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

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Message-ID: <20160928172530.GM1041@n2100.armlinux.org.uk>
Date:   Wed, 28 Sep 2016 18:25:30 +0100
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Eric Nelson <eric@...int.com>
Cc:     David Laight <David.Laight@...LAB.COM>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "andrew@...n.ch" <andrew@...n.ch>,
        "fugang.duan@....com" <fugang.duan@....com>,
        "otavio@...ystems.com.br" <otavio@...ystems.com.br>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "troy.kisky@...ndarydevices.com" <troy.kisky@...ndarydevices.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "u.kleine-koenig@...gutronix.de" <u.kleine-koenig@...gutronix.de>
Subject: Re: [PATCH 3/3] net: fec: align IP header in hardware

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.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up
according to speedtest.net.