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Date:   Fri, 11 Mar 2022 02:22:35 +0200
From:   Vladimir Oltean <olteanv@gmail.com>
To:     Tobias Waldekranz <tobias@waldekranz.com>
Cc:     davem@davemloft.net, kuba@kernel.org, Andrew Lunn <andrew@lunn.ch>,
        Vivien Didelot <vivien.didelot@gmail.com>,
        Florian Fainelli <f.fainelli@gmail.com>,
        Jiri Pirko <jiri@resnulli.us>,
        Ivan Vecera <ivecera@redhat.com>,
        Roopa Prabhu <roopa@nvidia.com>,
        Nikolay Aleksandrov <razor@blackwall.org>,
        Russell King <linux@armlinux.org.uk>,
        Petr Machata <petrm@nvidia.com>,
        Cooper Lees <me@cooperlees.com>,
        Ido Schimmel <idosch@nvidia.com>,
        Matt Johnston <matt@codeconstruct.com.au>,
        linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
        bridge@lists.linux-foundation.org
Subject: Re: [PATCH v2 net-next 07/10] net: dsa: Pass MST state changes to
 driver
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On Fri, Mar 11, 2022 at 12:59:54AM +0100, Tobias Waldekranz wrote:
> On Fri, Mar 11, 2022 at 01:08, Vladimir Oltean <olteanv@gmail.com> wrote:
> > On Thu, Mar 10, 2022 at 11:46:45PM +0100, Tobias Waldekranz wrote:
> >> On Thu, Mar 10, 2022 at 18:18, Vladimir Oltean <olteanv@gmail.com> wrote:
> >> > On Thu, Mar 10, 2022 at 05:05:35PM +0100, Tobias Waldekranz wrote:
> >> >> On Thu, Mar 10, 2022 at 12:35, Vladimir Oltean <olteanv@gmail.com> wrote:
> >> >> > On Thu, Mar 10, 2022 at 09:54:34AM +0100, Tobias Waldekranz wrote:
> >> >> >> >> +	if (!dsa_port_can_configure_learning(dp) || dp->learning) {
> >> >> >> >> +		switch (state->state) {
> >> >> >> >> +		case BR_STATE_DISABLED:
> >> >> >> >> +		case BR_STATE_BLOCKING:
> >> >> >> >> +		case BR_STATE_LISTENING:
> >> >> >> >> +			/* Ideally we would only fast age entries
> >> >> >> >> +			 * belonging to VLANs controlled by this
> >> >> >> >> +			 * MST.
> >> >> >> >> +			 */
> >> >> >> >> +			dsa_port_fast_age(dp);
> >> >> >> >
> >> >> >> > Does mv88e6xxx support this? If it does, you might just as well
> >> >> >> > introduce another variant of ds->ops->port_fast_age() for an msti.
> >> >> >> 
> >> >> >> You can limit ATU operations to a particular FID. So the way I see it we
> >> >> >> could either have:
> >> >> >> 
> >> >> >> int (*port_vlan_fast_age)(struct dsa_switch *ds, int port, u16 vid)
> >> >> >> 
> >> >> >> + Maybe more generic. You could imagine there being a way to trigger
> >> >> >>   this operation from userspace for example.
> >> >> >> - We would have to keep the VLAN<->MSTI mapping in the DSA layer in
> >> >> >>   order to be able to do the fan-out in dsa_port_set_mst_state.
> >> >> >> 
> >> >> >> or:
> >> >> >> 
> >> >> >> int (*port_msti_fast_age)(struct dsa_switch *ds, int port, u16 msti)
> >> >> >> 
> >> >> >> + Let's the mapping be an internal affair in the driver.
> >> >> >> - Perhaps, less generically useful.
> >> >> >> 
> >> >> >> Which one do you prefer? Or is there a hidden third option? :)
> >> >> >
> >> >> > Yes, I was thinking of "port_msti_fast_age". I don't see a cheap way of
> >> >> > keeping VLAN to MSTI associations in the DSA layer. Only if we could
> >> >> > retrieve this mapping from the bridge layer - maybe with something
> >> >> > analogous to br_vlan_get_info(), but br_mst_get_info(), and this gets
> >> >> > passed a VLAN_N_VID sized bitmap, which the bridge populates with ones
> >> >> > and zeroes.
> >> >> 
> >> >> That can easily be done. Given that, should we go for port_vlan_fast_age
> >> >> instead? port_msti_fast_age feels like an awkward interface, since I
> >> >> don't think there is any hardware out there that can actually perform
> >> >> that operation without internally fanning it out over all affected VIDs
> >> >> (or FIDs in the case of mv88e6xxx).
> >> >
> >> > Yup, yup. My previous email was all over the place with regard to the
> >> > available options, because I wrote it in multiple phases so it wasn't
> >> > chronologically ordered top-to-bottom. But port_vlan_fast_age() makes
> >> > the most sense if you can implement br_mst_get_info(). Same goes for
> >> > dsa_port_notify_bridge_fdb_flush().
> >> >
> >> >> > The reason why I asked for this is because I'm not sure of the
