=========================
NXP SJA1105 switch driver
=========================

Overview
========

The NXP SJA1105 is a family of 10 SPI-managed automotive switches:

- 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
- SJA1110A: Third generation, TTEthernet, SGMII, integrated 100base-T1 and
  100base-TX PHYs
- SJA1110B: Third generation, TTEthernet, SGMII, 100base-T1, 100base-TX
- SJA1110C: Third generation, TTEthernet, SGMII, 100base-T1, 100base-TX
- SJA1110D: Third generation, TTEthernet, SGMII, 100base-T1

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:

============================= ================== =============================
Table                          Mandatory          Reconfigurable
============================= ================== =============================
Schedule                       no                 no
Schedule entry points          if Scheduling      no
VL Lookup                      no                 no
VL Policing                    if VL Lookup       no
VL Forwarding                  if VL Lookup       no
L2 Lookup                      no                 no
L2 Policing                    yes                no
VLAN Lookup                    yes                yes
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
AVB Params                     no                 no
General Params                 yes                partially
Retagging                      no                 yes
xMII Params                    yes                no
SGMII                          no                 yes
============================= ================== =============================


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.

Switching features
==================

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 join a bridge with
``vlan_filtering 1``.

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
bridge with ``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).

Topology and loop detection through STP is supported.

Offloads
========

Time-aware scheduling
---------------------

The switch supports a variation of the enhancements for scheduled traffic
specified in IEEE 802.1Q-2018 (formerly 802.1Qbv). This means it can be used to
ensure deterministic latency for priority traffic that is sent in-band with its
gate-open event in the network schedule.

This capability can be managed through the tc-taprio offload ('flags 2'). The
difference compared to the software implementation of taprio is that the latter
would only be able to shape traffic originated from the CPU, but not
autonomously forwarded flows.

The device has 8 traffic classes, and maps incoming frames to one of them based
on the VLAN PCP bits (if no VLAN is present, the port-based default is used).
As described in the previous sections, depending on the value of
``vlan_filtering``, the EtherType recognized by the switch as being VLAN can
either be the typical 0x8100 or a custom value used internally by the driver
for tagging. Therefore, the switch ignores the VLAN PCP if used in standalone
or bridge mode with ``vlan_filtering=0``, as it will not recognize the 0x8100
EtherType. In these modes, injecting into a particular TX queue can only be
done by the DSA net devices, which populate the PCP field of the tagging header
on egress. Using ``vlan_filtering=1``, the behavior is the other way around:
offloaded flows can be steered to TX queues based on the VLAN PCP, but the DSA
net devices are no longer able to do that. To inject frames into a hardware TX
queue with VLAN awareness active, it is necessary to create a VLAN
sub-interface on the DSA conduit port, and send normal (0x8100) VLAN-tagged
towards the switch, with the VLAN PCP bits set appropriately.

Management traffic (having DMAC 01-80-C2-xx-xx-xx or 01-19-1B-xx-xx-xx) is the
notable exception: the switch always treats it with a fixed priority and
disregards any VLAN PCP bits even if present. The traffic class for management
traffic has a value of 7 (highest priority) at the moment, which is not
configurable in the driver.

Below is an example of configuring a 500 us cyclic schedule on egress port
``swp5``. The traffic class gate for management traffic (7) is open for 100 us,
and the gates for all other traffic classes are open for 400 us::

  #!/bin/bash

  set -e -u -o pipefail

  NSEC_PER_SEC="1000000000"

  gatemask() {
          local tc_list="$1"
          local mask=0

          for tc in ${tc_list}; do
                  mask=$((${mask} | (1 << ${tc})))
          done

          printf "%02x" ${mask}
  }

  if ! systemctl is-active --quiet ptp4l; then
          echo "Please start the ptp4l service"
          exit
  fi

  now=$(phc_ctl /dev/ptp1 get | gawk '/clock time is/ { print $5; }')
  # Phase-align the base time to the start of the next second.
  sec=$(echo "${now}" | gawk -F. '{ print $1; }')
  base_time="$(((${sec} + 1) * ${NSEC_PER_SEC}))"

  tc qdisc add dev swp5 parent root handle 100 taprio \
          num_tc 8 \
          map 0 1 2 3 5 6 7 \
          queues 1@0 1@1 1@2 1@3 1@4 1@5 1@6 1@7 \
          base-time ${base_time} \
          sched-entry S $(gatemask 7) 100000 \
          sched-entry S $(gatemask "0 1 2 3 4 5 6") 400000 \
          flags 2

It is possible to apply the tc-taprio offload on multiple egress ports. There
are hardware restrictions related to the fact that no gate event may trigger
simultaneously on two ports. The driver checks the consistency of the schedules
against this restriction and errors out when appropriate. Schedule analysis is
needed to avoid this, which is outside the scope of the document.

Routing actions (redirect, trap, drop)
--------------------------------------

The switch is able to offload flow-based redirection of packets to a set of
destination ports specified by the user. Internally, this is implemented by
making use of Virtual Links, a TTEthernet concept.

