Re: [PATCH net-next] af_mpls: fix undefined reference to ip6_route_output

[David Miller <davem@davemloft.net>]

From: roopa <roopa@cumulusnetworks.com>
Date: Thu, 23 Jul 2015 06:44:13 -0700

> On 7/23/15, 12:09 AM, David Miller wrote:
>> From: roopa <roopa@cumulusnetworks.com>
>> Date: Wed, 22 Jul 2015 13:38:31 -0700
>>
>>> I cant think of a way to fix the current problem with my patch...
>> I guess it's not obvious that adding CONFIG_MPLS_IPV6 would solve
>> the problem perfectly.
> I thought that was not an option because CONFIG_MPLS_ROUTING is
> already out and supports IPV6 when RTA_OIF is
> specified.
> 
> I did submit another version yesterday using IS_BUILTIN (sorry missed
> the v2 in the patch prefix).
> https://patchwork.ozlabs.org/patch/498903/
> 
> I can certainly introduce CONFIG_MPLS_IPV6, if that is a more
> acceptable.

Also, IS_BUILTIN() is the wrong check, because this works properly also
when both MPLS_ROUTING and IPV6 are modular.

You need to encode this in the Kconfig dependencies of a new option
(which uses the dependency expression "IPV6 || IPV6=n"), then you can
key off of that Kconfig symbol to determine whether you need to
provide those NOP fallbacks or not.  There really is no other option.