netdev - Re: [PATCH net-next 2/3] net: dsa: Allow default tag protocol to be overridden from DT

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Date:   Mon, 29 Mar 2021 10:15:59 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Tobias Waldekranz <tobias@...dekranz.com>, davem@...emloft.net,
        kuba@...nel.org, vivien.didelot@...il.com, f.fainelli@...il.com,
        netdev@...r.kernel.org, robh+dt@...nel.org,
        devicetree@...r.kernel.org
Subject: Re: [PATCH net-next 2/3] net: dsa: Allow default tag protocol to be
 overridden from DT

On Mon, Mar 29, 2021 at 12:04:39AM +0200, Andrew Lunn wrote:
> On Mon, Mar 29, 2021 at 12:53:09AM +0300, Vladimir Oltean wrote:
> > On Sun, Mar 28, 2021 at 05:52:43PM +0200, Andrew Lunn wrote:
> > > > +static int dsa_switch_setup_tag_protocol(struct dsa_switch *ds)
> > > > +{
> > > > +	const struct dsa_device_ops *tag_ops = ds->dst->tag_ops;
> > > > +	struct dsa_switch_tree *dst = ds->dst;
> > > > +	int port, err;
> > > > +
> > > > +	if (tag_ops->proto == dst->default_proto)
> > > > +		return 0;
> > > > +
> > > > +	if (!ds->ops->change_tag_protocol) {
> > > > +		dev_err(ds->dev, "Tag protocol cannot be modified\n");
> > > > +		return -EINVAL;
> > > > +	}
> > > > +
> > > > +	for (port = 0; port < ds->num_ports; port++) {
> > > > +		if (!(dsa_is_dsa_port(ds, port) || dsa_is_cpu_port(ds, port)))
> > > > +			continue;
> > >
> > > dsa_is_dsa_port() is interesting. Do we care about the tagging
> > > protocol on DSA ports? We never see that traffic?
> >
> > I believe this comes from me (see dsa_switch_tag_proto_match). I did not
> > take into consideration at that time the fact that Marvell switches can
> > translate between DSA and EDSA. So I assumed that every switch in the
> > tree needs a notification about the tagging protocol, not just the
> > top-most one.
>
> Hi Vladimir
>
> static int mv88e6xxx_setup_port_mode(struct mv88e6xxx_chip *chip, int port)
> {
>         if (dsa_is_dsa_port(chip->ds, port))
>                 return mv88e6xxx_set_port_mode_dsa(chip, port);
>
> So DSA ports, the ports connecting two switches together, are hard
> coded to use DSA.
>
>         if (dsa_is_user_port(chip->ds, port))
>                 return mv88e6xxx_set_port_mode_normal(chip, port);
>
>         /* Setup CPU port mode depending on its supported tag format */
>         if (chip->info->tag_protocol == DSA_TAG_PROTO_DSA)
>                 return mv88e6xxx_set_port_mode_dsa(chip, port);
>
>         if (chip->info->tag_protocol == DSA_TAG_PROTO_EDSA)
>                 return mv88e6xxx_set_port_mode_edsa(chip, port);
>
> CPU ports can be configured to DSA or EDSA.
>
> The switches seem happy to translate between DSA and EDSA as needed.

I agree, and this is a particular feature of Marvell, not something that
can be said in general. However, I have nothing against deleting the
dsa_is_dsa_port checks from dsa_switch_tag_proto_match and from Tobias'
patch, since nobody needs them at the moment.