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Date: Wed, 11 May 2022 00:06:25 +0200 From: Felix Fietkau <nbd@....name> To: Vladimir Oltean <olteanv@...il.com> Cc: Sean Wang <sean.wang@...iatek.com>, Landen Chao <Landen.Chao@...iatek.com>, DENG Qingfang <dqfext@...il.com>, Andrew Lunn <andrew@...n.ch>, Vivien Didelot <vivien.didelot@...il.com>, Florian Fainelli <f.fainelli@...il.com>, "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Matthias Brugger <matthias.bgg@...il.com>, netdev@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, linux-mediatek@...ts.infradead.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH v2] net: dsa: tag_mtk: add padding for tx packets 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 http://nbd.name/arp-working.pcap - Felix
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