Hi Vladimir,

On Fri, Jul 1, 2022 at 3:02 PM Vladimir Oltean [off-list ref] wrote:
[...]

quoted

Use FID 0 (which is also the "default" FID) when adding/removing an FDB
entry for the CPU port.

What does "default" FID even mean, and why is the default FID relevant?

The GSW140 datasheet [0] (which is for a newer IP than the one we are
targeting currently with the GSWIP driver - but I am not aware of any
older datasheets) page 78 mentions: "By default the FID is zero and
all entries belong to shared VLAN learning."
Further down you mention that I probably don't understand the problem,
which is probably true - so I'll cut things short here.

[...]

quoted

Fixes: 58c59ef9e930c4 ("net: dsa: lantiq: Add Forwarding Database access")

I guess you don't understand the problem. That commit can't be wrong,
since it dates from v5.2, but DSA only started calling port_fdb_add() on
a CPU port at all since commit
d5f19486cee7 ("net: dsa: listen for SWITCHDEV_{FDB,DEL}_ADD_TO_DEVICE on foreign bridge neighbors") (v5.12)
- and technically, that was opt-in, and the technique only started to become more widespread with commits
81a619f78759 ("net: dsa: include fdb entries pointing to bridge in the host fdb list"),
10fae4ac89ce ("net: dsa: include bridge addresses which are local in the host fdb list") and
3068d466a67e ("net: dsa: sync static FDB entries on foreign interfaces to hardware")
(all appeared in v5.14).

OK, this makes sense as we're not seeing these warnings in 5.10
Initially I thought that "just" the printk level was changed from
DEBUG to something higher - but it seems that this observation is
incorrect.

Also, the most recent application of the "port_fdb_add() on CPU ports" technique was introduced in commit
5e8a1e03aa4d ("net: dsa: install secondary unicast and multicast addresses as host FDB/MDB")
(v5.18). But that is also more or less opt-in, since the driver needs to
declare support for FDB isolation to make use of it.

I did find the FDB isolation changes in 5.18 but I am not sure yet how
to integrate it into the GSWIP driver.

quoted

Cc: stable@vger.kernel.org

We don't CC stable for patches that go through the "net" tree, the
networking maintainers send weekly pull requests and the patches get
automatically backported from there to the relevant and still-not-EOL
stable branches, based on the Fixes: tag. That's why it's important that
you fill that in correctly.

sorry, I'll clean it up in v2

quoted

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
---
This patch is "minimalistic" on purpose: the goal is to have it
backported to Linux 5.15. Linux 5.15 doesn't have dsa_fdb_present_in_other_db()
or struct dsa_db yet. Once this patch has been accepted I will work on
implementing FDB isolation for the Lantiq GSWIP driver.

Don't you want to go the other way around, first understand what is the
real problem, its impact and the correct solution, then figure out what
and how can be backported, and _where_?

I'm willing to help.

I thought that I understood the problem but this is clearly not the
case. Thanks for offering your help!

First it should be understood why DSA bothers to install FDB entries on
the CPU port in the first place. It does so because there is a largeish
class of switches where the MAC source addresses of traffic originating
from Linux are not learned by the hardware. As such, packets being targeted
_towards_ Linux interfaces will not find an entry in the FDB, and will
be flooded. This can be seen if you have a system with swp0 and swp1
both under br0, and the station attached to swp0 pings br0. The ICMP
requests will also be visible by the station attached to swp1.

It's hard to say whether this is the case or not for gswip, but this has
been going on for years and years. Not really a functional (connectivity)
problem, but nonetheless undesirable.

I think for GSWIP it's not flooding packets to all ports.
For testing I had one device connected to LAN1 (let's call this swp0,
in OpenWrt we actually name the port "lan1") and another one connected
to LAN4 (I'll again go with your example and call this swp1).
A ping from the device on swp0 to the IPv4 of br0 (called br-lan in
OpenWrt) was not visible in wireshark on the other device on swp1.
This is with and without this patch.

If needed I can re-test this with Linux 5.10 to make sure that none of
the DSA changes had any impact on this behavior.

[...]

Yet, in a strange way, it appears that it isn't the development of new
core features that draws people's attention, but the harmless kernel log
error messages. So in a way, I don't feel so bad that now I have your
attention?

To give you some insight how I found this warning:
I updated the Lantiq target in OpenWrt to Linux 5.15 - until then I
didn't think we had to make any GSWIP changes because "everything
worked fine".
The first feedback I got for this was basically "does the Ethernet
switch still work with the new warnings?".

Also there's no need to feel bad for this message. I'm having trouble
understanding how just one switch IP (GSWIP) works. Maintaining a
subsystem which fits many switch IPs must be a different beast.

In any case, I recommend you to first set up a test bench where you
actually see a difference between packets being flooded to the CPU vs
matching an FDB entry targeting it. Then read up a bit what the provided
dsa_db argument wants from port_fdb_add(). This conversation with Alvin
should explain a few things.
https://patchwork.kernel.org/project/netdevbpf/cover/20220302191417.1288145-1-vladimir.oltean@nxp.com/#24763870

I previously asked Hauke whether the RX tag (net/dsa/tag_gswip.c) has
some bit to indicate whether traffic is flooded - but to his knowledge
the switch doesn't provide this information.
So I am not sure what I can do in this case - do you have any pointers for me?

Also apologies if all of this is very obvious. So far I have only been
working on the xMII part of Ethernet drivers, meaning: I am totally
new to the FDB part.

Then have a patch (set) lifting the "return -EINVAL" from gswip *properly*.
And only then do we get to ask the questions "how bad are things for
linux-5.18.y? how bad are they for linux-5.15.y? what do we need to do?".

agreed


Thanks again for your time and all these valuable hints Vladimir!
Martin


[0] https://assets.maxlinear.com/web/documents/617930_gsw140_ds_rev1.11.pdf