drm/i915: Disable LTTPR support when the LTTPR rev < 1.4

By the specification the 0xF0000 - 0xF02FF range is only valid if the
LTTPR revision at 0xF0000 is at least 1.4. Disable the LTTPR support
otherwise.

Fixes: 7b2a4ab8 ("drm/i915: Switch to LTTPR transparent mode link training")
Cc: <stable@vger.kernel.org> # v5.11
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/20210317184901.4029798-4-imre.deak@intel.com

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

@@ -100,17 +100,23 @@ static bool intel_dp_read_lttpr_common_caps(struct intel_dp *intel_dp)
 		return false;
 
 	if (drm_dp_read_lttpr_common_caps(&intel_dp->aux,
-					  intel_dp->lttpr_common_caps) < 0) {
-		intel_dp_reset_lttpr_common_caps(intel_dp);
-		return false;
-	}
+					  intel_dp->lttpr_common_caps) < 0)
+		goto reset_caps;
 
 	drm_dbg_kms(&dp_to_i915(intel_dp)->drm,
 		    "LTTPR common capabilities: %*ph\n",
 		    (int)sizeof(intel_dp->lttpr_common_caps),
 		    intel_dp->lttpr_common_caps);
 
+	/* The minimum value of LT_TUNABLE_PHY_REPEATER_FIELD_DATA_STRUCTURE_REV is 1.4 */
+	if (intel_dp->lttpr_common_caps[0] < 0x14)
+		goto reset_caps;
+
 	return true;
+
+reset_caps:
+	intel_dp_reset_lttpr_common_caps(intel_dp);
+	return false;
 }
 
 static bool