net: dsa: mt7530: support setting MTU

MT7530/7531 has a global RX packet length register, which can be used
to set MTU.

Supported packet length values are 1522 (1518 if untagged), 1536,
1552, and multiple of 1024 (from 2048 to 15360).

Signed-off-by: DENG Qingfang <dqfext@gmail.com>
Link: https://lore.kernel.org/r/20201103050618.11419-1-dqfext@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/dsa/mt7530.c

@@ -1021,6 +1021,53 @@ mt7530_port_disable(struct dsa_switch *ds, int port)
 	mutex_unlock(&priv->reg_mutex);
 }
 
+static int
+mt7530_port_change_mtu(struct dsa_switch *ds, int port, int new_mtu)
+{
+	struct mt7530_priv *priv = ds->priv;
+	struct mii_bus *bus = priv->bus;
+	int length;
+	u32 val;
+
+	/* When a new MTU is set, DSA always set the CPU port's MTU to the
+	 * largest MTU of the slave ports. Because the switch only has a global
+	 * RX length register, only allowing CPU port here is enough.
+	 */
+	if (!dsa_is_cpu_port(ds, port))
+		return 0;
+
+	mutex_lock_nested(&bus->mdio_lock, MDIO_MUTEX_NESTED);
+
+	val = mt7530_mii_read(priv, MT7530_GMACCR);
+	val &= ~MAX_RX_PKT_LEN_MASK;
+
+	/* RX length also includes Ethernet header, MTK tag, and FCS length */
+	length = new_mtu + ETH_HLEN + MTK_HDR_LEN + ETH_FCS_LEN;
+	if (length <= 1522) {
+		val |= MAX_RX_PKT_LEN_1522;
+	} else if (length <= 1536) {
+		val |= MAX_RX_PKT_LEN_1536;
+	} else if (length <= 1552) {
+		val |= MAX_RX_PKT_LEN_1552;
+	} else {
+		val &= ~MAX_RX_JUMBO_MASK;
+		val |= MAX_RX_JUMBO(DIV_ROUND_UP(length, 1024));
+		val |= MAX_RX_PKT_LEN_JUMBO;
+	}
+
+	mt7530_mii_write(priv, MT7530_GMACCR, val);
+
+	mutex_unlock(&bus->mdio_lock);
+
+	return 0;
+}
+
+static int
+mt7530_port_max_mtu(struct dsa_switch *ds, int port)
+{
+	return MT7530_MAX_MTU;
+}
+
 static void
 mt7530_stp_state_set(struct dsa_switch *ds, int port, u8 state)
 {
@@ -2519,6 +2566,8 @@ static const struct dsa_switch_ops mt7530_switch_ops = {
 	.get_sset_count		= mt7530_get_sset_count,
 	.port_enable		= mt7530_port_enable,
 	.port_disable		= mt7530_port_disable,
+	.port_change_mtu	= mt7530_port_change_mtu,
+	.port_max_mtu		= mt7530_port_max_mtu,
 	.port_stp_state_set	= mt7530_stp_state_set,
 	.port_bridge_join	= mt7530_port_bridge_join,
 	.port_bridge_leave	= mt7530_port_bridge_leave,

drivers/net/dsa/mt7530.h

@@ -11,6 +11,9 @@
 #define MT7530_NUM_FDB_RECORDS		2048
 #define MT7530_ALL_MEMBERS		0xff
 
+#define MTK_HDR_LEN	4
+#define MT7530_MAX_MTU	(15 * 1024 - ETH_HLEN - ETH_FCS_LEN - MTK_HDR_LEN)
+
 enum mt753x_id {
 	ID_MT7530 = 0,
 	ID_MT7621 = 1,
@@ -289,6 +292,15 @@ enum mt7530_vlan_port_attr {
 #define MT7531_DBG_CNT(x)		(0x3018 + (x) * 0x100)
 #define  MT7531_DIS_CLR			BIT(31)
 
+#define MT7530_GMACCR			0x30e0
+#define  MAX_RX_JUMBO(x)		((x) << 2)
+#define  MAX_RX_JUMBO_MASK		GENMASK(5, 2)
+#define  MAX_RX_PKT_LEN_MASK		GENMASK(1, 0)
+#define  MAX_RX_PKT_LEN_1522		0x0
+#define  MAX_RX_PKT_LEN_1536		0x1
+#define  MAX_RX_PKT_LEN_1552		0x2
+#define  MAX_RX_PKT_LEN_JUMBO		0x3
+
 /* Register for MIB */
 #define MT7530_PORT_MIB_COUNTER(x)	(0x4000 + (x) * 0x100)
 #define MT7530_MIB_CCR			0x4fe0