[PATCH net-next 2/3] mctp: serial: enforce fixed MTU

[Jeremy Kerr <jk@codeconstruct.com.au>]

The current serial driver requires a maximum MTU of 68, and it doesn't
make sense to set a MTU below the MCTP-required baseline (of 68) either.

This change sets the min_mtu & max_mtu of the mctp netdev, essentially
disallowing changes. By using these instead of a ndo_change_mtu op, we
get the netlink extacks reported too.

Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
---
 drivers/net/mctp/mctp-serial.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/net/mctp/mctp-serial.c b/drivers/net/mctp/mctp-serial.c
index c958d773a82a..5687ad3220cd 100644
--- a/drivers/net/mctp/mctp-serial.c
+++ b/drivers/net/mctp/mctp-serial.c
@@ -410,7 +410,14 @@ static const struct net_device_ops mctp_serial_netdev_ops = {
 static void mctp_serial_setup(struct net_device *ndev)
 {
 	ndev->type = ARPHRD_MCTP;
+
+	/* we limit at the fixed MTU, which is also the MCTP-standard
+	 * baseline MTU, so is also our minimum
+	 */
 	ndev->mtu = MCTP_SERIAL_MTU;
+	ndev->max_mtu = MCTP_SERIAL_MTU;
+	ndev->min_mtu = MCTP_SERIAL_MTU;
+
 	ndev->hard_header_len = 0;
 	ndev->addr_len = 0;
 	ndev->tx_queue_len = DEFAULT_TX_QUEUE_LEN;
-- 
2.33.0