diff --git a/drivers/net/dsa/ocelot/felix.c b/drivers/net/dsa/ocelot/felix.c
index 3e07dc39007a..859196898a7d 100644
--- a/drivers/net/dsa/ocelot/felix.c
+++ b/drivers/net/dsa/ocelot/felix.c
@@ -1553,9 +1553,18 @@ static void felix_txtstamp(struct dsa_switch *ds, int port,
 static int felix_change_mtu(struct dsa_switch *ds, int port, int new_mtu)
 {
 	struct ocelot *ocelot = ds->priv;
+	struct ocelot_port *ocelot_port = ocelot->ports[port];
+	struct felix *felix = ocelot_to_felix(ocelot);
 
 	ocelot_port_set_maxlen(ocelot, port, new_mtu);
 
+	mutex_lock(&ocelot->tas_lock);
+
+	if (ocelot_port->taprio && felix->info->tas_guard_bands_update)
+		felix->info->tas_guard_bands_update(ocelot, port);
+
+	mutex_unlock(&ocelot->tas_lock);
+
 	return 0;
 }
 
diff --git a/drivers/net/dsa/ocelot/felix.h b/drivers/net/dsa/ocelot/felix.h
index 9e07eb7ee28d..deb8dde1fc19 100644
--- a/drivers/net/dsa/ocelot/felix.h
+++ b/drivers/net/dsa/ocelot/felix.h
@@ -53,6 +53,7 @@ struct felix_info {
 				    struct phylink_link_state *state);
 	int	(*port_setup_tc)(struct dsa_switch *ds, int port,
 				 enum tc_setup_type type, void *type_data);
+	void	(*tas_guard_bands_update)(struct ocelot *ocelot, int port);
 	void	(*port_sched_speed_set)(struct ocelot *ocelot, int port,
 					u32 speed);
 	struct regmap *(*init_regmap)(struct ocelot *ocelot,
diff --git a/drivers/net/dsa/ocelot/felix_vsc9959.c b/drivers/net/dsa/ocelot/felix_vsc9959.c
index 7573254274b3..27d8b56cc21c 100644
--- a/drivers/net/dsa/ocelot/felix_vsc9959.c
+++ b/drivers/net/dsa/ocelot/felix_vsc9959.c
@@ -1127,9 +1127,199 @@ static void vsc9959_mdio_bus_free(struct ocelot *ocelot)
 	mdiobus_free(felix->imdio);
 }
 
