Index: if_em.c
===================================================================
RCS file: /cvs/src/sys/dev/pci/if_em.c,v
diff -u -p -r1.378 if_em.c
--- if_em.c	31 Aug 2024 16:23:09 -0000	1.378
+++ if_em.c	29 Oct 2024 09:17:54 -0000
@@ -452,7 +452,7 @@ em_attach(struct device *parent, struct 
 	sc->hw.wait_autoneg_complete = WAIT_FOR_AUTO_NEG_DEFAULT;
 	sc->hw.autoneg_advertised = AUTONEG_ADV_DEFAULT;
 	sc->hw.tbi_compatibility_en = TRUE;
-	sc->sc_rx_buffer_len = EM_RXBUFFER_2048;
+	sc->sc_rx_buffer_len = EM_MCLBYTES;
 
 	sc->hw.phy_init_script = 1;
 	sc->hw.phy_reset_disable = FALSE;
@@ -780,7 +780,7 @@ em_ioctl(struct ifnet *ifp, u_long comma
 
 	case SIOCGIFRXR:
 		error = if_rxr_ioctl((struct if_rxrinfo *)ifr->ifr_data,
-		    NULL, EM_MCLBYTES, &sc->queues->rx.sc_rx_ring);
+		    NULL, sc->sc_rx_buffer_len, &sc->queues->rx.sc_rx_ring);
 		break;
 
 	case SIOCGIFSFFPAGE:
@@ -2712,16 +2712,18 @@ em_get_buf(struct em_queue *que, int i)
 
 	KASSERT(pkt->pkt_m == NULL);
 
-	m = MCLGETL(NULL, M_DONTWAIT, EM_MCLBYTES);
+	m = MCLGETL(NULL, M_DONTWAIT, sc->sc_rx_buffer_len + ETHER_ALIGN);
 	if (m == NULL) {
 		sc->mbuf_cluster_failed++;
 		return (ENOBUFS);
 	}
-	m->m_len = m->m_pkthdr.len = EM_MCLBYTES;
-	m_adj(m, ETHER_ALIGN);
+
+	m->m_data += ETHER_ALIGN;
+	m->m_len = m->m_pkthdr.len = sc->sc_rx_buffer_len;
 
 	error = bus_dmamap_load_mbuf(sc->sc_dmat, pkt->pkt_map,
-	    m, BUS_DMA_NOWAIT);
+	    m, BUS_DMA_NOWAIT |
+	    (ISSET(sc->sc_ac.ac_if.if_flags, IFF_LINK0) ? BUS_DMA_BUS4 : 0));
 	if (error) {
 		m_freem(m);
 		return (error);
@@ -2771,8 +2773,9 @@ em_allocate_receive_structures(struct em
 		for (i = 0; i < sc->sc_rx_slots; i++) {
 			pkt = &que->rx.sc_rx_pkts_ring[i];
 
-			error = bus_dmamap_create(sc->sc_dmat, EM_MCLBYTES, 1,
-			    EM_MCLBYTES, 0, BUS_DMA_NOWAIT, &pkt->pkt_map);
+			error = bus_dmamap_create(sc->sc_dmat,
+			    sc->sc_rx_buffer_len, 1, sc->sc_rx_buffer_len, 0,
+			    BUS_DMA_NOWAIT, &pkt->pkt_map);
 			if (error != 0) {
 				printf("%s: em_allocate_receive_structures: "
 				    "bus_dmamap_create failed; error %u\n",
@@ -2801,7 +2804,6 @@ em_setup_receive_structures(struct em_so
 {
 	struct ifnet *ifp = &sc->sc_ac.ac_if;
 	struct em_queue *que;
-	u_int lwm;
 
 	if (em_allocate_receive_structures(sc))
 		return (ENOMEM);
@@ -2814,8 +2816,9 @@ em_setup_receive_structures(struct em_so
 		que->rx.sc_rx_desc_tail = 0;
 		que->rx.sc_rx_desc_head = sc->sc_rx_slots - 1;
 
-		lwm = max(4, 2 * ((ifp->if_hardmtu / MCLBYTES) + 1));
-		if_rxr_init(&que->rx.sc_rx_ring, lwm, sc->sc_rx_slots);
+		if_rxr_init(&que->rx.sc_rx_ring,
+		    2 * howmany(ifp->if_hardmtu, sc->sc_rx_buffer_len) + 1,
+		    sc->sc_rx_slots - 1);
 
 		if (em_rxfill(que) == 0) {
 			printf("%s: unable to fill any rx descriptors\n",
Index: if_em.h
===================================================================
RCS file: /cvs/src/sys/dev/pci/if_em.h,v
diff -u -p -r1.83 if_em.h
--- if_em.h	16 Feb 2024 22:30:54 -0000	1.83
+++ if_em.h	29 Oct 2024 09:17:54 -0000
@@ -268,7 +268,7 @@ typedef int	boolean_t;
 #define EM_RXBUFFER_8192	8192
 #define EM_RXBUFFER_16384	16384
 
-#define EM_MCLBYTES		(EM_RXBUFFER_2048 + ETHER_ALIGN)
+#define EM_MCLBYTES		EM_RXBUFFER_2048
 
 #define EM_MAX_SCATTER		64
 #define EM_TSO_SIZE		65535