drm/i915: Re-apply "Perform link quality check, unconditionally during long pulse"

[ Upstream commit 3cf71bc9904d7ee4a25a822c5dcb54c7804ea388 ]

This re-applies the workaround for "some DP sinks, [which] are a
little nuts" from commit 1a36147bb939 ("drm/i915: Perform link
quality check unconditionally during long pulse").
It makes the secondary AOC E2460P monitor connected via DP to an
acer Veriton N4640G usable again.

This hunk was dropped in commit c85d200e8321 ("drm/i915: Move SST
DP link retraining into the ->post_hotplug() hook")

Fixes: c85d200e8321 ("drm/i915: Move SST DP link retraining into the ->post_hotplug() hook")
[Cleaned up commit message, added stable cc]
Signed-off-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Jan-Marek Glogowski <glogow@fbihome.de>
Cc: stable@vger.kernel.org
Link: https://patchwork.freedesktop.org/patch/msgid/20180825191035.3945-1-lyude@redhat.com
Signed-off-by: Sasha Levin <sashal@kernel.org>

drivers/gpu/drm/i915/intel_dp.c

@@ -4739,6 +4739,22 @@ intel_dp_long_pulse(struct intel_connector *connector,
 		 */
 		status = connector_status_disconnected;
 		goto out;
+	} else {
+		/*
+		 * If display is now connected check links status,
+		 * there has been known issues of link loss triggering
+		 * long pulse.
+		 *
+		 * Some sinks (eg. ASUS PB287Q) seem to perform some
+		 * weird HPD ping pong during modesets. So we can apparently
+		 * end up with HPD going low during a modeset, and then
+		 * going back up soon after. And once that happens we must
+		 * retrain the link to get a picture. That's in case no
+		 * userspace component reacted to intermittent HPD dip.
+		 */
+		struct intel_encoder *encoder = &dp_to_dig_port(intel_dp)->base;
+
+		intel_dp_retrain_link(encoder, ctx);
 	}
 
 	/*