Index: sys/arm/arm/nexus.c
===================================================================
--- sys/arm/arm/nexus.c
+++ 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
@@ -275,6 +286,20 @@
 #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
 nexus_config_intr(device_t dev, int irq, enum intr_trigger trig,
     enum intr_polarity pol)
Index: sys/arm/arm/nexusvar.h
===================================================================
--- sys/arm/arm/nexusvar.h
+++ sys/arm/arm/nexusvar.h
@@ -0,0 +1,35 @@
+/*-
+ * 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
Index: sys/arm/mv/mv_machdep.c
===================================================================
--- sys/arm/mv/mv_machdep.c
+++ sys/arm/mv/mv_machdep.c
@@ -50,6 +50,8 @@
 #include <vm/vm.h>
 #include <vm/pmap.h>
 
+#include <arm/arm/nexusvar.h>
+
 #include <machine/bus.h>
 #include <machine/fdt.h>
 #include <machine/machdep.h>
@@ -86,6 +88,42 @@
 #define MPP_SEL(pin,func)	(((func) & 0xf) <<		\
     (((pin) % MPP_PINS_PER_REG) * 4))
 
+static void
+mv_busdma_tag_init()
+{
+	phandle_t node;
+	bus_dma_tag_t dmat;
+
+	/*
+	 * If this platform has coherent DMA, create the parent DMA tag to pass
+	 * down the coherent flag to all busses and devices on the platform,
+	 * otherwise return without doing anything.
+	 *
+	 * [ I'm not sure what the right search is here, maybe it's the compat
+	 * on the root, or maybe search for the "marvell,armada380-mbus" on the
+	 * soc node, or whatever makes the most sense to you marvell guys ]
+	 */
+	if ((node = OF_finddevice("/soc")) == -1)
+		return;
+	if (!fdt_find_compatible(node, "marvell,armada380", 0))
+		return;
+
+	bus_dma_tag_create(NULL,	/* No parent tag */
+	    1, 0,			/* alignment, bounds */
+	    BUS_SPACE_MAXADDR,		/* lowaddr */
+	    BUS_SPACE_MAXADDR,		/* highaddr */
+	    NULL, NULL,			/* filter, filterarg */
+	    BUS_SPACE_MAXSIZE,		/* maxsize */
+	    BUS_SPACE_UNRESTRICTED,	/* nsegments */
+	    BUS_SPACE_MAXSIZE,		/* maxsegsize */
+	    BUS_DMA_COHERENT,		/* flags */
+	    NULL, NULL,			/* lockfunc, lockarg */
+	    &dmat);
+
+	nexus_set_dma_tag(dmat);
+}
+
+
 static int
 platform_mpp_init(void)
 {
@@ -269,6 +307,7 @@
 	if (armada38x_open_bootrom_win() != 0)
 		printf("WARNING: could not open window to bootROM\n");
 #endif
+	mv_busdma_tag_init();
 #endif
 }