netdev - Re: [PATCH] net: fix unaligned memory accesses in ASIX

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Message-ID: <49CB3F6D.2010508@st.com>
Date:	Thu, 26 Mar 2009 09:40:13 +0100
From:	Giuseppe CAVALLARO <peppe.cavallaro@...com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH] net: fix unaligned memory accesses in ASIX

David Miller wrote:
> From: Giuseppe CAVALLARO <peppe.cavallaro@...com>
> Date: Thu, 26 Mar 2009 07:52:39 +0100
>
>   
>> Move in memory all the frames with an incorrect alignment.
>> This is to prevent unaligned memory accesses into the upper layers.
>> Note 1: this is a penalty for some architecture like SH.
>> Note 2: indeed, this patch restores an old one posted three years ago
>> (http://marc.info/?l=linux-usb-devel&m=116578791817830&w=2).
>>
>> Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@...com>
>>     
>
> There are some gratuitous (and wrong) changes in here:
>
>   
>> -	u32  header;
>>     
>  ...
>   
>> +	unsigned int header;
>>     
>
> It's a u32 whether you like it or not.  Please don't
> change away from fixed sized types.
>   
I agree... it's a typo error.
> Also isn't there a way we can prefixed what this packet
> offset is going to be?  If so, we can adjust the skb_reserve()
> call the generic USB net code uses.
>   
This was my first test. I had tried to change/adjust headroom but no
success.

Unfortunately, unaligned memory accesses seems to depend on the Asix HW
that packs several incoming frames.
So when these frames are 'unpacked' within the fix-up function, and
pushed to the upper layer, they can have a wrong alignment, indeed.
When no frame is packed all works fine and the IP never works with
unaligned addresses.
I think, the skb_reserve could actually help us, if this last scenario
generated misaligned accesses.
Please let me know if I'm missing something.

Thanks for your feedback!
Regards,
Peppe
> Thanks.
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