[Intel-gfx] [PATCH] drm/i915: Perform link quality check unconditionally during long pulse

[ville.syrjala at linux.intel.com]

From: Ville Syrjälä <ville.syrjala at linux.intel.com>

Apparently some DP sinks are a little nuts and cause HPD to drop
intermittently during modesets. This happens eg. on an ASUS PB287Q.
In oder to recover from this we can't really use the previous
connector status to determine if the link needs retraining, so let's
just ignore that piece of information and do the retrain
unconditionally. We do of course still check whether the link is
supposed to be running or not.

Cc: stable at vger.kernel.org
Cc: Palmer Dabbelt <palmer at dabbelt.com>
Reported-by: Palmer Dabbelt <palmer at dabbelt.com>
References: https://lists.freedesktop.org/archives/intel-gfx/2017-February/119779.html
Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
---
 drivers/gpu/drm/i915/intel_dp.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
index 024798a9c016..37a746f7fbc3 100644
--- a/drivers/gpu/drm/i915/intel_dp.c
+++ b/drivers/gpu/drm/i915/intel_dp.c
@@ -4648,11 +4648,18 @@ intel_dp_long_pulse(struct intel_connector *intel_connector)
 		 */
 		status = connector_status_disconnected;
 		goto out;
-	} else if (connector->status == connector_status_connected) {
+	} else {
 		/*
-		 * If display was connected already and is still connected
-		 * check links status, there has been known issues of
-		 * link loss triggerring long pulse!!!!
+		 * If display is now connected check links status,
+		 * there has been known issues of link loss triggerring
+		 * long pulse.
+		 *
+		 * Some sinks (eg. ASUS PB287Q) seem to perform some
+		 * weird HPD ping pong during modesets. So we can apparely
+		 * 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.
 		 */
 		drm_modeset_lock(&dev->mode_config.connection_mutex, NULL);
 		intel_dp_check_link_status(intel_dp);
-- 
2.10.2