Index: uspace/drv/bus/usb/xhci/trb_ring.c
===================================================================
--- uspace/drv/bus/usb/xhci/trb_ring.c	(revision 0f79283b7df80ab620570d6d06d69dc4e747cdac)
+++ uspace/drv/bus/usb/xhci/trb_ring.c	(revision 2bff2cc2f4626dd9e056a1dd37bb2ff505dae69a)
@@ -376,2 +376,70 @@
 	return EOK;
 }
+
+void xhci_sw_ring_init(xhci_sw_ring_t *ring, size_t size)
+{
+	ring->begin = calloc(size, sizeof(xhci_trb_t));
+	ring->end = ring->begin + size;
+
+	ring->enqueue = ring->dequeue = ring->begin;
+
+	fibril_mutex_initialize(&ring->guard);
+	fibril_condvar_initialize(&ring->enqueued_cv);
+	fibril_condvar_initialize(&ring->dequeued_cv);
+
+	ring->running = true;
+}
+
+int xhci_sw_ring_enqueue(xhci_sw_ring_t *ring, xhci_trb_t *trb)
+{
+	assert(ring);
+	assert(trb);
+
+	fibril_mutex_lock(&ring->guard);
+	while (ring->running && TRB_CYCLE(*ring->enqueue))
+		fibril_condvar_wait(&ring->dequeued_cv, &ring->guard);
+
+	*ring->enqueue = *trb;
+	TRB_SET_CYCLE(*ring->enqueue, 1);
+	if (++ring->enqueue == ring->end)
+		ring->enqueue = ring->begin;
+	fibril_condvar_signal(&ring->enqueued_cv);
+	fibril_mutex_unlock(&ring->guard);
+
+	return ring->running ? EOK : EINTR;
+}
+
+int xhci_sw_ring_dequeue(xhci_sw_ring_t *ring, xhci_trb_t *trb)
+{
+	assert(ring);
+	assert(trb);
+
+	fibril_mutex_lock(&ring->guard);
+	while (ring->running && !TRB_CYCLE(*ring->dequeue))
+		fibril_condvar_wait(&ring->enqueued_cv, &ring->guard);
+
+	*trb = *ring->dequeue;
+	TRB_SET_CYCLE(*ring->dequeue, 0);
+	if (++ring->dequeue == ring->end)
+		ring->dequeue = ring->begin;
+	fibril_condvar_signal(&ring->dequeued_cv);
+	fibril_mutex_unlock(&ring->guard);
+
+	return ring->running ? EOK : EINTR;
+}
+
+void xhci_sw_ring_stop(xhci_sw_ring_t *ring)
+{
+	ring->running = false;
+	fibril_condvar_broadcast(&ring->enqueued_cv);
+	fibril_condvar_broadcast(&ring->dequeued_cv);
+}
+
+void xhci_sw_ring_fini(xhci_sw_ring_t *ring)
+{
+	free(ring->begin);
+}
+
+/**
+ * @}
+ */
Index: uspace/drv/bus/usb/xhci/trb_ring.h
===================================================================
--- uspace/drv/bus/usb/xhci/trb_ring.h	(revision 0f79283b7df80ab620570d6d06d69dc4e747cdac)
+++ uspace/drv/bus/usb/xhci/trb_ring.h	(revision 2bff2cc2f4626dd9e056a1dd37bb2ff505dae69a)
@@ -119,3 +119,29 @@
 int xhci_event_ring_dequeue(xhci_event_ring_t *, xhci_trb_t *);
 
+/**
+ * A TRB ring of which the software is both consumer and provider.
+ */
+typedef struct xhci_sw_ring {
+	xhci_trb_t *begin, *end;
+	xhci_trb_t *enqueue, *dequeue;
+
+	fibril_mutex_t guard;
+	fibril_condvar_t enqueued_cv, dequeued_cv;
+
+	bool running;
+} xhci_sw_ring_t;
+
+void xhci_sw_ring_init(xhci_sw_ring_t *, size_t);
+
+/* Both may block if the ring is full/empty. */
+int xhci_sw_ring_enqueue(xhci_sw_ring_t *, xhci_trb_t *);
+int xhci_sw_ring_dequeue(xhci_sw_ring_t *, xhci_trb_t *);
+
+void xhci_sw_ring_stop(xhci_sw_ring_t *);
+void xhci_sw_ring_fini(xhci_sw_ring_t *);
+
 #endif
+
+/**
+ * @}
+ */