The driver supports 2 types of keys for Virtual Links:

- VLAN-aware virtual links: these match on destination MAC address, VLAN ID and
  VLAN PCP.
- VLAN-unaware virtual links: these match on destination MAC address only.

The VLAN awareness state of the bridge (vlan_filtering) cannot be changed while
there are virtual link rules installed.

Composing multiple actions inside the same rule is supported. When only routing
actions are requested, the driver creates a "non-critical" virtual link. When
the action list also contains tc-gate (more details below), the virtual link
becomes "time-critical" (draws frame buffers from a reserved memory partition,
etc).

The 3 routing actions that are supported are "trap", "drop" and "redirect".

Example 1: send frames received on swp2 with a DA of 42:be:24:9b:76:20 to the
CPU and to swp3. This type of key (DA only) when the port's VLAN awareness
state is off::

  tc qdisc add dev swp2 clsact
  tc filter add dev swp2 ingress flower skip_sw dst_mac 42:be:24:9b:76:20 \
          action mirred egress redirect dev swp3 \
          action trap

Example 2: drop frames received on swp2 with a DA of 42:be:24:9b:76:20, a VID
of 100 and a PCP of 0::

  tc filter add dev swp2 ingress protocol 802.1Q flower skip_sw \
          dst_mac 42:be:24:9b:76:20 vlan_id 100 vlan_prio 0 action drop

Time-based ingress policing
---------------------------

The TTEthernet hardware abilities of the switch can be constrained to act
similarly to the Per-Stream Filtering and Policing (PSFP) clause specified in
IEEE 802.1Q-2018 (formerly 802.1Qci). This means it can be used to perform
tight timing-based admission control for up to 1024 flows (identified by a
tuple composed of destination MAC address, VLAN ID and VLAN PCP). Packets which
are received outside their expected reception window are dropped.

This capability can be managed through the offload of the tc-gate action. As
routing actions are intrinsic to virtual links in TTEthernet (which performs
explicit routing of time-critical traffic and does not leave that in the hands
of the FDB, flooding etc), the tc-gate action may never appear alone when
asking sja1105 to offload it. One (or more) redirect or trap actions must also
follow along.

Example: create a tc-taprio schedule that is phase-aligned with a tc-gate
schedule (the clocks must be synchronized by a 1588 application stack, which is
outside the scope of this document). No packet delivered by the sender will be
dropped. Note that the reception window is larger than the transmission window
(and much more so, in this example) to compensate for the packet propagation
delay of the link (which can be determined by the 1588 application stack).

Receiver (sja1105)::

  tc qdisc add dev swp2 clsact
  now=$(phc_ctl /dev/ptp1 get | awk '/clock time is/ {print $5}') && \
          sec=$(echo $now | awk -F. '{print $1}') && \
          base_time="$(((sec + 2) * 1000000000))" && \
          echo "base time ${base_time}"
  tc filter add dev swp2 ingress flower skip_sw \
          dst_mac 42:be:24:9b:76:20 \
          action gate base-time ${base_time} \
          sched-entry OPEN  60000 -1 -1 \
          sched-entry CLOSE 40000 -1 -1 \
          action trap

Sender::

  now=$(phc_ctl /dev/ptp0 get | awk '/clock time is/ {print $5}') && \
          sec=$(echo $now | awk -F. '{print $1}') && \
          base_time="$(((sec + 2) * 1000000000))" && \
          echo "base time ${base_time}"
  tc qdisc add dev eno0 parent root taprio \
          num_tc 8 \
          map 0 1 2 3 4 5 6 7 \
          queues 1@0 1@1 1@2 1@3 1@4 1@5 1@6 1@7 \
          base-time ${base_time} \
          sched-entry S 01  50000 \
          sched-entry S 00  50000 \
          flags 2

The engine used to schedule the ingress gate operations is the same that the
one used for the tc-taprio offload. Therefore, the restrictions regarding the
fact that no two gate actions (either tc-gate or tc-taprio gates) may fire at
the same time (during the same 200 ns slot) still apply.

To come in handy, it is possible to share time-triggered virtual links across
more than 1 ingress port, via flow blocks. In this case, the restriction of
firing at the same time does not apply because there is a single schedule in
the system, that of the shared virtual link::

  tc qdisc add dev swp2 ingress_block 1 clsact
  tc qdisc add dev swp3 ingress_block 1 clsact
  tc filter add block 1 flower skip_sw dst_mac 42:be:24:9b:76:20 \
          action gate index 2 \
          base-time 0 \
          sched-entry OPEN 50000000 -1 -1 \
          sched-entry CLOSE 50000000 -1 -1 \
          action trap

Hardware statistics for each flow are also available ("pkts" counts the number
of dropped frames, which is a sum of frames dropped due to timing violations,
lack of destination ports and MTU enforcement checks). Byte-level counters are
not available.

