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Date:   Thu,  3 Dec 2020 18:40:40 +0100
From:   Loic Poulain <loic.poulain@...aro.org>
To:     subashab@...eaurora.org, stranche@...eaurora.org
Cc:     netdev@...r.kernel.org, kuba@...nel.org,
        Loic Poulain <loic.poulain@...aro.org>
Subject: [PATCH] net: rmnet: Adjust virtual device MTU on real device capability

A submitted qmap/rmnet packet size can not be larger than the linked
interface (real_dev) MTU. This patch ensures that the rmnet virtual
iface MTU is configured according real device capability.

Signed-off-by: Loic Poulain <loic.poulain@...aro.org>
---
 drivers/net/ethernet/qualcomm/rmnet/rmnet_vnd.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/net/ethernet/qualcomm/rmnet/rmnet_vnd.c b/drivers/net/ethernet/qualcomm/rmnet/rmnet_vnd.c
index d58b51d..d1d7328 100644
--- a/drivers/net/ethernet/qualcomm/rmnet/rmnet_vnd.c
+++ b/drivers/net/ethernet/qualcomm/rmnet/rmnet_vnd.c
@@ -60,9 +60,14 @@ static netdev_tx_t rmnet_vnd_start_xmit(struct sk_buff *skb,
 
 static int rmnet_vnd_change_mtu(struct net_device *rmnet_dev, int new_mtu)
 {
+	struct rmnet_priv *priv = netdev_priv(rmnet_dev);
+
 	if (new_mtu < 0 || new_mtu > RMNET_MAX_PACKET_SIZE)
 		return -EINVAL;
 
+	if (priv->real_dev && (new_mtu + RMNET_NEEDED_HEADROOM) > priv->real_dev->mtu)
+		return -EINVAL;
+
 	rmnet_dev->mtu = new_mtu;
 	return 0;
 }
@@ -242,6 +247,9 @@ int rmnet_vnd_newlink(u8 id, struct net_device *rmnet_dev,
 
 	priv->real_dev = real_dev;
 
+	/* Align default MTU with real_dev MTU */
+	rmnet_vnd_change_mtu(rmnet_dev, real_dev->mtu - RMNET_NEEDED_HEADROOM);
+
 	rc = register_netdevice(rmnet_dev);
 	if (!rc) {
 		ep->egress_dev = rmnet_dev;
-- 
2.7.4

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