diff --git a/sys/dev/asmc/asmc.c b/sys/dev/asmc/asmc.c
--- a/sys/dev/asmc/asmc.c
+++ b/sys/dev/asmc/asmc.c
@@ -418,43 +418,50 @@
 	return (rv);
 }
 
+/*
+ * Try PIO first; fall back to MMIO for T2 Macs.
+ */
 static int
-asmc_attach(device_t dev)
+asmc_try_probe(device_t dev)
 {
-	int i, j;
-	int ret;
-	char name[2];
 	struct asmc_softc *sc = device_get_softc(dev);
-	struct sysctl_ctx_list *sysctlctx;
-	struct sysctl_oid *sysctlnode;
 
-	/*
-	 * Try MMIO first (T2 Macs expose SMC via memory-mapped I/O).
-	 * Fall back to standard I/O port if MMIO is not available.
-	 */
+	sc->sc_ioport = bus_alloc_resource_any(dev, SYS_RES_IOPORT,
+	    &sc->sc_rid_port, RF_ACTIVE);
+	if (sc->sc_ioport != NULL)
+		return (0);
+
 	sc->sc_rid_mem = 0;
 	sc->sc_iomem = bus_alloc_resource_any(dev, SYS_RES_MEMORY,
 	    &sc->sc_rid_mem, RF_ACTIVE);
 	if (sc->sc_iomem != NULL) {
 		if (asmc_mmio_probe(dev) == 0) {
-			sc->sc_is_mmio = 1;
+			sc->sc_is_mmio = true;
 			device_printf(dev, "using MMIO backend (T2)\n");
-		} else {
-			bus_release_resource(dev, SYS_RES_MEMORY,
-			    sc->sc_rid_mem, sc->sc_iomem);
-			sc->sc_iomem = NULL;
+			return (0);
 		}
+		bus_release_resource(dev, SYS_RES_MEMORY,
+		    sc->sc_rid_mem, sc->sc_iomem);
+		sc->sc_iomem = NULL;
 	}
 
-	if (!sc->sc_is_mmio) {
-		sc->sc_ioport = bus_alloc_resource_any(dev, SYS_RES_IOPORT,
-		    &sc->sc_rid_port, RF_ACTIVE);
-		if (sc->sc_ioport == NULL) {
-			device_printf(dev, "unable to allocate IO port\n");
-			ret = ENOMEM;
-			goto err;
-		}
-	}
+	device_printf(dev, "unable to allocate IO port\n");
+	return (ENOMEM);
+}
+
+static int
+asmc_attach(device_t dev)
+{
+	int i, j;
+	int ret;
+	char name[2];
+	struct asmc_softc *sc = device_get_softc(dev);
+	struct sysctl_ctx_list *sysctlctx;
+	struct sysctl_oid *sysctlnode;
+
+	ret = asmc_try_probe(dev);
+	if (ret != 0)
+		goto err;
 
 	sysctlctx = device_get_sysctl_ctx(dev);
 	sysctlnode = device_get_sysctl_tree(dev);
diff --git a/sys/dev/asmc/asmcmmio.c b/sys/dev/asmc/asmcmmio.c
--- a/sys/dev/asmc/asmcmmio.c
+++ b/sys/dev/asmc/asmcmmio.c
@@ -307,7 +307,7 @@
 		    sc->sc_iomem);
 		sc->sc_iomem = NULL;
 	}
-	sc->sc_is_mmio = 0;
+	sc->sc_is_mmio = false;
 	sc->sc_is_t2 = 0;
 }
 
diff --git a/sys/dev/asmc/asmcvar.h b/sys/dev/asmc/asmcvar.h
--- a/sys/dev/asmc/asmcvar.h
+++ b/sys/dev/asmc/asmcvar.h
@@ -56,7 +56,7 @@
 	/* MMIO backend (T2 Macs) */
 	int			sc_rid_mem;
 	struct resource		*sc_iomem;
-	int			sc_is_mmio;
+	bool			sc_is_mmio;
 	int			sc_is_t2;	/* T2 fan float + per-fan manual */
 	int 			sc_sms_intrtype;
 	struct taskqueue 	*sc_sms_tq;