netdev - Re: [PATCH 0/3]: net: dsa: mt7530: support MT7530 in the MT7621 SoC

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Date:   Mon, 3 Dec 2018 16:47:01 +1000
From:   Greg Ungerer <gerg@...nel.org>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     sean.wang@...iatek.com, vivien.didelot@...oirfairelinux.com,
        f.fainelli@...il.com, netdev@...r.kernel.org, blogic@...nwrt.org,
        neil@...wn.name
Subject: Re: [PATCH 0/3]: net: dsa: mt7530: support MT7530 in the MT7621 SoC

Hi Andrew,

On 30/11/18 11:33 pm, Andrew Lunn wrote:
>> 2. Maximal sized RX packets get silently dropped. So receive side packets
>>     that are large (perfect case is the all-but-last packets in a fragemented
>>     larger packet) appear to be dropped at the mt7621 ethernet MAC level.
>>     The 7530 MIB switch register counters show receive packets at the physical
>>     switch port side and at the CPU switch port - but I get no packets
>>     received or errors in the 7621 ethernet MAC. If I set the mtu of the
>>     server at the other end a little smaller (a few bytes is enough) then
>>     I get all the packets through. It seems like the DSA/VLAN tag bytes
>>     are causing a too large packet to get silently dropped somewhere.
> 
> Hi Gerg
> 
> Try increasing the MTU on the master device. Some hardware will reject
> receiving packets bigger than the MTU. But if you increase the MTU by
> the DSA header size, it will then receive the frame.

I tried increasing it on the master, and it fails to set:

  # ifconfig eth0 mut 1500
  # ifconfig eth0 mtu 1504
  eth0: mtu greater than device maximum
  SIOCSIFMTU: Invalid argument

Looking within that staging driver it seems to generously set the size
of the RX descriptor buffers, so there is enough room in them. And glancing
around the driver I don't see it using the MTU to restrict the receive
packaet size (though I may be missing it).


> I have a patchset i will be posting soon to do this automatically.

That would be good.

Regards
Greg