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

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Date:	Tue, 17 Jan 2012 09:04:41 +0100
From:	Johannes Berg <johannes@...solutions.net>
To:	Arvid Brodin <arvid.brodin@...a.com>
Cc:	netdev@...r.kernel.org
Subject: Re: HSR: Standard breaks alignment. Solution?

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.

johannes

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