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 <[e-mail]>
Cc: Jani Nikula <[e-mail]>
Cc: Ville Syrjala <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
Cc: Lucas De Marchi <[e-mail]>
Cc: <[e-mail]> # 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 <[e-mail]>
Tested-by: Alexander Wilson <[e-mail]>
Reviewed-by: Ville Syrjälä <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]
(cherry picked from commit 1e712535c51ab025ebc776d4405683d81521996d)
Signed-off-by: Rodrigo Vivi <[e-mail]>

drivers/gpu/drm/i915/intel_dp.c

···
 	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)
 {
···
 				    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 {

drivers/gpu/drm/i915/intel_dp_link_training.c

···
 	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;
 }