Re: [PATCH net-next 1/3] net: dsa: Configure the MTU for switch ports

[Florian Fainelli <f.fainelli@gmail.com>]

Hi Vladimir,

On 11/23/2019 11:48 AM, Vladimir Oltean wrote:
> It is useful be able to configure port policers on a switch to accept
> frames of various sizes:
> 
> - Increase the MTU for better throughput from the default of 1500 if it
>   is known that there is no 10/100 Mbps device in the network.
> - Decrease the MTU to limit the latency of high-priority frames under
>   congestion.
> 
> For DSA slave ports, this is mostly a pass-through callback, called
> through the regular ndo ops and at probe time (to ensure consistency
> across all supported switches).
> 
> The CPU port is called with an MTU equal to the largest configured MTU
> of the slave ports. The assumption is that the user might want to
> sustain a bidirectional conversation with a partner over any switch
> port.
> 
> The DSA master is configured the same as the CPU port, plus the tagger
> overhead. Since the MTU is by definition L2 payload (sans Ethernet
> header), it is up to each individual driver to figure out if it needs to
> do anything special for its frame tags on the CPU port (it shouldn't
> except in special cases). So the MTU does not contain the tagger
> overhead on the CPU port.
> However the MTU of the DSA master, minus the tagger overhead, is used as
> a proxy for the MTU of the CPU port, which does not have a net device.
> This is to avoid uselessly calling the .change_mtu function on the CPU
> port when nothing should change.
> 
> So it is safe to assume that the DSA master and the CPU port MTUs are
> apart by exactly the tagger's overhead in bytes.
> 
> Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
> ---

[snip]
> +static int dsa_slave_change_mtu(struct net_device *dev, int new_mtu)
> +{
> +	struct net_device *master = dsa_slave_to_master(dev);
> +	struct dsa_slave_priv *p = netdev_priv(dev);
> +	struct dsa_switch *ds = p->dp->ds;
> +	struct dsa_port *cpu_dp;
> +	int port = p->dp->index;
> +	int max_mtu = 0;
> +	int cpu_mtu;
> +	int err, i;
> +
> +	if (!ds->ops->change_mtu)
> +		return -EOPNOTSUPP;
> +
> +	err = ds->ops->change_mtu(ds, port, new_mtu);
> +	if (err < 0)
> +		return err;
> +
> +	dev->mtu = new_mtu;
> +
> +	for (i = 0; i < ds->num_ports; i++) {
> +		if (!dsa_is_user_port(ds, i))
> +			continue;
> +
> +		/* During probe, this function will be called for each slave
> +		 * device, while not all of them have been allocated. That's
> +		 * ok, it doesn't change what the maximum is, so ignore it.
> +		 */
> +		if (!dsa_to_port(ds, i)->slave)
> +			continue;
> +
> +		if (max_mtu < dsa_to_port(ds, i)->slave->mtu)
> +			max_mtu = dsa_to_port(ds, i)->slave->mtu;
> +	}
> +
> +	cpu_dp = dsa_to_port(ds, port)->cpu_dp;
> +
> +	max_mtu += cpu_dp->tag_ops->overhead;
> +	cpu_mtu = master->mtu;
> +
> +	if (max_mtu != cpu_mtu) {
> +		err = ds->ops->change_mtu(ds, dsa_upstream_port(ds, port),
> +					  max_mtu - cpu_dp->tag_ops->overhead);
> +		if (err < 0)
> +			return err;

Before changing and committing the slave_dev's MTU you should actually
perform these two operations first to make sure that you can honor the
user port MTU that is requested. Here, you would possibly leave an user
port configured for a MTU value that is unsupported by the upstream
port(s) and/or the CPU port and/or the DSA master device, which could
possibly break frame forwarding depending on what the switch is willing
to accept.

I had prepared a patch series with Murali doing nearly the same thing
and targeting Broadcom switches nearly a year ago but since I never got
feedback whether this worked properly for the use case he was after, I
did not submit it since I did not need it personally and found it to be
a nice can of worms.

Another thing that I had not gotten around testing was making sure that
when a slave_dev gets enslaved as a bridge port member, that bridge MTU
normalization would kick in and make sure that if you have say: port 0
configured with MTU 1500 and port 1 configured with MTU 9000, the bridge
would normalize to MTU 1500 as you would expect.

https://github.com/ffainelli/linux/commits/dsa-mtu

This should be a DSA switch fabric notifier IMHO because changing the
MTU on an user port implies changing the MTU on every DSA port in
between plus the CPU port. Your approach here works for the first
upstream port, but not for the ones in between, and there can be more,
as is common with the ZII devel Rev. B and C boards.
-- 
Florian