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

Overview
========

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:

============================= ================== =============================
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.

Traffic support
===============

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 ``ETH_P_8021Q``) are
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
change.
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:

+--------------------+------------+------------------+------------------+
|                    | Standalone |   Bridged with   |   Bridged with   |
|                    |    ports   | vlan_filtering 0 | vlan_filtering 1 |
+====================+============+==================+==================+
| Regular traffic    |     Yes    |       Yes        |  No (use master) |
+--------------------+------------+------------------+------------------+
| Management traffic |     Yes    |       Yes        |       Yes        |
|    (BPDU, PTP)     |            |                  |                  |
+--------------------+------------+------------------+------------------+

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 are enslaved to 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). 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 ``Documentation/devicetree/bindings/net/dsa/sja1105.txt``
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 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.
