netdev - PROBLEM: (DSA/Microchip): 802.1Q-Header lost on KSZ9477-DSA ingress without bridge

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Message-ID: <ad09e947263c44c48a1d2c01bcb4d90a@BK99MAIL02.bk.local>
Date:   Wed, 29 Jul 2020 06:05:10 +0000
From:   "Gaube, Marvin (THSE-TL1)" <Marvin.Gaube@...at.de>
To:     Woojung Huh <woojung.huh@...rochip.com>,
        ", Microchip Linux Driver Support" <UNGLinuxDriver@...rochip.com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: PROBLEM: (DSA/Microchip): 802.1Q-Header lost on KSZ9477-DSA ingress
 without bridge

Summary: 802.1Q-Header lost on KSZ9477-DSA ingress without bridge
Keywords: networking, dsa, microchip, 802.1q, vlan
Full description:

Hello,
we're trying to get 802.1Q-Tagged Ethernet Frames through an KSZ9477 DSA-enabled switch without creating a bridge on the kernel side.
Following setup:
Switchport 1 <-- KSZ9477 --> eth1 (CPU-Port) <---> lan1

No bridge is configured, only the interface directly. Untagged packets are working without problems. The Switch uses the ksz9477-DSA-Driver with Tail-Tagging ("DSA_TAG_PROTO_KSZ9477").
When sending packets with 802.1Q-Header (tagged VLAN) into the Switchport, I see them including the 802.1Q-Header on eth1.
They also appear on lan1, but with the 802.1Q-Header missing.
When I create an VLAN-Interface over lan1 (e.g. lan1.21), nothing arrives there.
The other way around, everything works fine: Packets transmitted into lan1.21 are appearing in 802.1Q-VLAN 21 on the Switchport 1.

I assume that is not the intended behavior.
I haven't found an obvious reason for this behavior yet, but I suspect the VLAN-Header gets stripped of anywhere around "dsa_switch_rcv" in net/dsa/dsa.c or "ksz9477_rcv" in net/dsa/tag_ksz.c.
Hints where the problem could be in detail are welcomed, I will try patches and looking into details.

Kernel Version: v5.4.51
Device Tree from example (https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/devicetree/bindings/net/dsa/ksz.txt?h=v5.4 )

Thanks
Marvin Gaube

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