[Bug 208947] amdgpu DisplayPort won't recognize all display modes after 5.9 merges

[bugzilla-daemon@xxxxxxxxxxxxxxxxxxx]

https://bugzilla.kernel.org/show_bug.cgi?id=208947

--- Comment #12 from Coleman Kane (ckane@xxxxxxxxxxxxxxx) ---
Ok I re-did the bisect and I think I found the actual commit that introduced
the regression. I have confirmed this by generating a reverse-patch and then
applying that to the latest staging-testing branch I had cloned at the
beginning of the week, and I was able to use the newer kernel without the bug
occurring.

The bug appears to have been introduced by commit
3fd20292c2352660155bbc11736dd014b2fc6e98:

diff --git a/drivers/gpu/drm/amd/display/dc/core/dc_link_dp.c
b/drivers/gpu/drm/amd/display/dc/core/dc_link_dp.c
index 5cb7b834e459..1a3dbed3becb 100644
--- a/drivers/gpu/drm/amd/display/dc/core/dc_link_dp.c
+++ b/drivers/gpu/drm/amd/display/dc/core/dc_link_dp.c
@@ -1133,6 +1133,45 @@ static inline enum link_training_result
perform_link_training_int(
        return status;
 }

+static enum link_training_result check_link_loss_status(
+       struct dc_link *link,
+       const struct link_training_settings *link_training_setting)
+{
+       enum link_training_result status = LINK_TRAINING_SUCCESS;
+       unsigned int lane01_status_address = DP_LANE0_1_STATUS;
+       union lane_status lane_status;
+       uint8_t dpcd_buf[4] = {0};
+       uint32_t lane;
+
+       core_link_read_dpcd(
+               link,
+               lane01_status_address,
+               (uint8_t *)(dpcd_buf),
+               sizeof(dpcd_buf));
+
+       /*parse lane status*/
+       for (lane = 0; lane < link->cur_link_settings.lane_count; lane++) {
+               /*
+                * check lanes status
+                */
+               lane_status.raw = get_nibble_at_index(&dpcd_buf[0], lane);
+
+               if (!lane_status.bits.CHANNEL_EQ_DONE_0 ||
+                       !lane_status.bits.CR_DONE_0 ||
+                       !lane_status.bits.SYMBOL_LOCKED_0) {
+                       /* if one of the channel equalization, clock
+                        * recovery or symbol lock is dropped
+                        * consider it as (link has been
+                        * dropped) dp sink status has changed
+                        */
+                       status = LINK_TRAINING_LINK_LOSS;
+                       break;
+               }
+       }
+
+       return status;
+}
+
 static void initialize_training_settings(
         struct dc_link *link,
        const struct dc_link_settings *link_setting,
@@ -1372,6 +1411,9 @@ static void print_status_message(
        case LINK_TRAINING_LQA_FAIL:
                lt_result = "LQA failed";
                break;
+       case LINK_TRAINING_LINK_LOSS:
+               lt_result = "Link loss";
+               break;
        default:
                break;
        }
@@ -1531,6 +1573,14 @@ enum link_training_result
dc_link_dp_perform_link_training(
                                status);
        }

+       /* delay 5ms after Main Link output idle pattern and then check
+        * DPCD 0202h.
+        */
+       if (link->connector_signal != SIGNAL_TYPE_EDP && status ==
LINK_TRAINING_SUCCESS) {
+               msleep(5);
+               status = check_link_loss_status(link, &lt_settings);
+       }
+
        /* 6. print status message*/
        print_status_message(link, &lt_settings, status);

diff --git a/drivers/gpu/drm/amd/display/include/link_service_types.h
b/drivers/gpu/drm/amd/display/include/link_service_types.h
index 4869d4562e4d..550f46e9b95f 100644
--- a/drivers/gpu/drm/amd/display/include/link_service_types.h
+++ b/drivers/gpu/drm/amd/display/include/link_service_types.h
@@ -66,6 +66,8 @@ enum link_training_result {
        /* other failure during EQ step */
        LINK_TRAINING_EQ_FAIL_EQ,
        LINK_TRAINING_LQA_FAIL,
+       /* one of the CR,EQ or symbol lock is dropped */
+       LINK_TRAINING_LINK_LOSS,
 };

 struct link_training_settings {

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