net: mscc: ocelot: create a helper for changing the port MTU

Since in an NPI/DSA setup, not all ports will have the same MTU, we need
to make sure the watermarks for pause frames and/or tail dropping logic
that existed in the driver is still coherent for the new MTU values.

We need to do this because the NPI (aka external CPU) port needs an
increased MTU for the DSA tag. This will be done in a future patch.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/mscc/ocelot.c

@@ -2097,11 +2097,32 @@ static int ocelot_init_timestamp(struct ocelot *ocelot)
 	return 0;
 }
 
-static void ocelot_init_port(struct ocelot *ocelot, int port)
+static void ocelot_port_set_mtu(struct ocelot *ocelot, int port, size_t mtu)
 {
 	struct ocelot_port *ocelot_port = ocelot->ports[port];
 	int atop_wm;
 
+	ocelot_port_writel(ocelot_port, mtu, DEV_MAC_MAXLEN_CFG);
+
+	/* Set Pause WM hysteresis
+	 * 152 = 6 * mtu / OCELOT_BUFFER_CELL_SZ
+	 * 101 = 4 * mtu / OCELOT_BUFFER_CELL_SZ
+	 */
+	ocelot_write_rix(ocelot, SYS_PAUSE_CFG_PAUSE_ENA |
+			 SYS_PAUSE_CFG_PAUSE_STOP(101) |
+			 SYS_PAUSE_CFG_PAUSE_START(152), SYS_PAUSE_CFG, port);
+
+	/* Tail dropping watermark */
+	atop_wm = (ocelot->shared_queue_sz - 9 * mtu) / OCELOT_BUFFER_CELL_SZ;
+	ocelot_write_rix(ocelot, ocelot_wm_enc(9 * mtu),
+			 SYS_ATOP, port);
+	ocelot_write(ocelot, ocelot_wm_enc(atop_wm), SYS_ATOP_TOT_CFG);
+}
+
+static void ocelot_init_port(struct ocelot *ocelot, int port)
+{
+	struct ocelot_port *ocelot_port = ocelot->ports[port];
+
 	INIT_LIST_HEAD(&ocelot_port->skbs);
 
 	/* Basic L2 initialization */
@@ -2122,8 +2143,7 @@ static void ocelot_init_port(struct ocelot *ocelot, int port)
 			   DEV_MAC_HDX_CFG);
 
 	/* Set Max Length and maximum tags allowed */
-	ocelot_port_writel(ocelot_port, VLAN_ETH_FRAME_LEN,
-			   DEV_MAC_MAXLEN_CFG);
+	ocelot_port_set_mtu(ocelot, port, VLAN_ETH_FRAME_LEN);
 	ocelot_port_writel(ocelot_port, DEV_MAC_TAGS_CFG_TAG_ID(ETH_P_8021AD) |
 			   DEV_MAC_TAGS_CFG_VLAN_AWR_ENA |
 			   DEV_MAC_TAGS_CFG_VLAN_LEN_AWR_ENA,
@@ -2133,20 +2153,6 @@ static void ocelot_init_port(struct ocelot *ocelot, int port)
 	ocelot_port_writel(ocelot_port, 0, DEV_MAC_FC_MAC_HIGH_CFG);
 	ocelot_port_writel(ocelot_port, 0, DEV_MAC_FC_MAC_LOW_CFG);
 
-	/* Set Pause WM hysteresis
-	 * 152 = 6 * VLAN_ETH_FRAME_LEN / OCELOT_BUFFER_CELL_SZ
-	 * 101 = 4 * VLAN_ETH_FRAME_LEN / OCELOT_BUFFER_CELL_SZ
-	 */
-	ocelot_write_rix(ocelot, SYS_PAUSE_CFG_PAUSE_ENA |
-			 SYS_PAUSE_CFG_PAUSE_STOP(101) |
-			 SYS_PAUSE_CFG_PAUSE_START(152), SYS_PAUSE_CFG, port);
-
-	/* Tail dropping watermark */
-	atop_wm = (ocelot->shared_queue_sz - 9 * VLAN_ETH_FRAME_LEN) / OCELOT_BUFFER_CELL_SZ;
-	ocelot_write_rix(ocelot, ocelot_wm_enc(9 * VLAN_ETH_FRAME_LEN),
-			 SYS_ATOP, port);
-	ocelot_write(ocelot, ocelot_wm_enc(atop_wm), SYS_ATOP_TOT_CFG);
-
 	/* Drop frames with multicast source address */
 	ocelot_rmw_gix(ocelot, ANA_PORT_DROP_CFG_DROP_MC_SMAC_ENA,
 		       ANA_PORT_DROP_CFG_DROP_MC_SMAC_ENA,