> >> >> > implications of flushing the entire FDB of the port for a single MSTP
> >> >> > state change. It would trigger temporary useless flooding in other MSTIs
> >> >> > at the very least. There isn't any backwards compatibility concern to
> >> >> > speak of, so we can at least try from the beginning to limit the
> >> >> > flushing to the required VLANs.
> >> >> 
> >> >> Aside from the performance implications of flows being temporarily
> >> >> flooded I don't think there are any.
> >> >> 
> >> >> I suppose if you've disabled flooding of unknown unicast on that port,
> >> >> you would loose the flow until you see some return traffic (or when one
> >> >> side gives up and ARPs). While somewhat esoteric, it would be nice to
> >> >> handle this case if the hardware supports it.
> >> >
> >> > If by "handle this case" you mean "flush only the affected VLANs", then
> >> > yes, I fully agree.
> >> >
> >> >> > What I didn't think about, and will be a problem, is
> >> >> > dsa_port_notify_bridge_fdb_flush() - we don't know the vid to flush.
> >> >> > The easy way out here would be to export dsa_port_notify_bridge_fdb_flush(),
> >> >> > add a "vid" argument to it, and let drivers call it. Thoughts?
> >> >> 
> >> >> To me, this seems to be another argument in favor of
> >> >> port_vlan_fast_age. That way you would know the VIDs being flushed at
> >> >> the DSA layer, and driver writers needn't concern themselves with having
> >> >> to remember to generate the proper notifications back to the bridge.
> >> >
> >> > See above.
> >> >
> >> >> > Alternatively, if you think that cross-flushing FDBs of multiple MSTIs
> >> >> > isn't a real problem, I suppose we could keep the "port_fast_age" method.
> >> >> 
> >> >> What about falling back to it if the driver doesn't support per-VLAN
> >> >> flushing? Flushing all entries will work in most cases, at the cost of
> >> >> some temporary flooding. Seems more useful than refusing the offload
> >> >> completely.
> >> >
> >> > So here's what I don't understand. Do you expect a driver other than
> >> > mv88e6xxx to do something remotely reasonable under a bridge with MSTP
> >> > enabled? The idea being to handle gracefully the case where a port is
> >> > BLOCKING in an MSTI but FORWARDING in another. Because if not, let's
> >> > just outright not offload that kind of bridge, and only concern
> >> > ourselves with what MST-capable drivers can do.
> >> 
> >> I think you're right. I was trying to make it easier for other driver
> >> writers, but it will just be more confusing and error prone.
> >> 
> >> Alright, so v3 will have something like this:
> >> 
> >> bool dsa_port_can_offload_mst(struct dsa_port *dp)
> >> {
> >> 	return ds->ops->vlan_msti_set &&
> >> 		ds->ops->port_mst_state_set &&
> >> 		ds->ops->port_vlan_fast_age &&
> >> 		dsa_port_can_configure_learning(dp);
> >> }
> >> 
> >> If this returns false, we have two options:
> >> 
> >> 1. Return -EOPNOTSUPP, which the bridge will be unable to discriminate
> >>    from a non-switchdev port saying "I have no idea what you're talking
> >>    about". I.e. the bridge will happily apply the config, but the
> >>    hardware won't match. I don't like this, but it lines up with most
> >>    other stuff.
> >> 
> >> 2. Return a hard error, e.g. -EINVAL/-ENOSYS. This will keep the bridge
> >>    in sync with the hardware and also gives some feedback to the
> >>    user. This seems like the better approach to me, but it is a new kind
> >>    of paradigm.
> >> 
> >> What do you think?
> >
> > Wait, what? It matters a lot where you place the call to
> > dsa_port_can_offload_mst(), too. You don't have to propagate a hard
> > error code, either, at least if you make dsa_port_bridge_join() return
> > -EOPNOTSUPP prior to calling switchdev_bridge_port_offload(), no?
> > DSA transforms this error code into 0, and dsa_port_offloads_bridge*()
> > starts returning false, which makes us ignore all MSTP related switchdev
> > notifiers.
> 
> Right. So we also need:
> 
> 1. A br_mst_enabled() that we can call from dsa_port_bridge_join to
>    validate the initial state.
> 
> 2. A switchdev attr event sent out when enabling/disabling MST on the
>    bridge, so that we can NAK the change.

