Re: [PATCH 3/3] net: fec: align IP header in hardware

[Russell King - ARM Linux <linux@armlinux.org.uk>]

On Wed, Sep 28, 2016 at 10:14:52AM -0700, Eric Nelson wrote:
> Thanks David,
> 
> On 09/28/2016 09:42 AM, David Laight wrote:
> > From reading this it seems that the effect of FEC_RACC_SHIFT16 is to
> > add two bytes of 'junk' to the start of every receive frame.
> 
> That's right. Two bytes of junk between the MAC header and the
> IP header.

That's wrong.  FEC_RACC_SHIFT16 adds two bytes to the _beginning_ of
the packet, not in the middle of the packet:

   7      RX FIFO Shift-16
 SHIFT16
          When this field is set, the actual frame data starts at bit 16
          of the first word read from the RX FIFO aligning the Ethernet
          payload on a 32-bit boundary.

          NOTE: This function only affects the FIFO storage and has no
                influence on the statistics, which use the actual length
                of the frame received.

          0    Disabled.
          1    Instructs the MAC to write two additional bytes in front
               of each frame received into the RX FIFO.

*in front* of the frame - that's before the Ethernet header.  Not between
the ethernet and IP headers.

-- 
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