topic Re: Can we disable SJA1105 DSA TAG? in In Vehicle Networking

Can we disable SJA1105 DSA TAG?

tartarter — Wed, 17 Dec 2025 08:32:49 GMT

We are currently using a SJA1105Q with three of its ports in use. After debugging the driver, we found that when port 4 is set to HOST PORT in DSA mode, the frames transmitted from port 3 to port 4 contain a DSA tag, such as 0xdadb 0x0403, which causes the frames to be identified as unknown ethertype. We do not need the DSA tag (host tag), and we require normal ethertype. Could you please advise if it is possible to remove this DSA tag or disable the DSA function? Can the SJ1105 support operation without adding a tag? Please help answer this question.

Re: Can we disable SJA1105 DSA TAG?

PavelL — Wed, 17 Dec 2025 12:45:45 GMT

Hello @tartarter ,

In Linux DSA mode, frames that traverse the HOST/CPU port are tagged by the driver so the kernel can demultiplex traffic per switch port. This is expected: the tag is only present on the CPU link; traffic on the user ports (lanX) is normal Ethernet.

Key points

The CPU/HOST port must carry a DSA (switch) tag in DSA mode.
On the user interfaces (lan1, lan2, lan3…), the kernel removes the tag; applications should bind to these lanX interfaces to see standard Ethertypes.
If you require no tag on the CPU link, the device must run as a stand‑alone L2 switch (i.e., without Linux DSA). This disables per‑port management via DSA and features like STP/PTP integration.
The exact Ethertype you observe on the CPU link can vary with kernel/tagger configuration; it is driver‑inserted, not a data‑plane tag added by the SJA1105 hardware.

Next step
If you share which interface you capture on (eth0 vs. lanX), your kernel version, and the relevant device‑tree snippet for the host/port 4, we can suggest an exact configuration so applications only see normal Ethernet on lanX.

Best regards,

Pavel

Re: Can we disable SJA1105 DSA TAG?

tartarter — Thu, 18 Dec 2025 10:03:26 GMT

Currently, we are using sja1105-tool to directly configure the SJA1105 chip to avoid using the DSA function. The current progress is that we can enable several ports of the SJA1105 to output RGMII CLK using manually constructed XML files. Our requirement is for port4, port2, and port3 to be interconnected, and for all three ports to be able to send and receive normal network type packets (without DSA TAG for both sending and receiving). Ideally, all five ports should be interconnected.
The attached file is the XML configuration we are currently using. Currently, the package for port4 cannot communicate with port3. Please help confirm whether the xml file is correct, and could you provide an XML that enables all ports to communicate with each other? Thank you.

Re: Can we disable SJA1105 DSA TAG?

PavelL — Thu, 18 Dec 2025 12:57:04 GMT

Hello @tartarter ,

Thank you for sharing your configuration.

At first, let me list the key manuals and tools useful for configuring the SJA1105PQRS switch (available on page https://www.nxp.com/products/SJA1105PQRS

Documentation – UM11040: Software user manual for SJA1105P/Q/R/S (static configuration tables, register map, CGU overview).
Documentation – AH1704: SJA1105PQRS Application Hints (clocking, RGMII/RMII specifics, example CGU setups, VLAN/L2 examples).
Software – SJA1105X Configuration Tools (SJA1105X_CONFIG‑GENTOOL): Python-based package to generate static configuration streams/HEX and examples.

Additional open-source references (Linux userspace tooling & format reference):

sja1105‑tool (OpenIL) – Linux userspace tool with XML import/export and SPI upload; docs include a precise mapping of XML fields to UM tables: GitHub repository, man page: XML config format, docs/man index.
Linux driver documentation (DSA) – explains which tables are mandatory and how L2/VLAN decisions are combined: kernel.org doc.

What is wrong in your XML

VLAN Lookup Table

vmemb_port = 0x1 - only port 0 is a member of the VLAN. For interconnecting ports 2–3–4, the membership mask must include those ports (0x1C). For all 5 ports, use 0x1F. (Bitmask semantics are defined in the sja1105‑tool XML format and UM11040.)
tag_port = 0x1 - egress tagging is enabled on port 0. For “normal” untagged traffic on egress, set tag_port = 0x0.

L2 Forwarding Table

The per‑port entries (indexes 0…4) have bc_domain = 0x0, which blocks broadcast/flood from each ingress port to all egress ports. Correct masks are “all except self”:
- port 0 → 0x1E, port 1 → 0x1D, port 2 → 0x1B, port 3 → 0x17, port 4 → 0x0F.
reach_port and fl_domain should match the broadcast domain when you don’t intend any isolation.

MAC Configuration Table

retag = 0x1 - retagging is enabled; for plain, non‑VLAN operation it is safer to disable (retag = 0x0).
vlanid = 0x22B (assigned pvid) is inconsistent with a single untagged VLAN setup; recommend pvid = 1 on all ports if you use one untagged VLAN.

xMII Mode Parameters

xmii_mode = [2 2 2 2 2] (RGMII) and phy_mac = [0 0 0 0 0] configure SJA1105 ports as MAC (switch drives external PHY) in RGMII mode - this matches the typical “switch - PHY” wiring.

Note on clocks (CGU): The static configuration alone is not sufficient - CGU must be configured (RGMII TX/RX clock sinks and, if needed, IDIV/PLL). Otherwise ports may output TXC, but the datapath will not operate correctly. CGU setup is not part of the static XML; it requires programming CGU registers as described in AH1704 (and earlier AH1601).

I recommend starting with basic L2 switching using the mandatory tables and a single untagged VLAN (e.g., VLAN ID 1 with tag_port = 0x0). This approach keeps the setup simple while ensuring the switch forwards ARP and unknown unicast via broadcast/flood domains. The Python SJA1105X_CONFIG‑GENTOOL examples use table field names that are identical to those in the UM11040 documentation and correspond to the sja1105-tool XML field names. So, the python tools might help you a lot.

At this point, I cannot provide XML tables - this goes beyond the scope of our complimentary service, moreover I do not have target hardware to validate the XML before sharing.

Please note that due to the upcoming Christmas holidays and scheduled vacation, I can provide next response after New Year.

Best regards,

Pavel

Re: Can we disable SJA1105 DSA TAG?

tartarter — Thu, 25 Dec 2025 09:11:32 GMT

Based on your suggestion, all ports were placed on the same VLAN, this VLAN was registered in the VLAN lookup table, and then the host port function of all ports was disabled. Eventually, the requirement was fulfilled. Thank you.