+
+ /*
+ * "Ring Buffer Use"
+ * Gen2 BSpec "1. Programming Environment" / 1.4.4.6
+ * Gen3 BSpec "1c Memory Interface Functions" / 2.3.4.5
+ * Gen4+ BSpec "1c Memory Interface and Command Stream" / 5.3.4.5
+ * "If the Ring Buffer Head Pointer and the Tail Pointer are on the
+ * same cacheline, the Head Pointer must not be greater than the Tail
+ * Pointer."
+ *
+ * We use ring->head as the last known location of the actual RING_HEAD,
+ * it may have advanced but in the worst case it is equally the same
+ * as ring->head and so we should never program RING_TAIL to advance
+ * into the same cacheline as ring->head.
+ */
+#define cacheline(a) round_down(a, CACHELINE_BYTES)
+ GEM_BUG_ON(cacheline(tail) == cacheline(ring->head) &&
+ tail < ring->head);
+#undef cacheline