Limitations
===========

The SJA1105 switch family always performs VLAN processing. When configured as
VLAN-unaware, frames carry a different VLAN tag internally, depending on
whether the port is standalone or under a VLAN-unaware bridge.

The virtual link keys are always fixed at {MAC DA, VLAN ID, VLAN PCP}, but the
driver asks for the VLAN ID and VLAN PCP when the port is under a VLAN-aware
bridge. Otherwise, it fills in the VLAN ID and PCP automatically, based on
whether the port is standalone or in a VLAN-unaware bridge, and accepts only
"VLAN-unaware" tc-flower keys (MAC DA).

The existing tc-flower keys that are offloaded using virtual links are no
longer operational after one of the following happens:

- port was standalone and joins a bridge (VLAN-aware or VLAN-unaware)
- port is part of a bridge whose VLAN awareness state changes
- port was part of a bridge and becomes standalone
- port was standalone, but another port joins a VLAN-aware bridge and this
  changes the global VLAN awareness state of the bridge

The driver cannot veto all these operations, and it cannot update/remove the
existing tc-flower filters either. So for proper operation, the tc-flower
filters should be installed only after the forwarding configuration of the port
has been made, and removed by user space before making any changes to it.

Device Tree bindings and board design
=====================================

This section references ``Documentation/devicetree/bindings/net/dsa/nxp,sja1105.yaml``
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.

RGMII fixed-link and internal delays
------------------------------------

As mentioned in the bindings document, the second generation of devices has
tunable delay lines as part of the MAC, which can be used to establish the
correct RGMII timing budget.
When powered up, these can shift the Rx and Tx clocks with a phase difference
between 73.8 and 101.7 degrees.
The catch is that the delay lines need to lock onto a clock signal with a
stable frequency. This means that there must be at least 2 microseconds of
silence between the clock at the old vs at the new frequency. Otherwise the
lock is lost and the delay lines must be reset (powered down and back up).
In RGMII the clock frequency changes with link speed (125 MHz at 1000 Mbps, 25
MHz at 100 Mbps and 2.5 MHz at 10 Mbps), and link speed might change during the
AN process.
In the situation where the switch port is connected through an RGMII fixed-link
to a link partner whose link state life cycle is outside the control of Linux
(such as a different SoC), then the delay lines would remain unlocked (and
inactive) until there is manual intervention (ifdown/ifup on the switch port).
The take-away is that in RGMII mode, the switch's internal delays are only
reliable if the link partner never changes link speeds, or if it does, it does
so in a way that is coordinated with the switch port (practically, both ends of
the fixed-link are under control of the same Linux system).
As to why would a fixed-link interface ever change link speeds: there are
Ethernet controllers out there which come out of reset in 100 Mbps mode, and
their driver inevitably needs to change the speed and clock frequency if it's
required to work at gigabit.

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 conduit'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.

By comparison, the SJA1110 supports an MDIO slave access point over which its
internal 100base-T1 PHYs can be accessed from the host. This is, however, not
used by the driver, instead the internal 100base-T1 and 100base-TX PHYs are
accessed through SPI commands, modeled in Linux as virtual MDIO buses.

The microcontroller attached to the SJA1110 port 0 also has an MDIO controller
operating in master mode, however the driver does not support this either,
since the microcontroller gets disabled when the Linux driver operates.
Discrete PHYs connected to the switch ports should have their MDIO interface
attached to an MDIO controller from the host system and not to the switch,
similar to SJA1105.

Port compatibility matrix
-------------------------

The SJA1105 port compatibility matrix is:

===== ============== ============== ==============
Port   SJA1105E/T     SJA1105P/Q     SJA1105R/S
===== ============== ============== ==============
0      xMII           xMII           xMII
1      xMII           xMII           xMII
2      xMII           xMII           xMII
3      xMII           xMII           xMII
4      xMII           xMII           SGMII
===== ============== ============== ==============


The SJA1110 port compatibility matrix is:

===== ============== ============== ============== ==============
Port   SJA1110A       SJA1110B       SJA1110C       SJA1110D
===== ============== ============== ============== ==============
0      RevMII (uC)    RevMII (uC)    RevMII (uC)    RevMII (uC)
1      100base-TX     100base-TX     100base-TX
       or SGMII                                     SGMII
2      xMII           xMII           xMII           xMII
       or SGMII                                     or SGMII
3      xMII           xMII           xMII
       or SGMII       or SGMII                      SGMII
       or 2500base-X  or 2500base-X                 or 2500base-X
4      SGMII          SGMII          SGMII          SGMII
       or 2500base-X  or 2500base-X  or 2500base-X  or 2500base-X
5      100base-T1     100base-T1     100base-T1     100base-T1
6      100base-T1     100base-T1     100base-T1     100base-T1
7      100base-T1     100base-T1     100base-T1     100base-T1
8      100base-T1     100base-T1     n/a            n/a
9      100base-T1     100base-T1     n/a            n/a
10     100base-T1     n/a            n/a            n/a
===== ============== ============== ============== ==============