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From:   Rasmus Villemoes <rasmus.villemoes@prevas.dk>
To:     Li Yang <leoyang.li@nxp.com>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Murali Krishna Policharla <murali.policharla@broadcom.com>,
        Florian Fainelli <f.fainelli@gmail.com>,
        Vladimir Oltean <vladimir.oltean@nxp.com>
Cc:     Zhao Qiang <qiang.zhao@nxp.com>,
        Rasmus Villemoes <rasmus.villemoes@prevas.dk>,
        netdev@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        linux-kernel@vger.kernel.org
Subject: [PATCH 01/20] ethernet: ucc_geth: set dev->max_mtu to 1518
Date:   Sat,  5 Dec 2020 20:17:24 +0100
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All the buffers and registers are already set up appropriately for an
MTU slightly above 1500, so we just need to expose this to the
networking stack. AFAICT, there's no need to implement .ndo_change_mtu
when the receive buffers are always set up to support the max_mtu.

This fixes several warnings during boot on our mpc8309-board with an
embedded mv88e6250 switch:

mv88e6085 mdio@e0102120:10: nonfatal error -34 setting MTU 1500 on port 0
...
mv88e6085 mdio@e0102120:10: nonfatal error -34 setting MTU 1500 on port 4
ucc_geth e0102000.ethernet eth1: error -22 setting MTU to 1504 to include DSA overhead

The last line explains what the DSA stack tries to do: achieving an MTU
of 1500 on-the-wire requires that the master netdevice connected to
the CPU port supports an MTU of 1500+the tagging overhead.

Fixes: bfcb813203e6 ("net: dsa: configure the MTU for switch ports")
Cc: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
---
 drivers/net/ethernet/freescale/ucc_geth.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/net/ethernet/freescale/ucc_geth.c b/drivers/net/ethernet/freescale/ucc_geth.c
index 714b501be7d0..380c1f09adaf 100644
--- a/drivers/net/ethernet/freescale/ucc_geth.c
+++ b/drivers/net/ethernet/freescale/ucc_geth.c
@@ -3889,6 +3889,7 @@ static int ucc_geth_probe(struct platform_device* ofdev)
 	INIT_WORK(&ugeth->timeout_work, ucc_geth_timeout_work);
 	netif_napi_add(dev, &ugeth->napi, ucc_geth_poll, 64);
 	dev->mtu = 1500;
+	dev->max_mtu = 1518;
 
 	ugeth->msg_enable = netif_msg_init(debug.msg_enable, UGETH_MSG_DEFAULT);
 	ugeth->phy_interface = phy_interface;
-- 
2.23.0