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Date:   Fri, 28 Oct 2022 21:32:19 +0300
From:   Sergei Antonov <>
        Vladimir Oltean <>,
        Sergei Antonov <>
Subject: [PATCH 2/3] net: ftmac100: report the correct maximum MTU of 1500

From: Vladimir Oltean <>

The driver uses the MAX_PKT_SIZE (1518) for both MTU reporting and for
TX. However, the 2 places do not measure the same thing.

On TX, skb->len measures the entire L2 packet length (without FCS, which
software does not possess). So the comparison against 1518 there is

What is not correct is the reporting of dev->max_mtu as 1518. Since MTU
measures L2 *payload* length (excluding L2 overhead) and not total L2
packet length, it means that the correct max_mtu supported by this
device is the standard 1500. Anything higher than that will be dropped
on RX currently.

To fix this, subtract VLAN_ETH_HLEN from MAX_PKT_SIZE when reporting the
max_mtu, since that is the difference between L2 payload length and
total L2 length as seen by software.

Signed-off-by: Vladimir Oltean <>
Signed-off-by: Sergei Antonov <>
 drivers/net/ethernet/faraday/ftmac100.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/faraday/ftmac100.c b/drivers/net/ethernet/faraday/ftmac100.c
index 8013f85fc148..7c571b4515a9 100644
--- a/drivers/net/ethernet/faraday/ftmac100.c
+++ b/drivers/net/ethernet/faraday/ftmac100.c
@@ -11,6 +11,7 @@
 #include <linux/dma-mapping.h>
 #include <linux/etherdevice.h>
 #include <linux/ethtool.h>
+#include <linux/if_vlan.h>
 #include <linux/init.h>
 #include <linux/interrupt.h>
 #include <linux/io.h>
@@ -1070,7 +1071,7 @@ static int ftmac100_probe(struct platform_device *pdev)
 	SET_NETDEV_DEV(netdev, &pdev->dev);
 	netdev->ethtool_ops = &ftmac100_ethtool_ops;
 	netdev->netdev_ops = &ftmac100_netdev_ops;
-	netdev->max_mtu = MAX_PKT_SIZE;
+	netdev->max_mtu = MAX_PKT_SIZE - VLAN_ETH_HLEN;
 	err = platform_get_ethdev_address(&pdev->dev, netdev);
 	if (err == -EPROBE_DEFER)

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