diff --git a/sys/dev/random/random_harvestq.c b/sys/dev/random/random_harvestq.c
--- a/sys/dev/random/random_harvestq.c
+++ b/sys/dev/random/random_harvestq.c
@@ -278,8 +278,15 @@
 		epoch_enter_preempt(rs_epoch, &et);
 	CK_LIST_FOREACH(rrs, &source_list, rrs_entries) {
 		for (i = 0; i < npools; i++) {
+			if (rrs->rrs_source->rs_read == NULL) {
+				/* Source pushes entropy asynchronously. */
+				continue;
+			}
 			n = rrs->rrs_source->rs_read(entropy, sizeof(entropy));
-			KASSERT((n <= sizeof(entropy)), ("%s: rs_read returned too much data (%u > %zu)", __func__, n, sizeof(entropy)));
+			KASSERT((n <= sizeof(entropy)),
+			    ("%s: rs_read returned too much data (%u > %zu)",
+			    __func__, n, sizeof(entropy)));
+
 			/*
 			 * Sometimes the HW entropy source doesn't have anything
 			 * ready for us.  This isn't necessarily untrustworthy.
@@ -870,20 +877,6 @@
 	random_harvestq_fast_process_event(&event);
 }
 
-void
-random_harvest_register_source(enum random_entropy_source source)
-{
-
-	hc_source_mask |= (1 << source);
-}
-
-void
-random_harvest_deregister_source(enum random_entropy_source source)
-{
-
-	hc_source_mask &= ~(1 << source);
-}
-
 void
 random_source_register(const struct random_source *rsource)
 {
@@ -894,10 +887,9 @@
 	rrs = malloc(sizeof(*rrs), M_ENTROPY, M_WAITOK);
 	rrs->rrs_source = rsource;
 
-	random_harvest_register_source(rsource->rs_source);
-
 	printf("random: registering fast source %s\n", rsource->rs_ident);
 
+	hc_source_mask |= (1 << rsource->rs_source);
 	RANDOM_HARVEST_LOCK();
 	CK_LIST_INSERT_HEAD(&source_list, rrs, rrs_entries);
 	RANDOM_HARVEST_UNLOCK();
@@ -910,8 +902,7 @@
 
 	KASSERT(rsource != NULL, ("invalid input to %s", __func__));
 
-	random_harvest_deregister_source(rsource->rs_source);
-
+	hc_source_mask &= ~(1 << rsource->rs_source);
 	RANDOM_HARVEST_LOCK();
 	CK_LIST_FOREACH(rrs, &source_list, rrs_entries)
 		if (rrs->rrs_source == rsource) {
diff --git a/sys/dev/random/randomdev.h b/sys/dev/random/randomdev.h
--- a/sys/dev/random/randomdev.h
+++ b/sys/dev/random/randomdev.h
@@ -52,7 +52,9 @@
 }
 #endif /* SYSCTL_DECL */
 
+#ifdef MALLOC_DECLARE
 MALLOC_DECLARE(M_ENTROPY);
+#endif
 
 extern bool random_bypass_before_seeding;
 extern bool read_random_bypassed_before_seeding;
diff --git a/sys/dev/tpm/tpm20.c b/sys/dev/tpm/tpm20.c
--- a/sys/dev/tpm/tpm20.c
+++ b/sys/dev/tpm/tpm20.c
@@ -25,8 +25,8 @@
  * POSSIBILITY OF SUCH DAMAGE.
  */
 
-#include <sys/cdefs.h>
 #include <sys/random.h>
+#include <dev/random/randomdev.h>
 
 #include "tpm20.h"
 
@@ -184,6 +184,13 @@
 	return (ENOTTY);
 }
 
+#ifdef TPM_HARVEST
+static const struct random_source random_tpm = {
+	.rs_ident = "TPM",
+	.rs_source = RANDOM_PURE_TPM,
+};
+#endif
+
 int
 tpm20_init(struct tpm_sc *sc)
 {
@@ -206,7 +213,7 @@
 		tpm20_release(sc);
 
 #ifdef TPM_HARVEST
-	random_harvest_register_source(RANDOM_PURE_TPM);
+	random_source_register(&random_tpm);
 	TIMEOUT_TASK_INIT(taskqueue_thread, &sc->harvest_task, 0,
 	    tpm20_harvest, sc);
 	taskqueue_enqueue_timeout(taskqueue_thread, &sc->harvest_task, 0);
@@ -223,7 +230,7 @@
 #ifdef TPM_HARVEST
 	if (device_is_attached(sc->dev))
 		taskqueue_drain_timeout(taskqueue_thread, &sc->harvest_task);
-	random_harvest_deregister_source(RANDOM_PURE_TPM);
+	random_source_deregister(&random_tpm);
 #endif
 
 	if (sc->buf != NULL)
diff --git a/sys/dev/vmgenc/vmgenc_acpi.c b/sys/dev/vmgenc/vmgenc_acpi.c
--- a/sys/dev/vmgenc/vmgenc_acpi.c
+++ b/sys/dev/vmgenc/vmgenc_acpi.c
@@ -56,6 +56,7 @@
 #include <contrib/dev/acpica/include/acpi.h>
 
 #include <dev/acpica/acpivar.h>
+#include <dev/random/randomdev.h>
 #include <dev/random/random_harvestq.h>
 #include <dev/vmgenc/vmgenc_acpi.h>
 
@@ -210,6 +211,11 @@
 
 }
 
+static const struct random_source random_vmgenid = {
+	.rs_ident = "VM Generation ID",
+	.rs_source = RANDOM_PURE_VMGENID,
+};
+
 static int
 vmgenc_attach(device_t dev)
 {
@@ -234,7 +240,7 @@
 	memcpy(sc->vmg_cache_guid, __DEVOLATILE(void *, sc->vmg_pguid),
 	    sizeof(sc->vmg_cache_guid));
 
-	random_harvest_register_source(RANDOM_PURE_VMGENID);
+	random_source_register(&random_vmgenid);
 	vmgenc_harvest_all(sc->vmg_cache_guid, sizeof(sc->vmg_cache_guid));
 
 	AcpiInstallNotifyHandler(h, ACPI_DEVICE_NOTIFY, vmgenc_notify, dev);
diff --git a/sys/sys/random.h b/sys/sys/random.h
--- a/sys/sys/random.h
+++ b/sys/sys/random.h
@@ -142,9 +142,6 @@
 		random_harvest_direct_(entropy, size, origin);
 }
 
-void random_harvest_register_source(enum random_entropy_source);
-void random_harvest_deregister_source(enum random_entropy_source);
-
 #if defined(RANDOM_ENABLE_UMA)
 #define random_harvest_fast_uma(a, b, c)	random_harvest_fast(a, b, c)
 #else /* !defined(RANDOM_ENABLE_UMA) */