[PATCH 3/6] drm/i915/dp: Ensure max link params are always valid

[Imre Deak <imre.deak@xxxxxxxxx>]

Atm until the DPCD for a connector is read the max link rate and lane
count params are invalid. If the connector is modeset, in
intel_dp_compute_config(), intel_dp_common_len_rate_limit(max_link_rate)
will return 0, leading to a intel_dp->common_rates[-1] access.

Fix the above by making sure the max link params are always valid.

The above access leads to an undefined behaviour by definition, though
not causing a user visible problem to my best knowledge, see the previous
patch why. Nevertheless it is an undefined behaviour and it triggers a
BUG() in CONFIG_UBSAN builds, hence CC:stable.

Cc: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Imre Deak <imre.deak@xxxxxxxxx>
---
 drivers/gpu/drm/i915/display/intel_dp.c | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
index 153ae944a354b..1935eb49f9574 100644
--- a/drivers/gpu/drm/i915/display/intel_dp.c
+++ b/drivers/gpu/drm/i915/display/intel_dp.c
@@ -1864,6 +1864,12 @@ void intel_dp_set_link_params(struct intel_dp *intel_dp,
 	intel_dp->lane_count = lane_count;
 }
 
+static void intel_dp_reset_max_link_params(struct intel_dp *intel_dp)
+{
+	intel_dp->max_link_lane_count = intel_dp_max_common_lane_count(intel_dp);
+	intel_dp->max_link_rate = intel_dp_max_common_rate(intel_dp);
+}
+
 /* Enable backlight PWM and backlight PP control. */
 void intel_edp_backlight_on(const struct intel_crtc_state *crtc_state,
 			    const struct drm_connector_state *conn_state)
@@ -2023,8 +2029,7 @@ void intel_dp_sync_state(struct intel_encoder *encoder,
 	if (intel_dp->dpcd[DP_DPCD_REV] == 0)
 		intel_dp_get_dpcd(intel_dp);
 
-	intel_dp->max_link_lane_count = intel_dp_max_common_lane_count(intel_dp);
-	intel_dp->max_link_rate = intel_dp_max_common_rate(intel_dp);
+	intel_dp_reset_max_link_params(intel_dp);
 }
 
 bool intel_dp_initial_fastset_check(struct intel_encoder *encoder,
@@ -2597,6 +2602,7 @@ intel_edp_init_dpcd(struct intel_dp *intel_dp)
 		intel_dp_set_sink_rates(intel_dp);
 
 	intel_dp_set_common_rates(intel_dp);
+	intel_dp_reset_max_link_params(intel_dp);
 
 	/* Read the eDP DSC DPCD registers */
 	if (DISPLAY_VER(dev_priv) >= 10)
@@ -4338,12 +4344,7 @@ intel_dp_detect(struct drm_connector *connector,
 	 * supports link training fallback params.
 	 */
 	if (intel_dp->reset_link_params || intel_dp->is_mst) {
-		/* Initial max link lane count */
-		intel_dp->max_link_lane_count = intel_dp_max_common_lane_count(intel_dp);
-
-		/* Initial max link rate */
-		intel_dp->max_link_rate = intel_dp_max_common_rate(intel_dp);
-
+		intel_dp_reset_max_link_params(intel_dp);
 		intel_dp->reset_link_params = false;
 	}
 
@@ -5011,6 +5012,7 @@ intel_dp_init_connector(struct intel_digital_port *dig_port,
 	intel_dp_set_source_rates(intel_dp);
 	intel_dp_set_default_sink_rates(intel_dp);
 	intel_dp_set_common_rates(intel_dp);
+	intel_dp_reset_max_link_params(intel_dp);
 
 	if (IS_VALLEYVIEW(dev_priv) || IS_CHERRYVIEW(dev_priv))
 		intel_dp->pps.active_pipe = vlv_active_pipe(intel_dp);
-- 
2.27.0