When CONFIG_IPV6 is not set, then the compilation of the lan966x driver
fails with the following error:

drivers/net/ethernet/microchip/lan966x/lan966x_main.c:444: undefined
reference to `ipv6_mc_check_mld'

The fix consists in adding #ifdef around this code.

Fixes: 47aeea0d57e80c ("net: lan966x: Implement the callback SWITCHDEV_ATTR_ID_BRIDGE_MC_DISABLED")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
---
 drivers/net/ethernet/microchip/lan966x/lan966x_main.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/net/ethernet/microchip/lan966x/lan966x_main.c b/drivers/net/ethernet/microchip/lan966x/lan966x_main.c
index d62484f14564..526dc41e98f8 100644
--- a/drivers/net/ethernet/microchip/lan966x/lan966x_main.c
+++ b/drivers/net/ethernet/microchip/lan966x/lan966x_main.c
@@ -439,10 +439,12 @@ static bool lan966x_hw_offload(struct lan966x *lan966x, u32 port,
 	    ip_hdr(skb)->protocol == IPPROTO_IGMP)
 		return false;
 
+#if IS_ENABLED(CONFIG_IPV6)
 	if (skb->protocol == htons(ETH_P_IPV6) &&
 	    ipv6_addr_is_multicast(&ipv6_hdr(skb)->daddr) &&
 	    !ipv6_mc_check_mld(skb))
 		return false;
+#endif
 
 	return true;
 }
-- 
2.33.0

On Wed, Feb 09, 2022 at 11:18:23AM +0100, Horatiu Vultur wrote:
> When CONFIG_IPV6 is not set, then the compilation of the lan966x driver
> fails with the following error:
> 
> drivers/net/ethernet/microchip/lan966x/lan966x_main.c:444: undefined
> reference to `ipv6_mc_check_mld'
> 
> The fix consists in adding #ifdef around this code.

It might be better to add a stub function for when IPv6 is
disabled. We try to avoid #if in C code.

	  Andrew

On Wed, 9 Feb 2022 14:54:23 +0100 Andrew Lunn wrote:
> On Wed, Feb 09, 2022 at 11:18:23AM +0100, Horatiu Vultur wrote:
> > When CONFIG_IPV6 is not set, then the compilation of the lan966x driver

compilation or linking?

> > fails with the following error:
> > 
> > drivers/net/ethernet/microchip/lan966x/lan966x_main.c:444: undefined
> > reference to `ipv6_mc_check_mld'
> > 
> > The fix consists in adding #ifdef around this code.  
> 
> It might be better to add a stub function for when IPv6 is
> disabled. We try to avoid #if in C code.

If it's linking we can do:

 	if (IS_ENABLED(CONFIG_IPV6) &&
	    skb->protocol == htons(ETH_P_IPV6) &&
 	    ipv6_addr_is_multicast(&ipv6_hdr(skb)->daddr) &&
 	    !ipv6_mc_check_mld(skb))
 		return false;

But beware that IPV6 can be a module, you may need a Kconfig dependency.

The 02/09/2022 18:06, Jakub Kicinski wrote:

Hi Andrew, Jakub
> 
> On Wed, 9 Feb 2022 14:54:23 +0100 Andrew Lunn wrote:
> > On Wed, Feb 09, 2022 at 11:18:23AM +0100, Horatiu Vultur wrote:
> > > When CONFIG_IPV6 is not set, then the compilation of the lan966x driver
> 
> compilation or linking?

It is a linking error. I will fix in the next version

> 
> > > fails with the following error:
> > >
> > > drivers/net/ethernet/microchip/lan966x/lan966x_main.c:444: undefined
> > > reference to `ipv6_mc_check_mld'
> > >
> > > The fix consists in adding #ifdef around this code.
> >
> > It might be better to add a stub function for when IPv6 is
> > disabled. We try to avoid #if in C code.

What do you think if I do something like this in the lan966x_main.h

---
#if IS_ENABLED(CONFIG_IPV6)
static inline bool lan966x_hw_offload_ipv6(struct sk_buff *skb)
{
	if (skb->protocol == htons(ETH_P_IPV6) &&
	    ipv6_addr_is_multicast(&ipv6_hdr(skb)->daddr) &&
	    !ipv6_mc_check_mld(skb))
		return false;

	return true;
}
#else
static inline bool lan966x_hw_offload_ipv6(struct sk_buff *skb)
{
	return false;
}
#endif
---

And then in lan966x_main.c just call this function.

> 
> If it's linking we can do:
> 
>         if (IS_ENABLED(CONFIG_IPV6) &&
>             skb->protocol == htons(ETH_P_IPV6) &&
>             ipv6_addr_is_multicast(&ipv6_hdr(skb)->daddr) &&
>             !ipv6_mc_check_mld(skb))
>                 return false;
> 
> But beware that IPV6 can be a module, you may need a Kconfig dependency.

I was also looking at other drivers on how they use 'ipv6_mc_check_mld'.
Then I have seen that drivers/net/amt.c and net/bridge/br_multicast.c
they wrap this function with #if.
But then there is net/batman-adv/multicast.c which doesn't do that and
it can compile and link without CONFIG_IPV6 and I just don't see how
that is working.

-- 
/Horatiu

> What do you think if I do something like this in the lan966x_main.h
> 
> ---
> #if IS_ENABLED(CONFIG_IPV6)
> static inline bool lan966x_hw_offload_ipv6(struct sk_buff *skb)
> {
> 	if (skb->protocol == htons(ETH_P_IPV6) &&
> 	    ipv6_addr_is_multicast(&ipv6_hdr(skb)->daddr) &&
> 	    !ipv6_mc_check_mld(skb))
> 		return false;
> 
> 	return true;
> }
> #else
> static inline bool lan966x_hw_offload_ipv6(struct sk_buff *skb)
> {
> 	return false;
> }
> #endif
> ---

The reason we prefer not to use #if is that it reduced compile testing
coverage. The block of code inside gets compiled a lot less.

> And then in lan966x_main.c just call this function.
> 
> > 
> > If it's linking we can do:
> > 
> >         if (IS_ENABLED(CONFIG_IPV6) &&
> >             skb->protocol == htons(ETH_P_IPV6) &&
> >             ipv6_addr_is_multicast(&ipv6_hdr(skb)->daddr) &&
> >             !ipv6_mc_check_mld(skb))
> >                 return false;

Jakub solution results in the code always being compiled, but the
IS_ENABLED(CONFIG_IPV6) gets turned into a constant 0 or 1. The
optimizer can then remove the whole block of code in the 0 case.

> I was also looking at other drivers on how they use 'ipv6_mc_check_mld'.
> Then I have seen that drivers/net/amt.c and net/bridge/br_multicast.c
> they wrap this function with #if.
> But then there is net/batman-adv/multicast.c which doesn't do that and
> it can compile and link without CONFIG_IPV6 and I just don't see how
> that is working.

Maybe it is to do with this at the end of net/ip6/Makefile

ifneq ($(CONFIG_IPV6),)
obj-$(CONFIG_NET_UDP_TUNNEL) += ip6_udp_tunnel.o
obj-y += mcast_snoop.o
endif