netdev - Re: wireless vs. alignment requirements

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Message-ID: <20071127103940.314c7844@freepuppy.rosehill>
Date:	Tue, 27 Nov 2007 10:39:40 -0800
From:	Stephen Hemminger <shemminger@...ux-foundation.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Herbert Xu <herbert@...dor.apana.org.au>,
	Johannes Berg <johannes@...solutions.net>, dsd@...too.org,
	davem@...emloft.net, netdev@...r.kernel.org,
	linux-wireless@...r.kernel.org
Subject: Re: wireless vs. alignment requirements

On Tue, 27 Nov 2007 09:16:07 -0800
"H. Peter Anvin" <hpa@...or.com> wrote:

> Stephen Hemminger wrote:
> > Herbert Xu wrote:
> >> On Sat, Nov 24, 2007 at 02:49:36PM +0100, Johannes Berg wrote:
> >>  
> >>> Right. I just didn't think that would be a valid value for an
> >>> architecture to set.
> >>>     
> >>
> >> OK.  Let me clarify this a bit more.  We require at least one
> >> of the following rules to be met:
> >>
> >> * the IPv4/IPv6 header is aligned by 8 bytes on reception;
> >> * or the platform provides unaligned exception handlers.
> >>
> >> So if your platform violates both rules then it won't work with
> >> the IP stack, simple as that.  Fortunately I don't think such a
> >> platform exists currently on Linux.
> >>
> >> Cheers,
> >>   
> > 
> > Then what about hardware that can't dma ethernet to non-aligned address.
> > Sky2 hardware breaks if DMA is not 8 byte aligned.  IMHO the IP stack 
> > should handle any alignment, and do the appropriate memove if the CPU requires 
> > alignment.
> 
> I wrote a patch for the IP stack to realign packets if necessary at one 
> point.  I should dredge it up again and submit it for collective flamage.
> 
> 	-hpa
> 

Is there any standard kernel config define for "this platform can't do
unaligned accesses"?

-- 
Stephen Hemminger <shemminger@...ux-foundation.org>
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