Index: sys/kern/subr_intr.c
===================================================================
--- sys/kern/subr_intr.c
+++ sys/kern/subr_intr.c
@@ -406,9 +406,21 @@
 
 	mtx_assert(&isrc_table_lock, MA_OWNED);
 
+	/* allocation requesting specific number */
+	if ((isrc->isrc_flags & INTR_ISRCF_USE_IRQ) != 0) {
+		if (isrc->isrc_irq >= intr_nirq)
+			return (EINVAL);
+		else if (irq_sources[isrc->isrc_irq] != NULL)
+			return (EEXIST);
+		irq_sources[isrc->isrc_irq] = isrc;
+		return (0);
+	}
+
+	/* table known full, don't bother */
 	if (irq_next_free >= intr_nirq)
 		return (ENOSPC);
 
+	/* usual approach, look for an empty entry */
 	for (irq = irq_next_free; irq < intr_nirq; irq++) {
 		if (irq_sources[irq] == NULL)
 			goto found;
@@ -472,7 +484,6 @@
 
 	bzero(isrc, sizeof(struct intr_irqsrc));
 	isrc->isrc_dev = dev;
-	isrc->isrc_irq = INTR_IRQ_INVALID;	/* just to be safe */
 	isrc->isrc_flags = flags;
 
 	va_start(ap, fmt);
Index: sys/sys/intr.h
===================================================================
--- sys/sys/intr.h
+++ sys/sys/intr.h
@@ -76,6 +76,7 @@
 #define INTR_ISRCF_IPI		0x01	/* IPI interrupt */
 #define INTR_ISRCF_PPI		0x02	/* PPI interrupt */
 #define INTR_ISRCF_BOUND	0x04	/* bound to a CPU */
+#define INTR_ISRCF_USE_IRQ	0x80	/* use passed in ->isrc_irq value */
 
 struct intr_pic;