Re: [PATCH v5 net-next 06/10] net: dsa: act as passthrough for bridge port flags

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

On 2/12/2021 7:15 AM, Vladimir Oltean wrote:
> From: Vladimir Oltean <vladimir.oltean@nxp.com>
> 
> There are multiple ways in which a PORT_BRIDGE_FLAGS attribute can be
> expressed by the bridge through switchdev, and not all of them can be
> emulated by DSA mid-layer API at the same time.
> 
> One possible configuration is when the bridge offloads the port flags
> using a mask that has a single bit set - therefore only one feature
> should change. However, DSA currently groups together unicast and
> multicast flooding in the .port_egress_floods method, which limits our
> options when we try to add support for turning off broadcast flooding:
> do we extend .port_egress_floods with a third parameter which b53 and
> mv88e6xxx will ignore? But that means that the DSA layer, which
> currently implements the PRE_BRIDGE_FLAGS attribute all by itself, will
> see that .port_egress_floods is implemented, and will report that all 3
> types of flooding are supported - not necessarily true.
> 
> Another configuration is when the user specifies more than one flag at
> the same time, in the same netlink message. If we were to create one
> individual function per offloadable bridge port flag, we would limit the
> expressiveness of the switch driver of refusing certain combinations of
> flag values. For example, a switch may not have an explicit knob for
> flooding of unknown multicast, just for flooding in general. In that
> case, the only correct thing to do is to allow changes to BR_FLOOD and
> BR_MCAST_FLOOD in tandem, and never allow mismatched values. But having
> a separate .port_set_unicast_flood and .port_set_multicast_flood would
> not allow the driver to possibly reject that.
> 
> Also, DSA doesn't consider it necessary to inform the driver that a
> SWITCHDEV_ATTR_ID_BRIDGE_MROUTER attribute was offloaded, because it
> just calls .port_egress_floods for the CPU port. When we'll add support
> for the plain SWITCHDEV_ATTR_ID_PORT_MROUTER, that will become a real
> problem because the flood settings will need to be held statefully in
> the DSA middle layer, otherwise changing the mrouter port attribute will
> impact the flooding attribute. And that's _assuming_ that the underlying
> hardware doesn't have anything else to do when a multicast router
> attaches to a port than flood unknown traffic to it.  If it does, there
> will need to be a dedicated .port_set_mrouter anyway.
> 
> So we need to let the DSA drivers see the exact form that the bridge
> passes this switchdev attribute in, otherwise we are standing in the
> way. Therefore we also need to use this form of language when
> communicating to the driver that it needs to configure its initial
> (before bridge join) and final (after bridge leave) port flags.
> 
> The b53 and mv88e6xxx drivers are converted to the passthrough API and
> their implementation of .port_egress_floods is split into two: a
> function that configures unicast flooding and another for multicast.
> The mv88e6xxx implementation is quite hairy, and it turns out that
> the implementations of unknown unicast flooding are actually the same
> for 6185 and for 6352:
> 
> behind the confusing names actually lie two individual bits:
> NO_UNKNOWN_MC -> FLOOD_UC = 0x4 = BIT(2)
> NO_UNKNOWN_UC -> FLOOD_MC = 0x8 = BIT(3)
> 
> so there was no reason to entangle them in the first place.
> 
> Whereas the 6185 writes to MV88E6185_PORT_CTL0_FORWARD_UNKNOWN of
> PORT_CTL0, which has the exact same bit index. I have left the
> implementations separate though, for the only reason that the names are
> different enough to confuse me, since I am not able to double-check with
> a user manual. The multicast flooding setting for 6185 is in a different
> register than for 6352 though.
> 
> Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
> ---
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
-- 
Florian