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

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Date:   Wed, 28 Sep 2016 11:01:10 -0700
From:   Eric Nelson <eric@...int.com>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
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

Thanks Russell,

On 09/28/2016 10:25 AM, Russell King - ARM Linux wrote:
> 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.
> 

I obviously mis-read this, and haven't dumped any packets to straighten
myself out.