[Intel-gfx] [PATCH 2/6] drm/i915/dp_mst: Disable link training fallback on MST links

[Imre Deak]

We calculate the MST available bandwidth using the link's maximum rate
and lane count. This bandwidth won't be recalculated in response to a
link training error and fallback setting, so modesets following a link
training error will calculate incorrect timing parameters (like the
transcoder's data M/N params or the number of MST TUs).

Prevent the above problem by disabling the link training fallback on MST
links for now, until the MST compute config can deal with changing link
parameters.

The misconfigured timing lead at least to a
'Timed out waiting for DP idle patterns'
error.

Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
Cc: Manasi Navare <manasi.d.navare at intel.com>
Signed-off-by: Imre Deak <imre.deak at intel.com>
---
 drivers/gpu/drm/i915/display/intel_dp.c | 25 ++++++++++++++++++-------
 1 file changed, 18 insertions(+), 7 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
index 42589cae766d..c585b002783a 100644
--- a/drivers/gpu/drm/i915/display/intel_dp.c
+++ b/drivers/gpu/drm/i915/display/intel_dp.c
@@ -468,6 +468,13 @@ int intel_dp_get_link_train_fallback_values(struct intel_dp *intel_dp,
 	struct drm_i915_private *i915 = dp_to_i915(intel_dp);
 	int index;
 
+	/*
+	 * TODO: Enable fallback on MST links once MST link compute can handle
+	 * the fallback params.
+	 */
+	if (intel_dp->is_mst)
+		return -1;
+
 	index = intel_dp_rate_index(intel_dp->common_rates,
 				    intel_dp->num_common_rates,
 				    link_rate);
@@ -6163,7 +6170,17 @@ intel_dp_detect(struct drm_connector *connector,
 		goto out;
 	}
 
-	if (intel_dp->reset_link_params) {
+	/* Read DP Sink DSC Cap DPCD regs for DP v1.4 */
+	if (INTEL_GEN(dev_priv) >= 11)
+		intel_dp_get_dsc_sink_cap(intel_dp);
+
+	intel_dp_configure_mst(intel_dp);
+
+	/*
+	 * TODO: Reset link params when switching to MST mode, until MST
+	 * 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);
 
@@ -6175,12 +6192,6 @@ intel_dp_detect(struct drm_connector *connector,
 
 	intel_dp_print_rates(intel_dp);
 
-	/* Read DP Sink DSC Cap DPCD regs for DP v1.4 */
-	if (INTEL_GEN(dev_priv) >= 11)
-		intel_dp_get_dsc_sink_cap(intel_dp);
-
-	intel_dp_configure_mst(intel_dp);
-
 	if (intel_dp->is_mst) {
 		/*
 		 * If we are in MST mode then this connector
-- 
2.23.1