[Intel-gfx] [PATCH] drm/i915/display/dp: Add fallback on LT failure for DP2.0

[Arun R Murthy]

For DP2.0 as per the Spec on LT failure we need to reduce the lane count
if the lane count is not equal to 1. If lane count is 1 then need to
retry with reducing the link rate.

Signed-off-by: Arun R Murthy <arun.r.mur...@intel.com>
---
 drivers/gpu/drm/i915/display/intel_dp.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_dp.c 
b/drivers/gpu/drm/i915/display/intel_dp.c
index 54ddc953e5bc..2b12ca45596d 100644
--- a/drivers/gpu/drm/i915/display/intel_dp.c
+++ b/drivers/gpu/drm/i915/display/intel_dp.c
@@ -657,8 +657,20 @@ int intel_dp_get_link_train_fallback_values(struct 
intel_dp *intel_dp,
                                    "Retrying Link training for eDP with same 
parameters\n");
                        return 0;
                }
-               intel_dp->max_link_rate = intel_dp_common_rate(intel_dp, index 
- 1);
-               intel_dp->max_link_lane_count = lane_count;
+               if (intel_dp->dpcd[DP_MAIN_LINK_CHANNEL_CODING] & 
DP_CAP_ANSI_128B132B) {
+                       if (lane_count > 1) {
+                               /* Reduce the lane count */
+                               intel_dp->max_link_lane_count = lane_count >> 1;
+                               intel_dp->max_link_rate = 
intel_dp_common_rate(intel_dp, index);
+                       } else {
+                               /* Reduce the link rate */
+                               intel_dp->max_link_rate = 
intel_dp_common_rate(intel_dp, index - 1);
+                               intel_dp->max_link_lane_count = 
intel_dp_max_common_lane_count(intel_dp);
+                       }
+               } else {
+                       intel_dp->max_link_rate = 
intel_dp_common_rate(intel_dp, index - 1);
+                       intel_dp->max_link_lane_count = lane_count;
+               }
        } else if (lane_count > 1) {
                if (intel_dp_is_edp(intel_dp) &&
                    !intel_dp_can_link_train_fallback_for_edp(intel_dp,
@@ -675,6 +687,10 @@ int intel_dp_get_link_train_fallback_values(struct 
intel_dp *intel_dp,
                return -1;
        }
 
+       drm_dbg_kms(&i915->drm,
+                   "Retrying Link training with link rate %d and lane count 
%d\n",
+                   intel_dp->max_link_rate, intel_dp->max_link_lane_count);
+
        return 0;
 }
 
-- 
2.25.1