Index: head/sys/arm/arm/nexus.c
===================================================================
--- head/sys/arm/arm/nexus.c
+++ head/sys/arm/arm/nexus.c
@@ -62,6 +62,8 @@
 #include <machine/resource.h>
 #include <machine/intr.h>
 
+#include <arm/arm/nexusvar.h>
+
 #ifdef FDT
 #include <machine/fdt.h>
 #include <dev/ofw/ofw_bus_subr.h>
@@ -87,6 +89,7 @@
 static	int nexus_activate_resource(device_t, device_t, int, int,
     struct resource *);
 static bus_space_tag_t nexus_get_bus_tag(device_t, device_t);
+static bus_dma_tag_t nexus_get_dma_tag(device_t dev, device_t child);
 #ifdef INTRNG
 #ifdef SMP
 static	int nexus_bind_intr(device_t, device_t, struct resource *, int);
@@ -112,6 +115,13 @@
     int icells, pcell_t *intr);
 #endif
 
+/*
+ * Normally NULL (which results in defaults which are handled in
+ * busdma_machdep), platform init code can use nexus_set_dma_tag() to set this
+ * to a tag that will be inherited by all busses and devices on the platform.
+ */
+static bus_dma_tag_t nexus_dma_tag;
+
 static device_method_t nexus_methods[] = {
 	/* Device interface */
 	DEVMETHOD(device_probe,		nexus_probe),
@@ -127,6 +137,7 @@
 	DEVMETHOD(bus_setup_intr,	nexus_setup_intr),
 	DEVMETHOD(bus_teardown_intr,	nexus_teardown_intr),
 	DEVMETHOD(bus_get_bus_tag,	nexus_get_bus_tag),
+	DEVMETHOD(bus_get_dma_tag,	nexus_get_dma_tag),
 #ifdef INTRNG
 	DEVMETHOD(bus_describe_intr,	nexus_describe_intr),
 #ifdef SMP
@@ -273,6 +284,20 @@
 #else
 		return((void *)1);
 #endif
+}
+
+static bus_dma_tag_t
+nexus_get_dma_tag(device_t dev, device_t child)
+{
+
+	return nexus_dma_tag;
+}
+
+void
+nexus_set_dma_tag(bus_dma_tag_t tag)
+{
+
+	nexus_dma_tag = tag;
 }
 
 static int
Index: head/sys/arm/arm/nexusvar.h
===================================================================
--- head/sys/arm/arm/nexusvar.h
+++ head/sys/arm/arm/nexusvar.h
@@ -0,0 +1,36 @@
+/*-
+ * Copyright (c) 2017 Ian Lepore <ian@freebsd.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ *
+ * $FreeBSD$
+ */
+
+#ifndef	_ARM_ARM_NEXUSVAR_H_
+#define	_ARM_ARM_NEXUSVAR_H_
+
+/* Set a platform busdma tag to be inherited by all busses and devices. */
+void nexus_set_dma_tag(bus_dma_tag_t _tag);
+
+#endif
+