+ /* Unmask the flip-done completion message. Note that the bspec says that
+ * we should do this for both the BCS and RCS, and that we must not unmask
+ * more than one flip event at any time (or ensure that one flip message
+ * can be sent by waiting for flip-done prior to queueing new flips).
+ * Experimentation says that BCS works despite DERRMR masking all
+ * flip-done completion events and that unmasking all planes at once
+ * for the RCS also doesn't appear to drop events. Setting the DERRMR
+ * to zero does lead to lockups within MI_DISPLAY_FLIP.
+ */
+ if (ring->id == RCS) {
+ intel_ring_emit(ring, MI_LOAD_REGISTER_IMM(1));
+ intel_ring_emit(ring, DERRMR);
+ intel_ring_emit(ring, ~(DERRMR_PIPEA_PRI_FLIP_DONE |
+ DERRMR_PIPEB_PRI_FLIP_DONE |
+ DERRMR_PIPEC_PRI_FLIP_DONE));
+ intel_ring_emit(ring, MI_STORE_REGISTER_MEM(1));
+ intel_ring_emit(ring, DERRMR);
+ intel_ring_emit(ring, ring->scratch.gtt_offset + 256);
+ }
+