+/* Extract shortest continuous gate open intervals in ns for each traffic class
+ * of a cyclic tc-taprio schedule. If a gate is always open, the duration is
+ * considered U64_MAX. If the gate is always closed, it is considered 0.
+ */
+static void vsc9959_tas_min_gate_lengths(struct tc_taprio_qopt_offload *taprio,
+					 u64 min_gate_len[OCELOT_NUM_TC])
+{
+	struct tc_taprio_sched_entry *entry;
+	u64 gate_len[OCELOT_NUM_TC];
+	int tc, i, n;
+
+	/* Initialize arrays */
+	for (tc = 0; tc < OCELOT_NUM_TC; tc++) {
+		min_gate_len[tc] = U64_MAX;
+		gate_len[tc] = 0;
+	}
+
+	/* If we don't have taprio, consider all gates as permanently open */
+	if (!taprio)
+		return;
+
+	n = taprio->num_entries;
+
+	/* Walk through the gate list twice to determine the length
+	 * of consecutively open gates for a traffic class, including
+	 * open gates that wrap around. We are just interested in the
+	 * minimum window size, and this doesn't change what the
+	 * minimum is (if the gate never closes, min_gate_len will
+	 * remain U64_MAX).
+	 */
+	for (i = 0; i < 2 * n; i++) {
+		entry = &taprio->entries[i % n];
+
+		for (tc = 0; tc < OCELOT_NUM_TC; tc++) {
+			if (entry->gate_mask & BIT(tc)) {
+				gate_len[tc] += entry->interval;
+			} else {
+				/* Gate closes now, record a potential new
+				 * minimum and reinitialize length
+				 */
+				if (min_gate_len[tc] > gate_len[tc])
+					min_gate_len[tc] = gate_len[tc];
+				gate_len[tc] = 0;
+			}
+		}
+	}
+}
+
+/* Update QSYS_PORT_MAX_SDU to make sure the static guard bands added by the
+ * switch (see the ALWAYS_GUARD_BAND_SCH_Q comment) are correct at all MTU
+ * values (the default value is 1518). Also, for traffic class windows smaller
+ * than one MTU sized frame, update QSYS_QMAXSDU_CFG to enable oversized frame
+ * dropping, such that these won't hang the port, as they will never be sent.
+ */
+static void vsc9959_tas_guard_bands_update(struct ocelot *ocelot, int port)
+{
+	struct ocelot_port *ocelot_port = ocelot->ports[port];
+	u64 min_gate_len[OCELOT_NUM_TC];
+	int speed, picos_per_byte;
+	u64 needed_bit_time_ps;
+	u32 val, maxlen;
+	u8 tas_speed;
+	int tc;
+
+	lockdep_assert_held(&ocelot->tas_lock);
+
+	val = ocelot_read_rix(ocelot, QSYS_TAG_CONFIG, port);
+	tas_speed = QSYS_TAG_CONFIG_LINK_SPEED_X(val);
+
+	switch (tas_speed) {
+	case OCELOT_SPEED_10:
+		speed = SPEED_10;
+		break;
+	case OCELOT_SPEED_100:
+		speed = SPEED_100;
+		break;
+	case OCELOT_SPEED_1000:
+		speed = SPEED_1000;
+		break;
+	case OCELOT_SPEED_2500:
+		speed = SPEED_2500;
+		break;
+	default:
+		return;
+	}
+
+	picos_per_byte = (USEC_PER_SEC * 8) / speed;
+
+	val = ocelot_port_readl(ocelot_port, DEV_MAC_MAXLEN_CFG);
+	/* MAXLEN_CFG accounts automatically for VLAN. We need to include it
+	 * manually in the bit time calculation, plus the preamble and SFD.
+	 */
+	maxlen = val + 2 * VLAN_HLEN;
+	/* Consider the standard Ethernet overhead of 8 octets preamble+SFD,
+	 * 4 octets FCS, 12 octets IFG.
+	 */
+	needed_bit_time_ps = (maxlen + 24) * picos_per_byte;
+
+	dev_dbg(ocelot->dev,
+		"port %d: max frame size %d needs %llu ps at speed %d\n",
+		port, maxlen, needed_bit_time_ps, speed);
+
+	vsc9959_tas_min_gate_lengths(ocelot_port->taprio, min_gate_len);
+
+	for (tc = 0; tc < OCELOT_NUM_TC; tc++) {
+		u32 max_sdu;
+
+		if (min_gate_len[tc] == U64_MAX /* Gate always open */ ||
+		    min_gate_len[tc] * 1000 > needed_bit_time_ps) {
+			/* Setting QMAXSDU_CFG to 0 disables oversized frame
+			 * dropping.
+			 */
+			max_sdu = 0;
+			dev_dbg(ocelot->dev,
+				"port %d tc %d min gate len %llu"
+				", sending all frames\n",
+				port, tc, min_gate_len[tc]);
+		} else {
+			/* If traffic class doesn't support a full MTU sized
+			 * frame, make sure to enable oversize frame dropping
+			 * for frames larger than the smallest that would fit.
+			 */
+			max_sdu = div_u64(min_gate_len[tc] * 1000,
+					  picos_per_byte);
+			/* A TC gate may be completely closed, which is a
+			 * special case where all packets are oversized.
+			 * Any limit smaller than 64 octets accomplishes this
+			 */
+			if (!max_sdu)
+				max_sdu = 1;
+			/* Take L1 overhead into account, but just don't allow
+			 * max_sdu to go negative or to 0. Here we use 20
+			 * because QSYS_MAXSDU_CFG_* already counts the 4 FCS
+			 * octets as part of packet size.
+			 */
+			if (max_sdu > 20)
+				max_sdu -= 20;
+			dev_info(ocelot->dev,
+				 "port %d tc %d min gate length %llu"
+				 " ns not enough for max frame size %d at %d"
+				 " Mbps, dropping frames over %d"
+				 " octets including FCS\n",
+				 port, tc, min_gate_len[tc], maxlen, speed,
+				 max_sdu);
+		}
+
+		/* ocelot_write_rix is a macro that concatenates
+		 * QSYS_MAXSDU_CFG_* with _RSZ, so we need to spell out
+		 * the writes to each traffic class
+		 */
+		switch (tc) {
+		case 0:
+			ocelot_write_rix(ocelot, max_sdu, QSYS_QMAXSDU_CFG_0,
+					 port);
+			break;
+		case 1:
+			ocelot_write_rix(ocelot, max_sdu, QSYS_QMAXSDU_CFG_1,
+					 port);
+			break;
+		case 2:
+			ocelot_write_rix(ocelot, max_sdu, QSYS_QMAXSDU_CFG_2,
+					 port);
+			break;
+		case 3:
+			ocelot_write_rix(ocelot, max_sdu, QSYS_QMAXSDU_CFG_3,
+					 port);
+			break;
+		case 4:
+			ocelot_write_rix(ocelot, max_sdu, QSYS_QMAXSDU_CFG_4,
+					 port);
+			break;
+		case 5:
+			ocelot_write_rix(ocelot, max_sdu, QSYS_QMAXSDU_CFG_5,
+					 port);
+			break;
+		case 6:
+			ocelot_write_rix(ocelot, max_sdu, QSYS_QMAXSDU_CFG_6,
+					 port);
+			break;
+		case 7:
+			ocelot_write_rix(ocelot, max_sdu, QSYS_QMAXSDU_CFG_7,
+					 port);
+			break;
+		}
+	}
+
+	ocelot_write_rix(ocelot, maxlen, QSYS_PORT_MAX_SDU, port);
+}
+
 static void vsc9959_sched_speed_set(struct ocelot *ocelot, int port,
 				    u32 speed)
 {
+	struct ocelot_port *ocelot_port = ocelot->ports[port];
 	u8 tas_speed;
 
