drm/i915: WARN if max vswing/pre-emphasis violates the DP spec

According to the DP spec a DPTX must support vswing/pre-emphasis
up to and including level 2. Level 3 is optional (actually DP 1.4a
seems to make even level 3 mandatory for HBR2/3, while leaving it
optional for RBR/HBR1).

WARN if out encoders' .voltage_max()/.preemph_max() return
an illegal value.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200709145845.18118-1-ville.syrjala@linux.intel.com

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

@@ -52,6 +52,7 @@ static u8 dp_voltage_max(u8 preemph)
 void intel_dp_get_adjust_train(struct intel_dp *intel_dp,
 			       const u8 link_status[DP_LINK_STATUS_SIZE])
 {
+	struct drm_i915_private *i915 = dp_to_i915(intel_dp);
 	u8 v = 0;
 	u8 p = 0;
 	int lane;
@@ -64,12 +65,20 @@ void intel_dp_get_adjust_train(struct intel_dp *intel_dp,
 	}
 
 	preemph_max = intel_dp->preemph_max(intel_dp);
+	drm_WARN_ON_ONCE(&i915->drm,
+			 preemph_max != DP_TRAIN_PRE_EMPH_LEVEL_2 &&
+			 preemph_max != DP_TRAIN_PRE_EMPH_LEVEL_3);
+
 	if (p >= preemph_max)
 		p = preemph_max | DP_TRAIN_MAX_PRE_EMPHASIS_REACHED;
 
 	v = min(v, dp_voltage_max(p));
 
 	voltage_max = intel_dp->voltage_max(intel_dp);
+	drm_WARN_ON_ONCE(&i915->drm,
+			 voltage_max != DP_TRAIN_VOLTAGE_SWING_LEVEL_2 &&
+			 voltage_max != DP_TRAIN_VOLTAGE_SWING_LEVEL_3);
+
 	if (v >= voltage_max)
 		v = voltage_max | DP_TRAIN_MAX_SWING_REACHED;