drm/i915/dp: Link train Fallback on eDP only if fallback link BW can fit panel's native mode

This patch fixes the original commit c0cfb10d9e1de49 ("drm/i915/edp:
Do not do link training fallback or prune modes on EDP") that causes
a blank screen in case of certain eDP panels (Eg: seen on Dell XPS13 9350)
where first link training fails and a retraining is required by falling
back to lower link rate/lane count.
In case of some panels they advertise higher link rate/lane count
than whats required for supporting the panel's native mode.
But we always link train at highest link rate/lane count for eDP
and if that fails we can still fallback to lower link rate/lane count
as long as the fallback link BW still fits the native mode to avoid
pruning the panel's native mode yet retraining at fallback values
to recover from a blank screen.

v3:
* Add const for fixed_mode (Ville)
v2:
* Send uevent if link failure on eDP unconditionally

Fixes: c0cfb10d9e1d ("drm/i915/edp: Do not do link training fallback or prune modes on EDP")
Cc: Clinton Taylor <clinton.a.taylor@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: <stable@vger.kernel.org> # v4.17+
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107489
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105338
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Tested-by: Alexander Wilson <alexander.wilson@ncf.edu>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181009212804.702-1-manasi.d.navare@intel.com
(cherry picked from commit 1e712535c51ab025ebc776d4405683d81521996d)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

2 files changed, 39 insertions, 17 deletions

diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
index 6b4c19123f2a..06e91d7b6d8e 100644
--- a/drivers/gpu/drm/i915/intel_dp.c
+++ b/drivers/gpu/drm/i915/intel_dp.c
@@ -557,6 +557,22 @@ static bool intel_dp_link_params_valid(struct intel_dp *intel_dp, int link_rate,
         return true;
 }
 
+static bool intel_dp_can_link_train_fallback_for_edp(struct intel_dp *intel_dp,
+                                                     int link_rate,
+                                                     uint8_t lane_count)
+{
+        const struct drm_display_mode *fixed_mode =
+                intel_dp->attached_connector->panel.fixed_mode;
+        int mode_rate, max_rate;
+
+        mode_rate = intel_dp_link_required(fixed_mode->clock, 18);
+        max_rate = intel_dp_max_data_rate(link_rate, lane_count);
+        if (mode_rate > max_rate)
+                return false;
+
+        return true;
+}
+
 int intel_dp_get_link_train_fallback_values(struct intel_dp *intel_dp,
                                             int link_rate, uint8_t lane_count)
 {
@@ -566,9 +582,23 @@ int intel_dp_get_link_train_fallback_values(struct intel_dp *intel_dp,
                                     intel_dp->num_common_rates,
                                     link_rate);
         if (index > 0) {
+                if (intel_dp_is_edp(intel_dp) &&
+                    !intel_dp_can_link_train_fallback_for_edp(intel_dp,
+                                                              intel_dp->common_rates[index - 1],
+                                                              lane_count)) {
+                        DRM_DEBUG_KMS("Retrying Link training for eDP with same parameters\n");
+                        return 0;
+                }
                 intel_dp->max_link_rate = intel_dp->common_rates[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,
+                                                              intel_dp_max_common_rate(intel_dp),
+                                                              lane_count >> 1)) {
+                        DRM_DEBUG_KMS("Retrying Link training for eDP with same parameters\n");
+                        return 0;
+                }
                 intel_dp->max_link_rate = intel_dp_max_common_rate(intel_dp);
                 intel_dp->max_link_lane_count = lane_count >> 1;
         } else {
diff --git a/drivers/gpu/drm/i915/intel_dp_link_training.c b/drivers/gpu/drm/i915/intel_dp_link_training.c
index a9f40985a621..30be0e39bd5f 100644
--- a/drivers/gpu/drm/i915/intel_dp_link_training.c
+++ b/drivers/gpu/drm/i915/intel_dp_link_training.c
@@ -367,22 +367,14 @@ intel_dp_start_link_train(struct intel_dp *intel_dp)
         return;
 
  failure_handling:
-        /* Dont fallback and prune modes if its eDP */
-        if (!intel_dp_is_edp(intel_dp)) {
-                DRM_DEBUG_KMS("[CONNECTOR:%d:%s] Link Training failed at link rate = %d, lane count = %d",
-                              intel_connector->base.base.id,
-                              intel_connector->base.name,
-                              intel_dp->link_rate, intel_dp->lane_count);
-                if (!intel_dp_get_link_train_fallback_values(intel_dp,
-                                                             intel_dp->link_rate,
-                                                             intel_dp->lane_count))
-                        /* Schedule a Hotplug Uevent to userspace to start modeset */
-                        schedule_work(&intel_connector->modeset_retry_work);
-        } else {
-                DRM_ERROR("[CONNECTOR:%d:%s] Link Training failed at link rate = %d, lane count = %d",
-                          intel_connector->base.base.id,
-                          intel_connector->base.name,
-                          intel_dp->link_rate, intel_dp->lane_count);
-        }
+        DRM_DEBUG_KMS("[CONNECTOR:%d:%s] Link Training failed at link rate = %d, lane count = %d",
+                      intel_connector->base.base.id,
+                      intel_connector->base.name,
+                      intel_dp->link_rate, intel_dp->lane_count);
+        if (!intel_dp_get_link_train_fallback_values(intel_dp,
+                                                     intel_dp->link_rate,
+                                                     intel_dp->lane_count))
+                /* Schedule a Hotplug Uevent to userspace to start modeset */
+                schedule_work(&intel_connector->modeset_retry_work);
         return;
 }