drm/i915: Reverse preemph vs. voltage swing preference

The DP spec says:
"When the combination of the requested pre-emphasis level and
 voltage swing exceeds the capability of a DPTX, the DPTX shall
 set the pre-emphasis level according to the request and use the
 highest voltage swing it can output with the given pre-emphasis level."
and
"When a DPTX reads a request beyond the limits of this Standard,
 the DPTX shall set the pre-emphasis level according to the request
 and set the highest voltage swing level it can output with the
 given pre-emphasis level. If a DPTX is requested for 9.5dB of
 pre-emphasis level (may be supported for a DPTX) and cannot support
 that level, it shall set the pre-emphasis level to the next
 highest level, 6dB."

Ie. we should first validate the pre-emphasis, and then select
the appropriate vswing for it.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200512174145.3186-6-ville.syrjala@linux.intel.comReviewed-by: Manasi Navare <manasi.d.navare@intel.com>

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

@@ -34,18 +34,18 @@ intel_dp_dump_link_status(const u8 link_status[DP_LINK_STATUS_SIZE])
 		      link_status[3], link_status[4], link_status[5]);
 }
 
-static u8 dp_pre_emphasis_max(u8 voltage_swing)
+static u8 dp_voltage_max(u8 preemph)
 {
-	switch (voltage_swing & DP_TRAIN_VOLTAGE_SWING_MASK) {
-	case DP_TRAIN_VOLTAGE_SWING_LEVEL_0:
-		return DP_TRAIN_PRE_EMPH_LEVEL_3;
-	case DP_TRAIN_VOLTAGE_SWING_LEVEL_1:
-		return DP_TRAIN_PRE_EMPH_LEVEL_2;
-	case DP_TRAIN_VOLTAGE_SWING_LEVEL_2:
-		return DP_TRAIN_PRE_EMPH_LEVEL_1;
-	case DP_TRAIN_VOLTAGE_SWING_LEVEL_3:
+	switch (preemph & DP_TRAIN_PRE_EMPHASIS_MASK) {
+	case DP_TRAIN_PRE_EMPH_LEVEL_0:
+		return DP_TRAIN_VOLTAGE_SWING_LEVEL_3;
+	case DP_TRAIN_PRE_EMPH_LEVEL_1:
+		return DP_TRAIN_VOLTAGE_SWING_LEVEL_2;
+	case DP_TRAIN_PRE_EMPH_LEVEL_2:
+		return DP_TRAIN_VOLTAGE_SWING_LEVEL_1;
+	case DP_TRAIN_PRE_EMPH_LEVEL_3:
 	default:
-		return DP_TRAIN_PRE_EMPH_LEVEL_0;
+		return DP_TRAIN_VOLTAGE_SWING_LEVEL_0;
 	}
 }
 
@@ -68,15 +68,15 @@ void intel_dp_get_adjust_train(struct intel_dp *intel_dp,
 			p = this_p;
 	}
 
-	voltage_max = intel_dp->voltage_max(intel_dp);
-	if (v >= voltage_max)
-		v = voltage_max | DP_TRAIN_MAX_SWING_REACHED;
-
-	preemph_max = min(intel_dp->preemph_max(intel_dp),
-			  dp_pre_emphasis_max(v));
+	preemph_max = intel_dp->preemph_max(intel_dp);
 	if (p >= preemph_max)
 		p = preemph_max | DP_TRAIN_MAX_PRE_EMPHASIS_REACHED;
 
+	voltage_max = min(intel_dp->voltage_max(intel_dp),
+			  dp_voltage_max(p));
+	if (v >= voltage_max)
+		v = voltage_max | DP_TRAIN_MAX_SWING_REACHED;
+
 	for (lane = 0; lane < 4; lane++)
 		intel_dp->train_set[lane] = v | p;
 }