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>

1 files changed, 9 insertions, 17 deletions

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;
 }