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Date:	Wed, 8 Oct 2014 06:43:42 +0200
From:	Lothar Waßmann <>
To:	David Miller <>
Subject: Re: [PATCH] net: fec: fix regression on i.MX28 introduced by
 rx_copybreak support


David Miller wrote:
> From: Lothar Waßmann <>
> Date: Tue,  7 Oct 2014 15:19:37 +0200
> > commit 1b7bde6d659d ("net: fec: implement rx_copybreak to improve rx performance")
> > introduced a regression for i.MX28. The swap_buffer() function doing
> > the endian conversion of the received data on i.MX28 may access memory
> > beyond the actual packet size in the DMA buffer. fec_enet_copybreak()
> > does not copy those bytes, so that the last bytes of a packet may be
> > filled with invalid data after swapping.
> > This will likely lead to checksum errors on received packets.
> > E.g. when trying to mount an NFS rootfs:
> > UDP: bad checksum. From to ulen 36
> > 
> > Do the byte swapping and copying to the new skb in one go if
> > necessary.
> > 
> > Signed-off-by: Lothar Waßmann <>
> Why don't you just round up the length fec_enet_copybreak() uses when
> need_swap is true?  Then you will end up mimicking the original behavior
> and not require this new helper function.
I wanted to eliminate the need to access the buffer twice (once for
copying and once for swapping).

> And in any case I agree with Sergei that if you do retain your approach,
> the new 'swap' argument to fec_enet_copybreak() should be a 'bool'.

> I'm really surprised there isn't a control register bit to adjust the
> endianness of the data DMA'd to/from the network.
This is a "feature" of the i.MX28.

Lothar Waßmann

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