net: dsa: qca8k: implement the port MTU callbacks

This switch has a single max frame size configuration register, so we
track the requested MTU for each port and apply the largest.

v2:
- Address review feedback from Vladimir Oltean
Signed-off-by: Jonathan McDowell <noodles@earth.li>
Acked-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/qca8k.c

@@ -670,6 +670,11 @@ qca8k_setup(struct dsa_switch *ds)
 		}
 	}
 
+	/* Setup our port MTUs to match power on defaults */
+	for (i = 0; i < QCA8K_NUM_PORTS; i++)
+		priv->port_mtu[i] = ETH_FRAME_LEN + ETH_FCS_LEN;
+	qca8k_write(priv, QCA8K_MAX_FRAME_SIZE, ETH_FRAME_LEN + ETH_FCS_LEN);
+
 	/* Flush the FDB table */
 	qca8k_fdb_flush(priv);
 
@@ -1098,6 +1103,30 @@ qca8k_port_disable(struct dsa_switch *ds, int port)
 	priv->port_sts[port].enabled = 0;
 }
 
+static int
+qca8k_port_change_mtu(struct dsa_switch *ds, int port, int new_mtu)
+{
+	struct qca8k_priv *priv = ds->priv;
+	int i, mtu = 0;
+
+	priv->port_mtu[port] = new_mtu;
+
+	for (i = 0; i < QCA8K_NUM_PORTS; i++)
+		if (priv->port_mtu[port] > mtu)
+			mtu = priv->port_mtu[port];
+
+	/* Include L2 header / FCS length */
+	qca8k_write(priv, QCA8K_MAX_FRAME_SIZE, mtu + ETH_HLEN + ETH_FCS_LEN);
+
+	return 0;
+}
+
+static int
+qca8k_port_max_mtu(struct dsa_switch *ds, int port)
+{
+	return QCA8K_MAX_MTU;
+}
+
 static int
 qca8k_port_fdb_insert(struct qca8k_priv *priv, const u8 *addr,
 		      u16 port_mask, u16 vid)
@@ -1174,6 +1203,8 @@ static const struct dsa_switch_ops qca8k_switch_ops = {
 	.set_mac_eee		= qca8k_set_mac_eee,
 	.port_enable		= qca8k_port_enable,
 	.port_disable		= qca8k_port_disable,
+	.port_change_mtu	= qca8k_port_change_mtu,
+	.port_max_mtu		= qca8k_port_max_mtu,
 	.port_stp_state_set	= qca8k_port_stp_state_set,
 	.port_bridge_join	= qca8k_port_bridge_join,
 	.port_bridge_leave	= qca8k_port_bridge_leave,

drivers/net/dsa/qca8k.h

@@ -13,6 +13,7 @@
 #include <linux/gpio.h>
 
 #define QCA8K_NUM_PORTS					7
+#define QCA8K_MAX_MTU					9000
 
 #define PHY_ID_QCA8337					0x004dd036
 #define QCA8K_ID_QCA8337				0x13
@@ -58,6 +59,7 @@
 #define   QCA8K_MDIO_MASTER_MAX_REG			32
 #define QCA8K_GOL_MAC_ADDR0				0x60
 #define QCA8K_GOL_MAC_ADDR1				0x64
+#define QCA8K_MAX_FRAME_SIZE				0x78
 #define QCA8K_REG_PORT_STATUS(_i)			(0x07c + (_i) * 4)
 #define   QCA8K_PORT_STATUS_SPEED			GENMASK(1, 0)
 #define   QCA8K_PORT_STATUS_SPEED_10			0
@@ -189,6 +191,7 @@ struct qca8k_priv {
 	struct device *dev;
 	struct dsa_switch_ops ops;
 	struct gpio_desc *reset_gpio;
+	unsigned int port_mtu[QCA8K_NUM_PORTS];
 };
 
 struct qca8k_mib_desc {