[Intel-gfx] [PATCH v3 1/6] drm/i915/dp_link_training: Add a final failing state to link training fallback

[Gil Dekel]

Instead of silently giving up when all link-training fallback values are
exhausted, this patch modifies the fallback's failure branch to reduces
both max_link_lane_count and max_link_rate to zero (0) and continues to
emit uevents until userspace stops attempting to modeset.

By doing so, we ensure the failing connector, which is in
link-status=Bad, has all its modes pruned (due to effectively having a
bandwidth of 0Gbps).

It is then the userspace's responsibility to ignore connectors with no
modes, even if they are marked as connected.

Change-Id: Ifc0f6a1ee15cc02da6d65a3eeb9e2cf4e8adb8ec
Signed-off-by: Gil Dekel <gildekel at chromium.org>
---
 drivers/gpu/drm/i915/display/intel_dp.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
index 49a34298b1834..2b8d2ee08a2b2 100644
--- a/drivers/gpu/drm/i915/display/intel_dp.c
+++ b/drivers/gpu/drm/i915/display/intel_dp.c
@@ -274,8 +274,12 @@ static int intel_dp_common_len_rate_limit(const struct intel_dp *intel_dp,

 static int intel_dp_common_rate(struct intel_dp *intel_dp, int index)
 {
+	/* This occurs when max link rate drops to 0 via link training fallback*/
+	if (index < 0)
+		return 0;
+
 	if (drm_WARN_ON(&dp_to_i915(intel_dp)->drm,
-			index < 0 || index >= intel_dp->num_common_rates))
+			index >= intel_dp->num_common_rates))
 		return 162000;

 	return intel_dp->common_rates[index];
@@ -316,6 +320,9 @@ static int intel_dp_max_common_lane_count(struct intel_dp *intel_dp)
 int intel_dp_max_lane_count(struct intel_dp *intel_dp)
 {
 	switch (intel_dp->max_link_lane_count) {
+	/* This occurs when max link lane count drops to 0 via link training fallback*/
+	case 0:
+		return 0;
 	case 1:
 	case 2:
 	case 4:
@@ -650,7 +657,14 @@ int intel_dp_get_link_train_fallback_values(struct intel_dp *intel_dp,
 		intel_dp->max_link_lane_count = lane_count >> 1;
 	} else {
 		drm_err(&i915->drm, "Link Training Unsuccessful\n");
-		return -1;
+		/*
+                 * Ensure all of the connector's modes are pruned in the next
+                 * probe by effectively reducing its bandwidth to 0 so userspace
+                 * can ignore it within the next modeset attempt.
+                 */
+		intel_dp->max_link_rate = 0;
+		intel_dp->max_link_lane_count = 0;
+		return 0;
 	}

 	return 0;
--
Gil Dekel, Software Engineer, Google / ChromeOS Display and Graphics