net: ixp4xx_eth: Support changing the MTU

As we don't specify the MTU in the driver, the framework
will fall back to 1500 bytes and this doesn't work very
well when we try to attach a DSA switch:

  eth1: mtu greater than device maximum
  ixp4xx_eth c800a000.ethernet eth1: error -22 setting
  MTU to 1504 to include DSA overhead

After locating an out-of-tree patch in OpenWrt I found
suitable code to set the MTU on the interface and ported
it and updated it. Now the MTU gets set properly.
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20231005-ixp4xx-eth-mtu-v4-1-08c66ed0bc69@linaro.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/xscale/ixp4xx_eth.c

@@ -64,7 +64,15 @@
 
 #define POOL_ALLOC_SIZE		(sizeof(struct desc) * (RX_DESCS + TX_DESCS))
 #define REGS_SIZE		0x1000
-#define MAX_MRU			1536 /* 0x600 */
+
+/* MRU is said to be 14320 in a code dump, the SW manual says that
+ * MRU/MTU is 16320 and includes VLAN and ethernet headers.
+ * See "IXP400 Software Programmer's Guide" section 10.3.2, page 161.
+ *
+ * FIXME: we have chosen the safe default (14320) but if you can test
+ * jumboframes, experiment with 16320 and see what happens!
+ */
+#define MAX_MRU			(14320 - VLAN_ETH_HLEN)
 #define RX_BUFF_SIZE		ALIGN((NET_IP_ALIGN) + MAX_MRU, 4)
 
 #define NAPI_WEIGHT		16
@@ -1183,6 +1191,54 @@ static void destroy_queues(struct port *port)
 	}
 }
 
+static int ixp4xx_do_change_mtu(struct net_device *dev, int new_mtu)
+{
+	struct port *port = netdev_priv(dev);
+	struct npe *npe = port->npe;
+	int framesize, chunks;
+	struct msg msg = {};
+
+	/* adjust for ethernet headers */
+	framesize = new_mtu + VLAN_ETH_HLEN;
+	/* max rx/tx 64 byte chunks */
+	chunks = DIV_ROUND_UP(framesize, 64);
+
+	msg.cmd = NPE_SETMAXFRAMELENGTHS;
+	msg.eth_id = port->id;
+
+	/* Firmware wants to know buffer size in 64 byte chunks */
+	msg.byte2 = chunks << 8;
+	msg.byte3 = chunks << 8;
+
+	msg.byte4 = msg.byte6 = framesize >> 8;
+	msg.byte5 = msg.byte7 = framesize & 0xff;
+
+	if (npe_send_recv_message(npe, &msg, "ETH_SET_MAX_FRAME_LENGTH"))
+		return -EIO;
+	netdev_dbg(dev, "set MTU on NPE %s to %d bytes\n",
+		   npe_name(npe), new_mtu);
+
+	return 0;
+}
+
+static int ixp4xx_eth_change_mtu(struct net_device *dev, int new_mtu)
+{
+	int ret;
+
+	/* MTU can only be changed when the interface is up. We also
+	 * set the MTU from dev->mtu when opening the device.
+	 */
+	if (dev->flags & IFF_UP) {
+		ret = ixp4xx_do_change_mtu(dev, new_mtu);
+		if (ret < 0)
+			return ret;
+	}
+
+	dev->mtu = new_mtu;
+
+	return 0;
+}
+
 static int eth_open(struct net_device *dev)
 {
 	struct port *port = netdev_priv(dev);
@@ -1233,6 +1289,8 @@ static int eth_open(struct net_device *dev)
 	if (npe_send_recv_message(port->npe, &msg, "ETH_SET_FIREWALL_MODE"))
 		return -EIO;
 
+	ixp4xx_do_change_mtu(dev, dev->mtu);
+
 	if ((err = request_queues(port)) != 0)
 		return err;
 
@@ -1375,6 +1433,7 @@ static int eth_close(struct net_device *dev)
 static const struct net_device_ops ixp4xx_netdev_ops = {
 	.ndo_open = eth_open,
 	.ndo_stop = eth_close,
+	.ndo_change_mtu = ixp4xx_eth_change_mtu,
 	.ndo_start_xmit = eth_xmit,
 	.ndo_set_rx_mode = eth_set_mcast_list,
 	.ndo_eth_ioctl = eth_ioctl,
@@ -1489,12 +1548,8 @@ static int ixp4xx_eth_probe(struct platform_device *pdev)
 	ndev->dev.dma_mask = dev->dma_mask;
 	ndev->dev.coherent_dma_mask = dev->coherent_dma_mask;
 
-	/* Maximum frame size is 16320 bytes and includes VLAN and
-	 * ethernet headers. See "IXP400 Software Programmer's Guide"
-	 * section 10.3.2, page 161.
-	 */
 	ndev->min_mtu = ETH_MIN_MTU;
-	ndev->max_mtu = 16320 - VLAN_ETH_HLEN;
+	ndev->max_mtu = MAX_MRU;
 
 	netif_napi_add_weight(ndev, &port->napi, eth_poll, NAPI_WEIGHT);