netdev - Re: wireless vs. alignment requirements

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Date:	Sat, 24 Nov 2007 09:33:36 +0100
From:	Johannes Berg <johannes@...solutions.net>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	dsd@...too.org, davem@...emloft.net, netdev@...r.kernel.org,
	linux-wireless@...r.kernel.org
Subject: Re: wireless vs. alignment requirements


> > Now, the IP stack actually assumes that its header is four-byte aligned
> > (see comment at NET_IP_ALIGN, although it is not said explicitly that
> > the alignment requirement for an IP header is four) so that is actually
> > something for the hardware/firmware (!) to do, for example Broadcom
> 
> Good point.  In fact IIRC we've always had the policy that drivers
> should do their best to generate aligned packets but it is not a
> requirement since on some platforms it's more important for the DMA
> to be aligned.

We still require four-byte alignment, no?

> So it's up the platform code to fix up any exceptions should they
> show up.
> 
> Daniel, what's the specific case that you had in mind with this
> patch?

Well. This goes back to a user reporting unaligned accesses on sparc64.
Davem thought this came from the ether addr comparisons but the user
later reported that the patch from davem didn't fix it, and I think
Daniel just made a sweep over all ether addr comparisons replacing them
with unaligned ones.

johannes

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