linux-kernel - Re: [PATCH net-next] net: lan966x: Fix when CONFIG

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Date:   Thu, 10 Feb 2022 14:14:27 +0100
From:   Andrew Lunn <andrew@...n.ch>
To:     Horatiu Vultur <horatiu.vultur@...rochip.com>
Cc:     Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, UNGLinuxDriver@...rochip.com,
        davem@...emloft.net, kernel test robot <lkp@...el.com>
Subject: Re: [PATCH net-next] net: lan966x: Fix when CONFIG_IPV6 is not set

> 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