netdev - [PATCH net-next v2 2/7] gtp: set initial MTU

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Date:   Wed,  3 Feb 2021 08:08:00 +0100
From:   Jonas Bonn <jonas@...rbonn.se>
To:     laforge@...monks.org, kuba@...nel.org, netdev@...r.kernel.org,
        pablo@...filter.org
Cc:     Jonas Bonn <jonas@...rbonn.se>
Subject: [PATCH net-next v2 2/7] gtp: set initial MTU

The GTP link is brought up with a default MTU of zero.  This can lead to
some rather unexpected behaviour for users who are more accustomed to
interfaces coming online with reasonable defaults.

This patch sets an initial MTU for the GTP link of 1500 less worst-case
tunnel overhead.

Signed-off-by: Jonas Bonn <jonas@...rbonn.se>
Acked-by: Harald Welte <laforge@...monks.org>
---
 drivers/net/gtp.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/drivers/net/gtp.c b/drivers/net/gtp.c
index 4c04e271f184..5a048f050a9c 100644
--- a/drivers/net/gtp.c
+++ b/drivers/net/gtp.c
@@ -612,11 +612,16 @@ static const struct net_device_ops gtp_netdev_ops = {
 
 static void gtp_link_setup(struct net_device *dev)
 {
+	unsigned int max_gtp_header_len = sizeof(struct iphdr) +
+					  sizeof(struct udphdr) +
+					  sizeof(struct gtp0_header);
+
 	dev->netdev_ops		= &gtp_netdev_ops;
 	dev->needs_free_netdev	= true;
 
 	dev->hard_header_len = 0;
 	dev->addr_len = 0;
+	dev->mtu = ETH_DATA_LEN - max_gtp_header_len;
 
 	/* Zero header length. */
 	dev->type = ARPHRD_NONE;
@@ -626,11 +631,7 @@ static void gtp_link_setup(struct net_device *dev)
 	dev->features	|= NETIF_F_LLTX;
 	netif_keep_dst(dev);
 
-	/* Assume largest header, ie. GTPv0. */
-	dev->needed_headroom	= LL_MAX_HEADER +
-				  sizeof(struct iphdr) +
-				  sizeof(struct udphdr) +
-				  sizeof(struct gtp0_header);
+	dev->needed_headroom	= LL_MAX_HEADER + max_gtp_header_len;
 }
 
 static int gtp_hashtable_new(struct gtp_dev *gtp, int hsize);
-- 
2.27.0