Re: [PATCH v4 net-next 02/11] net: dsa: mv88e6xxx: deny vid 0 on the CPU port and DSA links too

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

On 1/8/2021 4:01 PM, Vladimir Oltean wrote:
> From: Vladimir Oltean <vladimir.oltean@nxp.com>
> 
> mv88e6xxx apparently has a problem offloading VID 0, which the 8021q
> module tries to install as part of commit ad1afb003939 ("vlan_dev: VLAN
> 0 should be treated as "no vlan tag" (802.1p packet)"). That mv88e6xxx
> restriction seems to have been introduced by the "VTU GetNext VID-1
> trick to retrieve a single entry" - see commit 2fb5ef09de7c ("net: dsa:
> mv88e6xxx: extract single VLAN retrieval").
> 
> There is one more problem. The mv88e6xxx CPU port and DSA links do not
> report properly in the prepare phase what are the VLANs that they can
> offload. They'll say they can offload everything:
> 
> mv88e6xxx_port_vlan_prepare
> -> mv88e6xxx_port_check_hw_vlan:
> 
> 	/* DSA and CPU ports have to be members of multiple vlans */
> 	if (dsa_is_dsa_port(ds, port) || dsa_is_cpu_port(ds, port))
> 		return 0;
> 
> Except that if you actually try to commit to it, they'll error out and
> print this message:
> 
> [   32.802438] mv88e6085 d0032004.mdio-mii:12: p9: failed to add VLAN 0t
> 
> which comes from:
> 
> mv88e6xxx_port_vlan_add
> -> mv88e6xxx_port_vlan_join:
> 
> 	if (!vid)
> 		return -EOPNOTSUPP;
> 
> What prevents this condition from triggering in real life? The fact that
> when a DSA_NOTIFIER_VLAN_ADD is emitted, it never targets a DSA link
> directly. Instead, the notifier will always target either a user port or
> a CPU port. DSA links just happen to get dragged in by:
> 
> static bool dsa_switch_vlan_match(struct dsa_switch *ds, int port,
> 				  struct dsa_notifier_vlan_info *info)
> {
> 	...
> 	if (dsa_is_dsa_port(ds, port))
> 		return true;
> 	...
> }
> 
> So for every DSA VLAN notifier, during the prepare phase, it will just
> so happen that there will be somebody to say "no, don't do that".
> 
> This will become a problem when the switchdev prepare/commit transactional
> model goes away. Every port needs to think on its own. DSA links can no
> longer bluff and rely on the fact that the prepare phase will not go
> through to the end, because there will be no prepare phase any longer.
> 
> Fix this issue before it becomes a problem, by having the "vid == 0"
> check earlier than the check whether we are a CPU port / DSA link or not.
> Also, the "vid == 0" check becomes unnecessary in the .port_vlan_add
> callback, so we can remove it.
> 
> Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
-- 
Florian