netdev - Re: Problem with non aligned DMA in usbnet on ARM

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Date:	Wed, 11 Aug 2010 11:59:21 +0200
From:	Matthieu CASTET <matthieu.castet@...rot.com>
To:	Martin Fuzzey <mfuzzey@...il.com>
CC:	linux-usb <linux-usb@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: Problem with non aligned DMA in usbnet on ARM

Hi,

Martin Fuzzey a écrit :
> Hi all,

> So what is the correct fix for this problem?
> The options seem to be:
> 1) Remove skb_reserve() from usbnet
> But this will cause the ip header to be non aligned on platforms that
> don't have the DMA alignment requirement.
And cause lot's of unaligned access for the IP stack. This will slow 
down arm platform...

> 
> 2) Define NET_IP_ALIGN to 0 for ARM
> Seems a bit intrusive to me.
Again this will generate lot's of unaligned access.
> 
> 3) Change the HCD to copy if not aligned
> I'd like to avoid that.
> 
> 4) Change usbnet to use seperate buffers for the usb side and skb.
> Copy between them so both remain aligned
> Seems unlikely that the gain further up the stack from ensuring the ip
> header is aligned will offset the cost of the copy.
> 
> Any ideas?
I got the same problem on the gadget side : 
http://article.gmane.org/gmane.linux.usb.general/28700

A solution could be the hcd driver to tell it doesn't support unaligned 
transfert and in this case the usb stack/usb driver align it.

It is best to solve in usb driver, because for example in Asix case the 
driver already do copy (see asix_rx_fixup/asix_tx_fixup).


Matthieu

PS : what hcd driver do you use ?
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