drm/i915: consider the source max DP lane count too

Even if the panel claims it can support 4 lanes, there's the
possibility that the HW can't, so consider this while selecting the
max lane count.
Reviewed-by: NDamien Lespiau <damien.lespiau@intel.com>
Signed-off-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/intel_dp.c

@@ -121,6 +121,22 @@ intel_dp_max_link_bw(struct intel_dp *intel_dp)
 	return max_link_bw;
 }
 
+static u8 intel_dp_max_lane_count(struct intel_dp *intel_dp)
+{
+	struct intel_digital_port *intel_dig_port = dp_to_dig_port(intel_dp);
+	struct drm_device *dev = intel_dig_port->base.base.dev;
+	u8 source_max, sink_max;
+
+	source_max = 4;
+	if (HAS_DDI(dev) && intel_dig_port->port == PORT_A &&
+	    (intel_dig_port->saved_port_bits & DDI_A_4_LANES) == 0)
+		source_max = 2;
+
+	sink_max = drm_dp_max_lane_count(intel_dp->dpcd);
+
+	return min(source_max, sink_max);
+}
+
 /*
  * The units on the numbers in the next two are... bizarre.  Examples will
  * make it clearer; this one parallels an example in the eDP spec.
@@ -171,7 +187,7 @@ intel_dp_mode_valid(struct drm_connector *connector,
 	}
 
 	max_link_clock = drm_dp_bw_code_to_link_rate(intel_dp_max_link_bw(intel_dp));
-	max_lanes = drm_dp_max_lane_count(intel_dp->dpcd);
+	max_lanes = intel_dp_max_lane_count(intel_dp);
 
 	max_rate = intel_dp_max_data_rate(max_link_clock, max_lanes);
 	mode_rate = intel_dp_link_required(target_clock, 18);
@@ -751,7 +767,7 @@ intel_dp_compute_config(struct intel_encoder *encoder,
 	struct intel_crtc *intel_crtc = encoder->new_crtc;
 	struct intel_connector *intel_connector = intel_dp->attached_connector;
 	int lane_count, clock;
-	int max_lane_count = drm_dp_max_lane_count(intel_dp->dpcd);
+	int max_lane_count = intel_dp_max_lane_count(intel_dp);
 	/* Conveniently, the link BW constants become indices with a shift...*/
 	int max_clock = intel_dp_max_link_bw(intel_dp) >> 3;
 	int bpp, mode_rate;