So far, so good. This, to me, is analogous to the way in which a hypothetical
VLAN-unaware switchdev driver wouldn't deny VLAN additions or removals,
but it would only accept a VLAN-unaware bridge, and refuse to transition
into a VLAN-aware one. So even though we wouldn't deny the bridge from
keeping state that would have effect when VLAN awareness is on, we
would just deny the bridge from making that state active. Same with MSTP
awareness in my view - don't deny MSTI migrations, per-MSTI port state
changes etc, just the ability to turn on MSTP awareness.

In practice I have only seen things done the other way around - the
dpaa2-switch driver refuses VLAN-unaware bridges, so it doesn't need to
handle ignoring VLAN switchdev notifiers - a slightly simpler task.
Also, the concept of unoffloaded uppers seems to be pretty unique to DSA
so far, among switchdev drivers.

> > The important part will be to make sure that MSTP is enabled for this
> > bridge from the get-go (that being the only case in which we can offload
> > an MSTP aware bridge), and refusing to offload dynamic changes to its
> > MSTP state. I didn't re-check now, but I think I remember there being
> 
> Hang on though. Won't that mean that this sequence...
> 
> ip link add dev br0 type bridge \
>     vlan_filtering 1 vlan_default_pvid 0 mst_enable 1
> ip link set dev swp1 master br0
> 
> ...will work, but offloading will be disabled on swp0; whereas this
> sequence...
> 
> ip link add dev br0 type bridge \
>     vlan_filtering 1 vlan_default_pvid 0
> ip link set dev swp1 master br0
> ip link set dev br0 type bridge mst_enable 1
> 
> ...will fail on the final command? Even though they are logically
> equivalent? But maybe that's just the way the cookie crumbles.

Well, they could be made equivalent for academic purposes, if you're
prepared to dynamically unoffload a bridge port from the MST awareness
notifier, be my guest, I never tried it... I suppose we could try it, in
theory it's just a call to dsa_port_pre_bridge_leave() +
dsa_port_bridge_leave() after all. But it's effort to be spent in work
and testing, and I'm not sure whether anyone will see the benefit or use
case. During initial bridge join, at least it's an established code
path, the drivers which don't implement ds->ops->port_bridge_join() have
exercised it. Alvin Šipraga has fixed a few bugs related to rtl8365mb
and this after some recent rework, it should work just fine now.

> > limitations even in the software bridge related to dynamic MSTP mode
> > changes anyway - there had to not be any port VLANs, which IIUC means
> > that you actually need to _delete_ the port PVIDs which are automatically
> > created before you could change the MSTP mode.
> 
> There are some ergonomic issues there, yes. I might look at it again and
> see if there is some reasonable way of allowing the mode to be changed
> even when VLANs are present.
> 
> > This is the model, what's wrong with it? I said "don't offload the
> > bridge", not "don't offload specific MSTP operations".
> 
> Nothing is wrong, I just couldn't see the whole picture.
> 
> This is the way.