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Subject: Re: [PATCH net v1] Revert "lan743x: trim all 4 bytes of the FCS; not
 just 2"
To:     George McCollister <george.mccollister@gmail.com>,
        Sven Van Asbroeck <thesven73@gmail.com>
Cc:     Bryan Whitehead <bryan.whitehead@microchip.com>,
        David S Miller <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Microchip Linux Driver Support <UNGLinuxDriver@microchip.com>,
        netdev <netdev@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>
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From:   Heiner Kallweit <hkallweit1@gmail.com>
Message-ID: <820ed30b-90f4-2cba-7197-6c6136d2e04e@gmail.com>
Date:   Thu, 8 Apr 2021 20:22:34 +0200
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On 08.04.2021 20:00, George McCollister wrote:
> On Thu, Apr 8, 2021 at 12:46 PM Sven Van Asbroeck <thesven73@gmail.com> wrote:
>>
>> Hi George,
>>
>> On Thu, Apr 8, 2021 at 1:36 PM George McCollister
>> <george.mccollister@gmail.com> wrote:
>>>
>>> Can you explain the difference in behavior with what I was observing
>>> on the LAN7431?
>>
>> I'm not using DSA in my application, so I cannot test or replicate
>> what you were observing. It would be great if we could work together
>> and settle on a solution that is acceptable to both of us.
> 
> Sounds good.
> 
>>
>>> I'll retest but if this is reverted I'm going to start
>>> seeing 2 extra bytes on the end of frames and it's going to break DSA
>>> with the LAN7431 again.
>>>
>>
>> Seen from my point of view, your patch is a regression. But perhaps my
>> patch set is a regression for you? Catch 22...
>>
>> Would you be able to identify which patch broke your DSA behaviour?
>> Was it one of mine? Perhaps we can start from there.
> 
> Yes, first I'm going to confirm that what is in the net branch still
> works (unlikely but perhaps something else could have broken it since
> last I tried it).
> Then I'll confirm the patch which I believe broke it actually did and
> report back.
> 
>>
>> Sven

Just an idea:
RX_HEAD_PADDING is an alias for NET_IP_ALIGN that can have two values:
0 and 2
The two systems you use may have different NET_IP_ALIGN values.
This could explain the behavior. Then what I proposed should work
for both of you: frame_length - ETH_FCS_LEN