp->train_set[0];
+	char phy_name[10];
 
-	drm_dbg_kms(&dev_priv->drm, "Using vswing level %d%s\n",
+	drm_dbg_kms(&dev_priv->drm, "Using vswing level %d%s, pre-emphasis level %d%s, at %s\n",
 		    train_set & DP_TRAIN_VOLTAGE_SWING_MASK,
-		    train_set & DP_TRAIN_MAX_SWING_REACHED ? " (max)" : "");
-	drm_dbg_kms(&dev_priv->drm, "Using pre-emphasis level %d%s\n",
+		    train_set & DP_TRAIN_MAX_SWING_REACHED ? " (max)" : "",
 		    (train_set & DP_TRAIN_PRE_EMPHASIS_MASK) >>
 		    DP_TRAIN_PRE_EMPHASIS_SHIFT,
 		    train_set & DP_TRAIN_MAX_PRE_EMPHASIS_REACHED ?
-		    " (max)" : "");
+		    " (max)" : "",
+		    intel_dp_phy_name(dp_phy, phy_name, sizeof(phy_name)));
 
-	intel_dp->set_signal_levels(intel_dp, crtc_state);
+	if (intel_dp_phy_is_downstream_of_source(intel_dp, dp_phy))
+		intel_dp->set_signal_levels(intel_dp, crtc_state);
 }
 
 static bool
@@ -359,7 +362,7 @@ intel_dp_reset_link_train(struct intel_dp *intel_dp,
 			  u8 dp_train_pat)
 {
 	memset(intel_dp->train_set, 0, sizeof(intel_dp->train_set));
-	intel_dp_set_signal_levels(intel_dp, crtc_state);
+	intel_dp_set_signal_levels(intel_dp, crtc_state, dp_phy);
 	return intel_dp_set_link_train(intel_dp, crtc_state, dp_phy, dp_train_pat);
 }
 
@@ -373,7 +376,7 @@ intel_dp_update_link_train(struct intel_dp *intel_dp,
 			    DP_TRAINING_LANE0_SET_PHY_REPEATER(dp_phy);
 	int ret;
 
-	intel_dp_set_signal_levels(intel_dp, crtc_state);
+	intel_dp_set_signal_levels(intel_dp, crtc_state, dp_phy);
 
 	ret = drm_dp_dpcd_write(&intel_dp->aux, reg,
 				intel_dp->train_set, crtc_state->lane_count);

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

@@ -18,7 +18,8 @@ void intel_dp_get_adjust_train(struct intel_dp *intel_dp,
 			       enum drm_dp_phy dp_phy,
 			       const u8 link_status[DP_LINK_STATUS_SIZE]);
 void intel_dp_set_signal_levels(struct intel_dp *intel_dp,
-				const struct intel_crtc_state *crtc_state);
+				const struct intel_crtc_state *crtc_state,
+				enum drm_dp_phy dp_phy);
 void intel_dp_start_link_train(struct intel_dp *intel_dp,
 			       const struct intel_crtc_state *crtc_state);
 void intel_dp_stop_link_train(struct intel_dp *intel_dp,