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Date:	Wed, 8 Oct 2014 08:45:31 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Lothar Waßmann' <LW@...O-electronics.de>
CC:	'Eric Dumazet' <eric.dumazet@...il.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	"Russell King" <rmk+kernel@....linux.org.uk>,
	Frank Li <Frank.Li@...escale.com>,
	"Fabio Estevam" <fabio.estevam@...escale.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] net: fec: fix regression on i.MX28 introduced by
 rx_copybreak support

From: Lothar Waßmann
> David Laight wrote:
> > From: Eric Dumazet
> > > On Tue, 2014-10-07 at 15:19 +0200, Lothar Wamann wrote:
> > > > 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 192.168.1.225:111 to 192.168.100.73:44662 ulen 36
> > > >
> > > > Do the byte swapping and copying to the new skb in one go if
> > > > necessary.
> > > >
> > > > Signed-off-by: Lothar Wamann <LW@...O-electronics.de>
> > > > ---
> > > >  drivers/net/ethernet/freescale/fec_main.c |   25 +++++++++++++++++++++----
> > > >  1 file changed, 21 insertions(+), 4 deletions(-)
> > > >
> > > > diff --git a/drivers/net/ethernet/freescale/fec_main.c b/drivers/net/ethernet/freescale/fec_main.c
> > > > index 87975b5..eaaebad 100644
> > > > --- a/drivers/net/ethernet/freescale/fec_main.c
> > > > +++ b/drivers/net/ethernet/freescale/fec_main.c
> > > > @@ -339,6 +339,18 @@ static void *swap_buffer(void *bufaddr, int len)
> > > >  	return bufaddr;
> > > >  }
> > > >
> > > > +static void *swap_buffer2(void *dst_buf, void *src_buf, int len)
> > > > +{
> > > > +	int i;
> > > > +	unsigned int *src = src_buf;
> > > > +	unsigned int *dst = dst_buf;
> > > > +
> > > > +	for (i = 0; i < DIV_ROUND_UP(len, 4); i++, src++, dst++)
> > > > +		*dst = cpu_to_be32(*src);
> > >
> > > No need for the DIV :
> > >
> > > 	for (i = 0; i < len; i += sizeof(*dst), src++, dst++)
> > > 		*dst = cpu_to_be32(*src);
> > >
> > > Also are you sure both src/dst are aligned to word boundaries, or is
> > > this architecture OK with possible misalignment ?
> >
> > I wondered about that as well.
> > I wouldn't have expected ppc to support misaligned transfers, and you'd also
> > want to make sure that cpu_to_be(*src) was using a byte-swapping instruction.
> > Hmmm... cpu_to_be() doesn't sound like the right 'swap' macro name.
> >
> ??? So what is cpu_to_be32() then?
> The new swap function is an exact copy of the original one already in
> use except for the fact that it uses distinct source and destination
> buffers.

cpu_to_be32() is for converting a 'cpu' endianness value to 'big-endian'.
Here you are processing receive data - so you probably want be32_to_cpu().
(Yes, I know they are functionally identical....)

Alternatively, since these aren't actually 32bit numbers and you know
whether you want to swap, something like the __swab32p() from swab.h
- but I'm not entirely sure that one is expected to be used.

Clearly you are well inside the ppc 'endianness' hell.

	David

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