The Cadence DSI requires negative syncs from the incoming video signal,
but at the moment that requirement is not expressed in any way. If the
crtc decides to use positive syncs, things break down.

Use the adjusted_mode in atomic_check to set the sync flags to negative
ones.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
---
 drivers/gpu/drm/bridge/cadence/cdns-dsi-core.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/gpu/drm/bridge/cadence/cdns-dsi-core.c b/drivers/gpu/drm/bridge/cadence/cdns-dsi-core.c
index 8a320bd4d34d..53322407c1b0 100644
--- a/drivers/gpu/drm/bridge/cadence/cdns-dsi-core.c
+++ b/drivers/gpu/drm/bridge/cadence/cdns-dsi-core.c
@@ -992,6 +992,11 @@ static int cdns_dsi_bridge_atomic_check(struct drm_bridge *bridge,
 	struct cdns_dsi_bridge_state *dsi_state = to_cdns_dsi_bridge_state(bridge_state);
 	const struct drm_display_mode *mode = &crtc_state->mode;
 	struct cdns_dsi_cfg *dsi_cfg = &dsi_state->dsi_cfg;
+	struct drm_display_mode *adjusted_crtc_mode = &crtc_state->adjusted_mode;
+
+	/* cdns-dsi requires negative syncs */
+	adjusted_crtc_mode->flags &= ~(DRM_MODE_FLAG_PHSYNC | DRM_MODE_FLAG_PVSYNC);
+	adjusted_crtc_mode->flags |= DRM_MODE_FLAG_NHSYNC | DRM_MODE_FLAG_NVSYNC;
 
 	return cdns_dsi_check_conf(dsi, mode, dsi_cfg, false);
 }

-- 
2.43.0