Make "netif_set_default" optional

[kripton]

I'm currently trying to port https://github.com/OpenLightingProject/rp2040-dmxsun to the Pico-W. of course with the plan to enable the WiFi interface as well.
At the moment, lwIP is already in use with one network interface (TinyUSB's CDC NCM code to appear to the USB host as an USB ethernet interface). I already copied over pico-sdk's pico_cyw43_arch (https://github.com/kripton/rp2040-dmxsun/tree/4f3e76310f3861f8f7e384d4e3e6091997211c83/lib/pico_cyw43_arch_nolwip) since that unconditionally re-initialized the lwIP-stack (https://github.com/raspberrypi/pico-sdk/blob/5e9a5e827b3265fca53529883abdcc87608abad5/src/rp2_common/pico_cyw43_arch/cyw43_arch_poll.c#L59). Which is okay if the cyw43 is the only network interface on a board.
However, even after that, when using the cyw43 as AP (easiest to test for now), the DHCP server on the USB ethernet-interface stopped working.
If I comment out line

cyw43-driver/src/cyw43_lwip.c

Line 206 in 83e0c70

netif_set_default(n);

, the DHCP server works again. It would be nice if that line was guarded by an #ifdef so users of this driver can use it while having another netif as default.

I can provide a PR for that in the coming days.

[peterharperuk]

Which dhcp server has the problem? It sounds like an issue with the dhcp server implementation in that it's just using the default netif. I had to fix a similar issue recently bluekitchen/btstack@88cd673

All these dhcp servers look suspiciously similar.

[kripton]

Hi, indeed the code of the DHCP server I use is derived from that code base. It already had the detection, which netif the packet came in integrated but it missed the "send on the specific interface"-part. With your changes, the DHCP server works even if cyw43-driver sets the default interface. I can work with that, thanks for the explanations.
I will now re-work the DHCP server so it can actually serve different IP ranges on different interfaces :)

So if you think setting the default interface in any case, just close this issue. In my opinion it wouldn't hurt making it optional, but I can still re-set another interface as default at the end of the init code ;)

UPDATE: Some typos which changed the meaning in the previous versions have been corrected

[peterharperuk]

it wouldn't hurt making it optional

True

[dpgeorge]

I agree these things (like setting the default netif) should be configurable. In fact we plan to rework the whole cyw43_cb_tcpip_init() function so that it can be completely replaced by the user if needed.

So for now please work around the issue by resetting the default netif after this cyw43 driver has started up.

[peterharperuk]

we plan to rework the whole cyw43_cb_tcpip_init() function so that it can be completely replaced

That would be good. It might be better if there was no lwip in the driver at all.