--- midi.c.orig	2022-01-10 10:38:43.659213000 +0900
+++ midi.c	2022-01-10 10:55:01.655948000 +0900
@@ -60,6 +60,7 @@
 #include <sys/sbuf.h>
 #include <sys/kobj.h>
 #include <sys/module.h>
+#include <sys/limits.h>
 
 #ifdef HAVE_KERNEL_OPTION_HEADERS
 #include "opt_snd.h"
@@ -265,8 +266,8 @@
  */
 
 /*
- * Register a new rmidi device. cls midi_if interface unit == 0 means
- * auto-assign new unit number unit != 0 already assigned a unit number, eg.
+ * Register a new rmidi device. cls midi_if interface unit == -1 means
+ * auto-assign new unit number unit >= 0 already assigned a unit number, eg.
  * not the first channel provided by this device. channel,	sub-unit
  * cookie is passed back on MPU calls Typical device drivers will call with
  * unit=0, channel=1..(number of channels) and cookie=soft_c and won't care
@@ -291,26 +292,32 @@
 	 * Protect against call with existing unit/channel or auto-allocate a
 	 * new unit number.
 	 */
-	i = -1;
-	TAILQ_FOREACH(m, &midi_devs, link) {
-		mtx_lock(&m->lock);
-		if (unit != 0) {
-			if (m->unit == unit && m->channel == channel) {
-				mtx_unlock(&m->lock);
-				goto err0;
-			}
-		} else {
+	if( unit < 0 ) {
+		i = -1;
+		TAILQ_FOREACH(m, &midi_devs, link) {
+			mtx_lock(&m->lock);
 			/*
 			 * Find a better unit number
 			 */
 			if (m->unit > i)
 				i = m->unit;
+			mtx_unlock(&m->lock);
 		}
-		mtx_unlock(&m->lock);
-	}
-
-	if (unit == 0)
+		if( i == INT_MAX ) {
+			goto err0;
+		}
 		unit = i + 1;
+	}
+	else {
+		TAILQ_FOREACH(m, &midi_devs, link) {
+			mtx_lock(&m->lock);
+			if (m->unit == unit && m->channel == channel) {
+				mtx_unlock(&m->lock);
+				goto err0;
+			}
+			mtx_unlock(&m->lock);
+		}
+	}
 
 	MIDI_DEBUG(1, printf("midiinit #2: unit %d/%d.\n", unit, channel));
 	m = malloc(sizeof(*m), M_MIDI, M_WAITOK | M_ZERO);
--- midi.h.orig	2022-01-10 10:39:11.196563000 +0900
+++ midi.h	2022-01-10 10:39:48.935932000 +0900
@@ -43,6 +43,8 @@
 
 #define MIDI_TYPE unsigned char
 
+#define MIDI_UNIT_AUTO_ALLOC (-1)
+
 struct snd_midi;
 
 struct snd_midi *
--- mpu401.c.orig	2022-01-10 10:39:04.555631000 +0900
+++ mpu401.c	2022-01-10 10:40:13.074777000 +0900
@@ -193,7 +193,7 @@
 	m->cookie = cookie;
 	m->flags = 0;
 
-	m->mid = midi_init(&mpu401_class, 0, 0, m);
+	m->mid = midi_init(&mpu401_class, MIDI_UNIT_AUTO_ALLOC, 0, m);
 	if (!m->mid)
 		goto err;
 	*cb = mpu401_intr;