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")
Cc: stable@vger.kernel.org # v6.4+
Assisted-by: Pi:gpt-5.6-sol
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
---
 drivers/gpu/drm/amd/display/dc/dc.h                           | 1 +
 .../amd/display/dc/link/protocols/link_edp_panel_control.c    | 4 +++-
 2 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 4ecd5699ac5e..0db55a73da0c 100644
--- a/drivers/gpu/drm/amd/display/dc/dc.h
+++ b/drivers/gpu/drm/amd/display/dc/dc.h
@@ -1912,6 +1912,7 @@ struct dc_scratch_space {
 	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_edp_panel_control.c b/drivers/gpu/drm/amd/display/dc/link/protocols/link_edp_panel_control.c
index 9883216dcc9d..cd90576bc6f0 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
@@ -93,6 +93,7 @@ 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("%d eDP panel mode supported: %d, enabled: %d\n",
 				 link->link_index,
 				 link->dpcd_caps.panel_mode_edp,
@@ -1300,7 +1301,8 @@ void edp_set_panel_assr(struct dc_link *link, struct pipe_ctx *pipe_ctx,
 
 		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;
 	}
 }
-- 
2.55.0