[PATCH 1/1][Unstable/OEM-5.10] UBUNTU: SAUCE: drm/i915/dp: Prevent setting LTTPR mode if no LTTPR is detected

[Koba Ko]

From: Imre Deak <imre.deak at intel.com>

BugLink: https://bugs.launchpad.net/bugs/1910211  

Signed-off-by: Imre Deak <imre.deak at intel.com>
Reference: https://gitlab.freedesktop.org/drm/intel/-/issues/2801#note_740352
Signed-off-by: Koba Ko <koba.ko at canonical.com>
---
 .../drm/i915/display/intel_dp_link_training.c | 36 ++++++++-----------
 1 file changed, 15 insertions(+), 21 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_dp_link_training.c b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
index 91d3979902d0..db4862760011 100644
--- a/drivers/gpu/drm/i915/display/intel_dp_link_training.c
+++ b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
@@ -34,18 +34,6 @@ intel_dp_dump_link_status(const u8 link_status[DP_LINK_STATUS_SIZE])
 		      link_status[3], link_status[4], link_status[5]);
 }
 
-static int intel_dp_lttpr_count(struct intel_dp *intel_dp)
-{
-	int count = drm_dp_lttpr_count(intel_dp->lttpr_common_caps);
-
-	/*
-	 * Pretend no LTTPRs in case of LTTPR detection error, or
-	 * if too many (>8) LTTPRs are detected. This translates to link
-	 * training in transparent mode.
-	 */
-	return count <= 0 ? 0 : count;
-}
-
 static void intel_dp_reset_lttpr_count(struct intel_dp *intel_dp)
 {
 	intel_dp->lttpr_common_caps[DP_PHY_REPEATER_CNT -
@@ -142,6 +130,17 @@ int intel_dp_lttpr_init(struct intel_dp *intel_dp)
 		return 0;
 
 	ret = intel_dp_read_lttpr_common_caps(intel_dp);
+	if (!ret)
+		return 0;
+
+	lttpr_count = drm_dp_lttpr_count(intel_dp->lttpr_common_caps);
+	/*
+	 * Prevent setting LTTPR transparent mode explicitly if no LTTPRs are
+	 * detected as this breaks link training at least on the Dell WD19TB
+	 * dock.
+	 */
+	if (lttpr_count == 0)
+		return 0;
 
 	/*
 	 * See DP Standard v2.0 3.6.6.1. about the explicit disabling of
@@ -150,17 +149,12 @@ int intel_dp_lttpr_init(struct intel_dp *intel_dp)
 	 */
 	intel_dp_set_lttpr_transparent_mode(intel_dp, true);
 
-	if (!ret)
-		return 0;
-
-	lttpr_count = intel_dp_lttpr_count(intel_dp);
-
 	/*
 	 * In case of unsupported number of LTTPRs or failing to switch to
 	 * non-transparent mode fall-back to transparent link training mode,
 	 * still taking into account any LTTPR common lane- rate/count limits.
 	 */
-	if (lttpr_count == 0)
+	if (lttpr_count < 0)
 		return 0;
 
 	if (!intel_dp_set_lttpr_transparent_mode(intel_dp, false)) {
@@ -222,11 +216,11 @@ intel_dp_phy_is_downstream_of_source(struct intel_dp *intel_dp,
 				     enum drm_dp_phy dp_phy)
 {
 	struct drm_i915_private *i915 = dp_to_i915(intel_dp);
-	int lttpr_count = intel_dp_lttpr_count(intel_dp);
+	int lttpr_count = drm_dp_lttpr_count(intel_dp->lttpr_common_caps);
 
-	drm_WARN_ON_ONCE(&i915->drm, lttpr_count == 0 && dp_phy != DP_PHY_DPRX);
+	drm_WARN_ON_ONCE(&i915->drm, lttpr_count <= 0 && dp_phy != DP_PHY_DPRX);
 
-	return lttpr_count == 0 || dp_phy == DP_PHY_LTTPR(lttpr_count - 1);
+	return lttpr_count <= 0 || dp_phy == DP_PHY_LTTPR(lttpr_count - 1);
 }
 
 static u8 intel_dp_phy_voltage_max(struct intel_dp *intel_dp,
-- 
2.25.1