vrx518_tc: allow larger MTU to support RFC4638

vrx518_tc currently sets the interface maximum MTU to the ethernet default
of 1500 bytes by default via ether_setup() called from ptm_setup().

To support 1508 byte baby jumbo frames (RFC4638) for PPPoE connections
over VDSL links as already supported by the VR9 ltq_ptm driver ([1], [2])
set the interface maximum MTU to MAX_MTU.

MAX_MTU is defined in dcdp/inc/tc_common.h to 2002 bytes and this value is
used in ptm_change_mtu() and elsewhere as the maximum MTU, however this is
short circuited by checks against the interface maximum MTU.

[1]: https://forum.openwrt.org/t/fritzbox-7530-and-rfc4638-baby-jumbo-frames/181327
[2]: https://git.openwrt.org/?p=openwrt/openwrt.git;a=commit;h=8a2a20e71e2909f84dab47e51dfda9e292a6c1ae

Signed-off-by: Andrew MacIntyre <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/16856
Signed-off-by: Robert Marko <[email protected]>
(cherry picked from commit ca53f2d430ce1f9ff5a560291de1e93380963417)

package/kernel/lantiq/vrx518_tc/patches/206-dcdp-ptm_tc-allow-larger-mtu-to-support-rfc4638.patch

@@ -0,0 +1,34 @@
+From 3983dc1674fec43beb8ce9d9bfdd6302fef86eae Mon Sep 17 00:00:00 2001
+From: Andrew MacIntyre <[email protected]>
+Date: Mon, 4 Nov 2024 22:41:25 +1100
+Subject: [PATCH] vrx518_tc: allow larger MTU to support RFC4638
+
+vrx518_tc currently sets the interface maximum MTU to the ethernet default
+of 1500 bytes by default via ether_setup() called from ptm_setup().
+
+To support 1508 byte baby jumbo frames (RFC4638) for PPPoE connections
+over VDSL links as already supported by the VR9 ltq_ptm driver ([1], [2])
+set the interface maximum MTU to MAX_MTU.
+
+MAX_MTU is defined in dcdp/inc/tc_common.h to 2002 bytes and this value is
+used in ptm_change_mtu() and elsewhere as the maximum MTU, however this is
+short circuited by checks against the interface maximum MTU.
+
+[1]: https://forum.openwrt.org/t/fritzbox-7530-and-rfc4638-baby-jumbo-frames/181327
+[2]: https://git.openwrt.org/?p=openwrt/openwrt.git;a=commit;h=8a2a20e71e2909f84dab47e51dfda9e292a6c1ae
+
+Signed-off-by: Andrew MacIntyre <[email protected]>
+---
+ dcdp/ptm_tc.c | 1 +
+ 1 file changed, 1 insertion(+)
+
+--- a/dcdp/ptm_tc.c	2023-12-17 16:11:22.503500398 +1100
++++ b/dcdp/ptm_tc.c	2024-01-26 16:48:49.881623474 +1100
+@@ -657,6 +657,7 @@ static void ptm_setup(struct net_device
+ 	ether_setup(dev);
+ 	dev->watchdog_timeo = 10 * HZ;
+ 	dev->netdev_ops = &g_ptm_netdev_ops;
++	dev->max_mtu = MAX_MTU;
+ }
+ 
+ static int ptm_dev_init(struct tc_priv *tc_priv, u32 id,