--- sys/sys/random.h
+++ sys/sys/random.h
@@ -45,6 +45,7 @@
 	RANDOM_NET,
 	RANDOM_INTERRUPT,
 	RANDOM_PURE,
+	RANDOM_DEVICE,
 	ENTROPYSOURCE
 };
 void random_harvest(void *, u_int, u_int, u_int, enum esource);
--- sys/dev/random/randomdev_soft.c
+++ sys/dev/random/randomdev_soft.c
@@ -303,7 +303,7 @@
 	KASSERT(origin == RANDOM_START || origin == RANDOM_WRITE ||
             origin == RANDOM_KEYBOARD || origin == RANDOM_MOUSE ||
             origin == RANDOM_NET || origin == RANDOM_INTERRUPT ||
-            origin == RANDOM_PURE,
+            origin == RANDOM_PURE || origin == RANDOM_DEVICE,
 	    ("random_harvest_internal: origin %d invalid\n", origin));
 
 	/* Lockless read to avoid lock operations if fifo is full. */
--- sys/kern/subr_bus.c
+++ sys/kern/subr_bus.c
@@ -44,6 +44,7 @@
 #include <sys/condvar.h>
 #include <sys/queue.h>
 #include <machine/bus.h>
+#include <sys/random.h>
 #include <sys/rman.h>
 #include <sys/selinfo.h>
 #include <sys/signalvar.h>
@@ -53,6 +54,7 @@
 #include <sys/bus.h>
 #include <sys/interrupt.h>
 
+#include <machine/cpu.h>
 #include <machine/stdarg.h>
 
 #include <vm/uma.h>
@@ -2738,6 +2740,33 @@
 	return (device_attach(dev));
 }
 
+static int
+highbit(uint64_t val)
+{
+	int bit;
+
+	for (bit = -1; val > 0; bit++)
+		val = val >> 1;
+	return (bit);
+}
+
+static void
+device_harvest_random(uint64_t attachtime)
+{
+	int entropybits;
+
+	/*
+	 * Discard top ten bits. This means we expect the attach
+	 * time to vary by ~0.1%. In practise it varies by at least
+	 * few percent, but we want to stay on the safe side.
+	 */
+	entropybits = highbit((uintmax_t)attachtime) - 9;
+	if (entropybits > 0) {
+		random_harvest(&attachtime, sizeof(attachtime), entropybits, 0,
+		    RANDOM_DEVICE);
+	}
+}
+
 /**
  * @brief Attach a device driver to a device
  *
@@ -2760,8 +2789,10 @@
 int
 device_attach(device_t dev)
 {
+	uint64_t attachtime;
 	int error;
 
+	attachtime = get_cyclecount();
 	device_sysctl_init(dev);
 	if (!device_is_quiet(dev))
 		device_print_child(dev->parent, dev);
@@ -2784,6 +2815,8 @@
 		dev->state = DS_ATTACHED;
 	dev->flags &= ~DF_DONENOMATCH;
 	devadded(dev);
+	device_harvest_random(get_cyclecount() - attachtime);
+
 	return (0);
 }