Index: share/man/man9/bus_space.9
===================================================================
--- share/man/man9/bus_space.9
+++ share/man/man9/bus_space.9
@@ -52,7 +52,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd July 7, 2020
+.Dd April ??, 2021
 .Dt BUS_SPACE 9
 .Os
 .Sh NAME
@@ -876,6 +876,11 @@
 If this
 flag is not specified, the system may map the space in whatever way is
 most convenient.
+.It Dv BUS_SPACE_MAP_NONPOSTED
+Try to map the space using non-posted device memory.
+This is to support buses and devices where mapping with posted device
+memory is unsupported or broken.
+This flag is currently only available on arm64.
 .El
 .Pp
 Not all combinations of flags make sense or are supported with all
Index: sys/arm64/arm64/bus_machdep.c
===================================================================
--- sys/arm64/arm64/bus_machdep.c
+++ sys/arm64/arm64/bus_machdep.c
@@ -99,9 +99,13 @@
 generic_bs_map(void *t, bus_addr_t bpa, bus_size_t size, int flags,
     bus_space_handle_t *bshp)
 {
+	vm_memattr_t ma;
 	void *va;
 
-	va = pmap_mapdev(bpa, size);
+	ma = VM_MEMATTR_DEVICE;
+	if (flags == BUS_SPACE_MAP_NONPOSTED)
+		ma = VM_MEMATTR_DEVICE_NP;
+	va = pmap_mapdev_attr(bpa, size, ma);
 	if (va == NULL)
 		return (ENOMEM);
 	*bshp = (bus_space_handle_t)va;
Index: sys/arm64/include/bus.h
===================================================================
--- sys/arm64/include/bus.h
+++ sys/arm64/include/bus.h
@@ -85,6 +85,7 @@
 #define	BUS_SPACE_MAP_CACHEABLE		0x01
 #define	BUS_SPACE_MAP_LINEAR		0x02
 #define	BUS_SPACE_MAP_PREFETCHABLE	0x04
+#define	BUS_SPACE_MAP_NONPOSTED		0x08
 
 #define	BUS_SPACE_UNRESTRICTED	(~0)
 
Index: sys/arm64/include/vm.h
===================================================================
--- sys/arm64/include/vm.h
+++ sys/arm64/include/vm.h
@@ -37,6 +37,7 @@
 #define	VM_MEMATTR_DEVICE_nGnRE		4
 
 #define	VM_MEMATTR_DEVICE		VM_MEMATTR_DEVICE_nGnRnE
+#define	VM_MEMATTR_DEVICE_NP		VM_MEMATTR_DEVICE_nGnRnE
 
 #ifdef _KERNEL
 /* If defined vmstat will try to use both of these in a switch statement */