[PATCH 6.6.y] drm/amd/display: Preserve eDP mode on initial ASSR failure

[Sasha Levin <sashal@kernel.org>]

From: Arthur Heymans <arthur@aheymans.xyz>

[ Upstream commit 5e08102d1d336593160d1434261d50b0c0f6cbe6 ]

Commit 56d8ce9d8c17 ("drm/amd/display: Apply correct panel mode when
reinitializing hardware") made ASSR failure fall back to the default panel
mode unless eDP mode had previously been applied.

However, dc_link is zero-initialized and DP_PANEL_MODE_DEFAULT is zero.
Before the first call to dp_set_panel_mode(), panel_mode therefore looks
like a previously applied default mode. If ASSR setup fails during the
first link training attempt, the driver incorrectly trains the eDP link
using the default scrambling mode.

On a Google Vilboz Chromebook running self-built coreboot firmware and
PSP verstage, this leaves the panel mostly black with corrupted output
along the top edge.

Track whether panel_mode has actually been initialized, and only consult
the saved mode after it has been applied. This retains the recovery
behavior while preserving eDP mode during initial link training.

Fixes: 56d8ce9d8c17 ("drm/amd/display: Apply correct panel mode when reinitializing hardware")
Assisted-by: Pi:gpt-5.6-sol
Reviewed-by: George Zhang <george.zhang@amd.com>
Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit ccb6c2702033f1002b45807fb3cc938760f26f78)
Cc: stable@vger.kernel.org # 6.4.x
[ sashal: Applied the ASSR guard in perform_link_training_with_retries() instead of the absent edp_set_panel_assr() helper. ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 drivers/gpu/drm/amd/display/dc/dc.h                            | 1 +
 .../gpu/drm/amd/display/dc/link/protocols/link_dp_training.c   | 3 ++-
 .../drm/amd/display/dc/link/protocols/link_edp_panel_control.c | 1 +
 3 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/amd/display/dc/dc.h b/drivers/gpu/drm/amd/display/dc/dc.h
index 88e64b280c90f..1f4292336692e 100644
--- a/drivers/gpu/drm/amd/display/dc/dc.h
+++ b/drivers/gpu/drm/amd/display/dc/dc.h
@@ -1512,6 +1512,7 @@ struct dc_link {
 	struct ddc_service *ddc;
 
 	enum dp_panel_mode panel_mode;
+	bool panel_mode_initialized;
 	bool aux_mode;
 
 	/* Private to DC core */
diff --git a/drivers/gpu/drm/amd/display/dc/link/protocols/link_dp_training.c b/drivers/gpu/drm/amd/display/dc/link/protocols/link_dp_training.c
index 539f57858dfec..aa46e0e84ce4f 100644
--- a/drivers/gpu/drm/amd/display/dc/link/protocols/link_dp_training.c
+++ b/drivers/gpu/drm/amd/display/dc/link/protocols/link_dp_training.c
@@ -1608,7 +1608,8 @@ bool perform_link_training_with_retries(
 				 */
 				bool result;
 				result = cp_psp->funcs.enable_assr(cp_psp->handle, link);
-				if (!result && link->panel_mode != DP_PANEL_MODE_EDP)
+				if (!result && link->panel_mode_initialized &&
+				    link->panel_mode != DP_PANEL_MODE_EDP)
 					panel_mode = DP_PANEL_MODE_DEFAULT;
 			}
 		}
diff --git a/drivers/gpu/drm/amd/display/dc/link/protocols/link_edp_panel_control.c b/drivers/gpu/drm/amd/display/dc/link/protocols/link_edp_panel_control.c
index d4d92da153ece..9f7ace40092ba 100644
--- a/drivers/gpu/drm/amd/display/dc/link/protocols/link_edp_panel_control.c
+++ b/drivers/gpu/drm/amd/display/dc/link/protocols/link_edp_panel_control.c
@@ -88,6 +88,7 @@ void dp_set_panel_mode(struct dc_link *link, enum dp_panel_mode panel_mode)
 	}
 
 	link->panel_mode = panel_mode;
+	link->panel_mode_initialized = true;
 	DC_LOG_DETECTION_DP_CAPS("Link: %d eDP panel mode supported: %d "
 		 "eDP panel mode enabled: %d \n",
 		 link->link_index,
-- 
2.53.0