From 8eebae090b9b06e2ff6c2338bb8b13a54e845252 Mon Sep 17 00:00:00 2001
From: John Baldwin <jhb@FreeBSD.org>
Date: Thu, 15 Sep 2005 17:09:42 +0000
Subject: [PATCH] Add a memory barrier for PREWRITE operations to ensure all
 writes by the CPU have drained before further writes to kick off the
 operation.

MFC after:	1 week
---
 sys/alpha/alpha/busdma_machdep.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/sys/alpha/alpha/busdma_machdep.c b/sys/alpha/alpha/busdma_machdep.c
index ee2ee410fd7..a7eb1c5ad40 100644
--- a/sys/alpha/alpha/busdma_machdep.c
+++ b/sys/alpha/alpha/busdma_machdep.c
@@ -891,6 +891,10 @@ _bus_dmamap_sync(bus_dma_tag_t dmat, bus_dmamap_t map, bus_dmasync_op_t op)
 			}
 		}
 	}
+
+	/* Ensure any pending writes have drained. */
+	if (op & (BUS_DMASYNC_PREWRITE))
+		alpha_mb();
 }
 
 static void