NXP SJA1105 switch driver¶
The NXP SJA1105 is a family of 6 devices:
- SJA1105E: First generation, no TTEthernet
- SJA1105T: First generation, TTEthernet
- SJA1105P: Second generation, no TTEthernet, no SGMII
- SJA1105Q: Second generation, TTEthernet, no SGMII
- SJA1105R: Second generation, no TTEthernet, SGMII
- SJA1105S: Second generation, TTEthernet, SGMII
These are SPI-managed automotive switches, with all ports being gigabit capable, and supporting MII/RMII/RGMII and optionally SGMII on one port.
Being automotive parts, their configuration interface is geared towards set-and-forget use, with minimal dynamic interaction at runtime. They require a static configuration to be composed by software and packed with CRC and table headers, and sent over SPI.
The static configuration is composed of several configuration tables. Each table takes a number of entries. Some configuration tables can be (partially) reconfigured at runtime, some not. Some tables are mandatory, some not:
|Schedule entry points||if Scheduling||no|
|VL Policing||if VL Lookup||no|
|VL Forwarding||if VL Lookup||no|
|L2 Forwarding||yes||partially (fully on P/Q/R/S)|
|MAC Config||yes||partially (fully on P/Q/R/S)|
|Schedule Params||if Scheduling||no|
|Schedule Entry Points Params||if Scheduling||no|
|VL Forwarding Params||if VL Forwarding||no|
|L2 Lookup Params||no||partially (fully on P/Q/R/S)|
|L2 Forwarding Params||yes||no|
|Clock Sync Params||no||no|
Also the configuration is write-only (software cannot read it back from the switch except for very few exceptions).
The driver creates a static configuration at probe time, and keeps it at all times in memory, as a shadow for the hardware state. When required to change a hardware setting, the static configuration is also updated. If that changed setting can be transmitted to the switch through the dynamic reconfiguration interface, it is; otherwise the switch is reset and reprogrammed with the updated static configuration.
The switches do not support switch tagging in hardware. But they do support
customizing the TPID by which VLAN traffic is identified as such. The switch
driver is leveraging
CONFIG_NET_DSA_TAG_8021Q by requesting that special
VLANs (with a custom TPID of
ETH_P_EDSA instead of
installed on its ports when not in
vlan_filtering mode. This does not
interfere with the reception and transmission of real 802.1Q-tagged traffic,
because the switch does no longer parse those packets as VLAN after the TPID
The TPID is restored when
vlan_filtering is requested by the user through
the bridge layer, and general IP termination becomes no longer possible through
the switch netdevices in this mode.
The switches have two programmable filters for link-local destination MACs. These are used to trap BPDUs and PTP traffic to the master netdevice, and are further used to support STP and 1588 ordinary clock/boundary clock functionality.
The following traffic modes are supported over the switch netdevices:
|Regular traffic||Yes||Yes||No (use master)|
The driver supports the configuration of L2 forwarding rules in hardware for port bridging. The forwarding, broadcast and flooding domain between ports can be restricted through two methods: either at the L2 forwarding level (isolate one bridge’s ports from another’s) or at the VLAN port membership level (isolate ports within the same bridge). The final forwarding decision taken by the hardware is a logical AND of these two sets of rules.
The hardware tags all traffic internally with a port-based VLAN (pvid), or it
decodes the VLAN information from the 802.1Q tag. Advanced VLAN classification
is not possible. Once attributed a VLAN tag, frames are checked against the
port’s membership rules and dropped at ingress if they don’t match any VLAN.
This behavior is available when switch ports are enslaved to a bridge with
Normally the hardware is not configurable with respect to VLAN awareness, but
by changing what TPID the switch searches 802.1Q tags for, the semantics of a
vlan_filtering 0 can be kept (accept all traffic, tagged or
untagged), and therefore this mode is also supported.
Segregating the switch ports in multiple bridges is supported (e.g. 2 + 2), but
all bridges should have the same level of VLAN awareness (either both have
vlan_filtering 0, or both 1). Also an inevitable limitation of the fact
that VLAN awareness is global at the switch level is that once a bridge with
vlan_filtering enslaves at least one switch port, the other un-bridged
ports are no longer available for standalone traffic termination.
Topology and loop detection through STP is supported.
L2 FDB manipulation (add/delete/dump) is currently possible for the first generation devices. Aging time of FDB entries, as well as enabling fully static management (no address learning and no flooding of unknown traffic) is not yet configurable in the driver.
A special comment about bridging with other netdevices (illustrated with an example):
A board has eth0, eth1, swp0@eth1, swp1@eth1, swp2@eth1, swp3@eth1. The switch ports (swp0-3) are under br0. It is desired that eth0 is turned into another switched port that communicates with swp0-3.
If br0 has vlan_filtering 0, then eth0 can simply be added to br0 with the intended results. If br0 has vlan_filtering 1, then a new br1 interface needs to be created that enslaves eth0 and eth1 (the DSA master of the switch ports). This is because in this mode, the switch ports beneath br0 are not capable of regular traffic, and are only used as a conduit for switchdev operations.
Device Tree bindings and board design¶
This section references
and aims to showcase some potential switch caveats.
RMII PHY role and out-of-band signaling¶
In the RMII spec, the 50 MHz clock signals are either driven by the MAC or by an external oscillator (but not by the PHY). But the spec is rather loose and devices go outside it in several ways. Some PHYs go against the spec and may provide an output pin where they source the 50 MHz clock themselves, in an attempt to be helpful. On the other hand, the SJA1105 is only binary configurable - when in the RMII MAC role it will also attempt to drive the clock signal. To prevent this from happening it must be put in RMII PHY role. But doing so has some unintended consequences. In the RMII spec, the PHY can transmit extra out-of-band signals via RXD[1:0]. These are practically some extra code words (/J/ and /K/) sent prior to the preamble of each frame. The MAC does not have this out-of-band signaling mechanism defined by the RMII spec. So when the SJA1105 port is put in PHY role to avoid having 2 drivers on the clock signal, inevitably an RMII PHY-to-PHY connection is created. The SJA1105 emulates a PHY interface fully and generates the /J/ and /K/ symbols prior to frame preambles, which the real PHY is not expected to understand. So the PHY simply encodes the extra symbols received from the SJA1105-as-PHY onto the 100Base-Tx wire. On the other side of the wire, some link partners might discard these extra symbols, while others might choke on them and discard the entire Ethernet frames that follow along. This looks like packet loss with some link partners but not with others. The take-away is that in RMII mode, the SJA1105 must be let to drive the reference clock if connected to a PHY.
MDIO bus and PHY management¶
The SJA1105 does not have an MDIO bus and does not perform in-band AN either. Therefore there is no link state notification coming from the switch device. A board would need to hook up the PHYs connected to the switch to any other MDIO bus available to Linux within the system (e.g. to the DSA master’s MDIO bus). Link state management then works by the driver manually keeping in sync (over SPI commands) the MAC link speed with the settings negotiated by the PHY.