[PATCH v2] net: dsa: tag_mtk: add padding for tx packets

[Vladimir Oltean]

On Wed, May 11, 2022 at 12:06:25AM +0200, Felix Fietkau wrote:
>
> On 10.05.22 18:52, Vladimir Oltean wrote:
> > On Tue, May 10, 2022 at 04:52:16PM +0200, Felix Fietkau wrote:
> > >
> > > On 10.05.22 14:37, Vladimir Oltean wrote:
> > > > On Tue, May 10, 2022 at 11:40:13AM +0200, Felix Fietkau wrote:
> > > > > Padding for transmitted packets needs to account for the special tag.
> > > > > With not enough padding, garbage bytes are inserted by the switch at the
> > > > > end of small packets.
> > > > > I don't think padding bytes are guaranteed to be zeroes. Aren't
> > > they
> > > > discarded? What is the issue?
> > > With the broken padding, ARP requests are silently discarded on the receiver
> > > side in my test. Adding the padding explicitly fixes the issue.
> > >
> > > - Felix
> >
> > Ok, I'm not going to complain too much about the patch, but I'm still
> > curious where are the so-called "broken" packets discarded.
> > I think the receiving MAC should be passing up to software a buffer
> > without the extra padding beyond the L2 payload length (at least that's
> > the behavior I'm familiar with).
>
> I don't know where exactly these packets are discarded.
> After digging through the devices I used during the tests, I just found some
> leftover pcap files that show the differences in the received packets. Since
> the packets are bigger after my patch, I can't rule out that packet size
> instead of the padding may have made a difference here in getting the ARP
> requests accepted by the receiver.
>
> I've extracted the ARP requests and you can find them here:
> http://nbd.name/arp-broken.pcap

arp-broken.pcap was collected at the receiver MAC, right? So the packets
actually exited the switch?

> http://nbd.name/arp-working.pcap
>
> - Felix

It sounds as if this is masking a problem on the receiver end, because
not only does my enetc port receive the packet, it also replies to the
ARP request.

pc # sudo tcpreplay -i eth1 arp-broken.pcap
root at debian:~# ip addr add 192.168.42.1/24 dev eno0
root at debian:~# tcpdump -i eno0 -e -n --no-promiscuous-mode arp
tcpdump: verbose output suppressed, use -v[v]... for full protocol decode
listening on eno0, link-type EN10MB (Ethernet), snapshot length 262144 bytes
22:18:58.846753 f4:d4:88:5e:6f:d2 > ff:ff:ff:ff:ff:ff, ethertype ARP (0x0806), length 60: Request who-has 192.168.42.1 tell 192.168.42.173, length 46
22:18:58.846806 00:04:9f:05:f4:ab > f4:d4:88:5e:6f:d2, ethertype ARP (0x0806), length 42: Reply 192.168.42.1 is-at 00:04:9f:05:f4:ab, length 28
^C
2 packets captured
2 packets received by filter
0 packets dropped by kernel

What MAC/driver has trouble with these packets? Is there anything wrong
in ethtool stats? Do they even reach software? You can also use
"dropwatch -l kas" for some hints if they do.