[PATCH v2 16/21] drm/i915/dp_mst: Enable link training fallback for MST

Imre Deak
2024-05-20T18:59:15Z

Reduce the link parameters after a link training failure for MST
outputs, similarly to how this is done for SST.

For now allow the reduction only by staying in the 8b/10b vs. 128b/132b
mode. Enabling the mode switch is left for a follow-up patchset, after
taking measures ensuring that the mode switch happens properly. In
particular a rediscovery of the whole MST topology may be required for
such a switch, see the References below.

Link: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/10970
Signed-off-by: Imre Deak <imre.deak@intel.com>
---
drivers/gpu/drm/i915/display/intel_dp.c | 6 +-----
.../gpu/drm/i915/display/intel_dp_link_training.c | 13 ++++---------
2 files changed, 5 insertions(+), 14 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
index 34d64fe3302ef..c8d940a2ef7af 100644
--- a/drivers/gpu/drm/i915/display/intel_dp.c
+++ b/drivers/gpu/drm/i915/display/intel_dp.c
@@ -5844,11 +5844,7 @@ intel_dp_detect(struct drm_connector *connector,

intel_dp_mst_configure(intel_dp);

- /*
- * TODO: Reset link params when switching to MST mode, until MST
- * supports link training fallback params.
- */
- if (intel_dp->reset_link_params || intel_dp->is_mst) {
+ if (intel_dp->reset_link_params) {
intel_dp_reset_link_params(intel_dp);
intel_dp->reset_link_params = false;
}
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 375f59afd4dec..9a59a28ca36d2 100644
--- a/drivers/gpu/drm/i915/display/intel_dp_link_training.c
+++ b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
@@ -1124,6 +1124,10 @@ static int reduce_link_rate(struct intel_dp *intel_dp, int current_rate)

new_rate = intel_dp_common_rate(intel_dp, rate_index - 1);

+ /* TODO: Make switching from UHBR to non-UHBR rates work. */
+ if (drm_dp_is_uhbr_rate(current_rate) != drm_dp_is_uhbr_rate(new_rate))
+ return -1;
+
return new_rate;
}

@@ -1142,15 +1146,6 @@ static int intel_dp_get_link_train_fallback_values(struct intel_dp *intel_dp,
int new_link_rate;
int new_lane_count;

- /*
- * TODO: Enable fallback on MST links once MST link compute can handle
- * the fallback params.
- */
- if (intel_dp->is_mst) {
- drm_err(&i915->drm, "Link Training Unsuccessful\n");
- return -1;
- }
-
if (intel_dp_is_edp(intel_dp) && !intel_dp->use_max_params) {
drm_dbg_kms(&i915->drm,
"Retrying Link training for eDP with max parameters\n");
--
2.43.3