Debug log the sink's MST capability, including single-stream sideband
messaging support, and the decision whether to enable MST mode or not.

Cc: Arun R Murthy <arun.r.mur...@intel.com>
Cc: Ville Syrjälä <ville.syrj...@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nik...@intel.com>
---
 drivers/gpu/drm/i915/display/intel_dp.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_dp.c 
b/drivers/gpu/drm/i915/display/intel_dp.c
index e0b8ee6bde79..4dd9c50226d1 100644
--- a/drivers/gpu/drm/i915/display/intel_dp.c
+++ b/drivers/gpu/drm/i915/display/intel_dp.c
@@ -4008,18 +4008,27 @@ static bool intel_dp_mst_detect(struct intel_dp 
*intel_dp)
 {
        struct drm_i915_private *i915 = dp_to_i915(intel_dp);
        struct intel_encoder *encoder = &dp_to_dig_port(intel_dp)->base;
-       bool sink_can_mst = drm_dp_read_mst_cap(&intel_dp->aux, intel_dp->dpcd) 
== DP_MST_CAPABLE;
+       enum drm_dp_mst_mode sink_mst_mode;
+       const char *mst_mode_str;
+
+       sink_mst_mode = drm_dp_read_mst_cap(&intel_dp->aux, intel_dp->dpcd);
+
+       if (sink_mst_mode == DP_MST_SIDEBAND_MSG)
+               mst_mode_str = "single-stream sideband messaging";
+       else
+               mst_mode_str = str_yes_no(sink_mst_mode == DP_MST_CAPABLE);
 
        intel_dp->is_mst = i915->display.params.enable_dp_mst &&
                intel_dp_mst_source_support(intel_dp) &&
-               sink_can_mst;
+               sink_mst_mode == DP_MST_CAPABLE;
 
        drm_dbg_kms(&i915->drm,
-                   "[ENCODER:%d:%s] MST support: port: %s, sink: %s, modparam: 
%s\n",
+                   "[ENCODER:%d:%s] MST support: port: %s, sink: %s, modparam: 
%s -> enable: %s\n",
                    encoder->base.base.id, encoder->base.name,
                    str_yes_no(intel_dp_mst_source_support(intel_dp)),
-                   str_yes_no(sink_can_mst),
-                   str_yes_no(i915->display.params.enable_dp_mst));
+                   mst_mode_str,
+                   str_yes_no(i915->display.params.enable_dp_mst),
+                   str_yes_no(intel_dp->is_mst));
 
        return intel_dp->is_mst;
 }
-- 
2.39.2

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