netdev - Re: [PATCH] net: ethernet: ti: cpsw: allow to configure min tx packet size

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Date:   Thu, 29 Nov 2018 13:05:47 +0000
From:   "Lad, Prabhakar" <prabhakar.csengg@...il.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Grygorii Strashko <grygorii.strashko@...com>,
        "David S. Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>, Sekhar Nori <nsekhar@...com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux OMAP Mailing List <linux-omap@...r.kernel.org>
Subject: Re: [PATCH] net: ethernet: ti: cpsw: allow to configure min tx packet size

Hi Andrew,

On Mon, Nov 26, 2018 at 2:27 AM Andrew Lunn <andrew@...n.ch> wrote:
>
> On Sun, Nov 25, 2018 at 05:43:15PM -0600, Grygorii Strashko wrote:
> > For proper VLAN packets forwarding CPSW driver uses min tx packet size of
> > 64bytes (VLAN_ETH_ZLEN, excluding ETH_FCS) which was corrected by
> > commit 9421c9015047 ("net: ethernet: ti: cpsw: fix min eth packet size").
> >
> > Unfortunately, this breaks some industrial automation protocols, as
> > reported by TI customers [1], which can work only with min TX packet size
> > from 60 byte (ecluding FCS).
>
> Hi Grygorii
>
> excluding...
>
> > Hence, introduce module boot parameter "tx_packet_min" to allow configure
> > min TX packet size at boot time.
>
> Module parameters are generally not liked.
>
> What actually happens here with this lower limit? Does the hardware
> send runt packets? Does the protocol actually require runt packets?
>
Yes it does send runt packets,  and also get Rx align errors. you can
find the ethtool dump at [1].

[1] https://e2e.ti.com/support/processors/f/791/t/719557?Linux-AM5728-EtherCAT-packets-dropped

Cheers,
--Prabhakar Lad