 	switch (speed) {
@@ -1154,6 +1344,13 @@ static void vsc9959_sched_speed_set(struct ocelot *ocelot, int port,
 		       QSYS_TAG_CONFIG_LINK_SPEED(tas_speed),
 		       QSYS_TAG_CONFIG_LINK_SPEED_M,
 		       QSYS_TAG_CONFIG, port);
+
+	mutex_lock(&ocelot->tas_lock);
+
+	if (ocelot_port->taprio)
+		vsc9959_tas_guard_bands_update(ocelot, port);
+
+	mutex_unlock(&ocelot->tas_lock);
 }
 
 static void vsc9959_new_base_time(struct ocelot *ocelot, ktime_t base_time,
@@ -1210,6 +1407,8 @@ static int vsc9959_qos_port_tas_set(struct ocelot *ocelot, int port,
 		taprio_offload_free(ocelot_port->taprio);
 		ocelot_port->taprio = NULL;
 
+		vsc9959_tas_guard_bands_update(ocelot, port);
+
 		mutex_unlock(&ocelot->tas_lock);
 		return 0;
 	}
@@ -1284,6 +1483,7 @@ static int vsc9959_qos_port_tas_set(struct ocelot *ocelot, int port,
 		goto err;
 
 	ocelot_port->taprio = taprio_offload_get(taprio);
+	vsc9959_tas_guard_bands_update(ocelot, port);
 
 err:
 	mutex_unlock(&ocelot->tas_lock);
@@ -2303,6 +2503,7 @@ static const struct felix_info felix_info_vsc9959 = {
 	.port_modes		= vsc9959_port_modes,
 	.port_setup_tc		= vsc9959_port_setup_tc,
 	.port_sched_speed_set	= vsc9959_sched_speed_set,
+	.tas_guard_bands_update	= vsc9959_tas_guard_bands_update,
 	.init_regmap		= ocelot_regmap_init,
 };
 
