netdev - Re: [PATCH net-next 1/3] net: bgmac: Pad packets to a minimum size

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Message-ID: <b526788d-7bcd-8ac5-d23f-538a580f3314@gmail.com>
Date:   Thu, 9 Nov 2017 17:25:11 -0800
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     netdev@...r.kernel.org, davem@...emloft.net,
        vivien.didelot@...oirfairelinux.com
Subject: Re: [PATCH net-next 1/3] net: bgmac: Pad packets to a minimum size

On 11/09/2017 05:06 PM, Andrew Lunn wrote:
> On Thu, Nov 09, 2017 at 03:03:16PM -0800, Florian Fainelli wrote:
>> On 11/09/2017 02:37 PM, Andrew Lunn wrote:
>>> On Thu, Nov 09, 2017 at 02:26:04PM -0800, Florian Fainelli wrote:
>>>> In preparation for enabling Broadcom tags with b53, pad packets to a
>>>> minimum size of 64 bytes (sans FCS) in order for the Broadcom switch to
>>>> accept ingressing frames. Without this, we would typically be able to
>>>> DHCP, but not resolve with ARP because packets are too small and get
>>>> rejected by the switch.
>>>
>>> Hi Florian
>>>
>>> Is the MAC sending runt packets in its default configuration? Is this
>>> a general issue, and not just an issue when there is a switch directly
>>> attached?
>>
>> The MAC is sending 64 bytes (with FCS) padded packets by default, but
>> this apparently gets mis-calculated when Broadcom tags are enabled, such
>> that we need to pad before to avoid that.
> 
> Hi Florian
> 
> Ah, so maybe when the tag is stripped off it then becomes a runt
> packet and so gets dropped.

Yes, that is exactly what I observed.
-- 
Florian