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

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Date:	Wed, 11 Aug 2010 13:13:32 -0700
From:	Greg KH <greg@...ah.com>
To:	Martin Fuzzey <mfuzzey@...il.com>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	linux-usb <linux-usb@...r.kernel.org>, netdev@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: Problem with non aligned DMA in usbnet on ARM

On Wed, Aug 11, 2010 at 09:07:27PM +0200, Martin Fuzzey wrote:
> Greg KH wrote:
> >> So the question is are hcds expected to accept arbitarilly aligned but
> >> heap allocated pointers (such as the result of kmalloc() + 1)?
> >>     
> >
> > It sounds like your HCD doesn't like this, so perhaps we should make
> > that rule :)
> >
> > If you allocate the urb with a kmalloc() call with no offset, does it
> > all work properly? 
> Yes
> >  The driver should be calling usb_alloc_urb() which
> > does this automatically for them, right?  Or is it trying to allocate
> > things on its own somehow?
> >
> >   
> It's not the URB itself (which is allocated by usb_alloc_urb) but rather
> the buffer pointer within the URB that causes the problem.

Doh, you are right, sorry about that.

> It's the asix driver (or more exactly the usbnet core used by that driver).
> It does (rx_submit() in drivers/net/usb/usbnet.c):
> 
> urb = usb_alloc_urb();
> skb = alloc_skb (...);
> skb_reserve (skb, NET_IP_ALIGN);
> usb_fill_bulk_urb (urb,...  skb->data);
> usb_submit_urb(urb)
> 
> skb->data as returned by alloc_skb() is aligned
> but skb_reserve adds 2.
> 
> Thus removing the skb_reserve() call makes it work.
> BUT if I do that the IP header is no longer aligned so accesses further
> up the network stack have to be fixed up by exception handlers which is
> expensive (even with hcds which don't require this)

Can you fix this in the host controller driver?

thanks,

greg k-h
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