netdev - Re: HSR: Standard breaks alignment. Solution?

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Message-ID: <1326789125.2564.54.camel@edumazet-laptop>
Date:	Tue, 17 Jan 2012 09:32:05 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Johannes Berg <johannes@...solutions.net>
Cc:	Arvid Brodin <arvid.brodin@...a.com>, netdev@...r.kernel.org
Subject: Re: HSR: Standard breaks alignment. Solution?

Le mardi 17 janvier 2012 à 09:04 +0100, Johannes Berg a écrit :
> On Tue, 2012-01-17 at 00:10 +0100, Arvid Brodin wrote:
> > As I've written before here, I'm trying to add support for the HSR protocol
> > ("High-availability Seamless Redundancy") to the linux kernel. The protocol is
> > specified in IEC-62439-3, and involves adding a protocol tag after the ethhdr
> > on outgoing frames, and stripping it again on reception, much like VLAN.
> > 
> > This HSR tag is 6 bytes long, which breaks 32-bit header alignment and causes
> > an Oops and a kernel panic in icmp_echo on the receiving side of pings (here,
> > exactly: http://lxr.linux.no/#linux+v2.6.37/net/ipv4/icmp.c#L838 )
> > 
> > If I add two bytes of padding to the HSR tag everything works beautifully. But
> > of course that breaks any pretense of standard compliance.
> > 
> > Is there some way to fix this without having to memmove the whole frame payload
> > 2 bytes on reception?
> 
> I don't think there's any other choice, but you can use
> CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS to see whether you actually need
> to do it.

Or test if NET_IP_ALIGN is 0, it might be more explicit.



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