[PATCH 2/3] drm/display: hdmi: Use committed state to decide on SCDC resync

[Cristian Ciocaltea <cristian.ciocaltea@collabora.com>]

drm_connector_hdmi_sync_scdc() uses the scrambler_enabled flag to decide
whether the sink-side SCDC status is worth checking on a hotplug/detect
event.  That flag tracks the source-side scrambler status, which is not
necessarily in sync with what the committed mode actually requires.

In particular, a source driver may deliberately set scrambler_enabled at
init to force a disable at boot and resync hardware left enabled by the
firmware, as vc4 does.  If the pipeline doesn't get disabled afterwards,
e.g. because no display was active at probe time, the flag remains
stale.  A subsequent detect (e.g. an HPD event) then finds scrambling
disabled on the sink while the flag claims otherwise, and triggers a
CRTC reset.  This results in a one-time spurious flicker on an otherwise
correctly configured non-scrambled mode.

Verify conn_state->hdmi.scrambler_needed instead, as it is computed
during atomic check and reliably describes the requirements of the
active mode.

Fixes: d653818324ed ("drm/display: hdmi: Add HDMI 2.0 scrambling management helpers")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/all/sashiko-outbox-170670@kernel.org/
Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
---
 drivers/gpu/drm/display/drm_hdmi_helper.c | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/display/drm_hdmi_helper.c b/drivers/gpu/drm/display/drm_hdmi_helper.c
index 872f7e32e8d2..974d5de6d380 100644
--- a/drivers/gpu/drm/display/drm_hdmi_helper.c
+++ b/drivers/gpu/drm/display/drm_hdmi_helper.c
@@ -653,7 +653,6 @@ EXPORT_SYMBOL(drm_connector_hdmi_disable_scrambling);
 int drm_connector_hdmi_sync_scdc(struct drm_connector *connector, bool plugged,
 				 struct drm_modeset_acquire_ctx *ctx)
 {
-	struct drm_connector_hdmi *hdmi;
 	struct drm_connector_state *conn_state;
 	struct drm_device *dev;
 	struct drm_crtc *crtc;
@@ -663,7 +662,6 @@ int drm_connector_hdmi_sync_scdc(struct drm_connector *connector, bool plugged,
 	if (!connector || !plugged || !ctx)
 		return 0;
 
-	hdmi = &connector->hdmi;
 	dev = connector->dev;
 
 	/*
@@ -677,14 +675,21 @@ int drm_connector_hdmi_sync_scdc(struct drm_connector *connector, bool plugged,
 
 	/* TODO: Also handle HDMI 2.1 FRL link training */
 
-	if (!drm_connector_hdmi_scrambler_supported(connector) ||
-	    !READ_ONCE(hdmi->scrambler_enabled))
-		return 0;
-
 	conn_state = connector->state;
 	if (!conn_state)
 		return 0;
 
+	/*
+	 * Rely on the committed connector state rather than on the current
+	 * scrambler status: a source driver may force scrambler_enabled at
+	 * init to get a disable at boot (e.g. vc4), and that value would
+	 * remain stale until the first disable actually happens, leading to
+	 * a spurious CRTC reset whenever the active mode needs no scrambling.
+	 */
+	if (!drm_connector_hdmi_scrambler_supported(connector) ||
+	    !conn_state->hdmi.scrambler_needed)
+		return 0;
+
 	crtc = conn_state->crtc;
 	if (!crtc)
 		return 0;

-- 
2.55.0