Re: dsa: sja1105: regression after patch: "net: dsa: configure the MTU for switch ports"

[Andrew Lunn]

> > The code which is causing problems seems to be this one:
> > 
> >     mtu_limit = min_t(int, master->max_mtu, dev->max_mtu);
> >     old_master_mtu = master->mtu;
> >     new_master_mtu = largest_mtu + cpu_dp->tag_ops->overhead;
> >     if (new_master_mtu > mtu_limit)
> >         return -ERANGE;
> > 
> > called from
> > 
> >     rtnl_lock();
> >     ret = dsa_slave_change_mtu(slave_dev, ETH_DATA_LEN);
> >     rtnl_unlock();
> >     if (ret && ret != -EOPNOTSUPP) {
> >         dev_err(ds->dev, "error %d setting MTU on port %d\n",
> >             ret, port->index);
> >         goto out_free;
> >     }
> > 
> > Before this patch, it was silently failing, now it's preventing the
> > probing of the ports which I might agree with you is not better.
> > Andrew warned about this, and I guess that during probe, we should
> > warn but ignore any nonzero return code, not just EOPNOTSUPP. I'll
> > send a patch out shortly to correct this.
> > 
> > Out of curiosity, what DSA master port do you have? Does it not
> > support an MTU of 1504 bytes? Does MTU-sized traffic pass correctly
> > through your interface? (you can test with iperf3)
> 
> It is FEC@iMX6QP attached to the port 4 of the sja1105 switch.
> I'll try to make some tests tomorrow.

Ah, interesting. I've been testing recently on a Vybrid, so also
FEC. I had the warning, but it kept going.

I don't particularly like this warning in this case. We have hardware
which happy works, but is now issuing a warning on boot. I would
prefer if it warned when only trying to configure an MTU bigger than
the minimum needed for DSA, i.e. only the jumbo use case.

    Andrew