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

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Date:   Fri, 10 Nov 2017 02:06:25 +0100
From:   Andrew Lunn <andrew@...n.ch>
To:     Florian Fainelli <f.fainelli@...il.com>
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 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.

   Andrew