From: Andrew Lunn <andrew@lunn.ch>
Date: Sat, 11 Jul 2020 22:32:06 +0200
Subject: net: fec: Set max MTU size to allow the MTU to be changed
Git-commit: 5919305351475dda04c0f6f28f388c89ffafcd2b
Patch-mainline: v5.9-rc1
References: jsc#SLE-16106

The FEC allocates 2K buffers, but looses some of it due to
alignment. It can however support an MTU bigger than the default. This
is particularly interesting when used in combination with Ethernet
switches supporting DSA, which have extra headers. The DSA core will
try to increase the MTU to support these extra headers. If the max
size defaults to that of standard Ethernet we get a warning. By
setting the max to what the driver actually supports, we avoid this
warning.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
---
 drivers/net/ethernet/freescale/fec_main.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/net/ethernet/freescale/fec_main.c b/drivers/net/ethernet/freescale/fec_main.c
index 795693874e31..a93381284f08 100644
--- a/drivers/net/ethernet/freescale/fec_main.c
+++ b/drivers/net/ethernet/freescale/fec_main.c
@@ -3678,6 +3678,8 @@ fec_probe(struct platform_device *pdev)
 	fec_enet_clk_enable(ndev, false);
 	pinctrl_pm_select_sleep_state(&pdev->dev);
 
+	ndev->max_mtu = PKT_MAXBUF_SIZE - ETH_HLEN - ETH_FCS_LEN;
+
 	ret = register_netdev(ndev);
 	if (ret)
 		goto failed_register;