netdev - Re: wireless vs. alignment requirements

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Message-ID: <474C50D7.5010901@zytor.com>
Date:	Tue, 27 Nov 2007 09:16:07 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Stephen Hemminger <shemminger@...ux-foundation.org>
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

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

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