Architecture

This document describes the Distributed Switch Architecture (DSA) subsystem design principles, limitations, interactions with other subsystems, and how to develop drivers for this subsystem as well as a TODO for developers interested in joining the effort.

Design principles

The Distributed Switch Architecture is a subsystem which was primarily designed to support Marvell Ethernet switches (MV88E6xxx, a.k.a Linkstreet product line) using Linux, but has since evolved to support other vendors as well.

The original philosophy behind this design was to be able to use unmodified Linux tools such as bridge, iproute2, ifconfig to work transparently whether they configured/queried a switch port network device or a regular network device.

An Ethernet switch is typically comprised of multiple front-panel ports, and one or more CPU or management port. The DSA subsystem currently relies on the presence of a management port connected to an Ethernet controller capable of receiving Ethernet frames from the switch. This is a very common setup for all kinds of Ethernet switches found in Small Home and Office products: routers, gateways, or even top-of-the rack switches. This host Ethernet controller will be later referred to as “master” and “cpu” in DSA terminology and code.

The D in DSA stands for Distributed, because the subsystem has been designed with the ability to configure and manage cascaded switches on top of each other using upstream and downstream Ethernet links between switches. These specific ports are referred to as “dsa” ports in DSA terminology and code. A collection of multiple switches connected to each other is called a “switch tree”.

For each front-panel port, DSA will create specialized network devices which are used as controlling and data-flowing endpoints for use by the Linux networking stack. These specialized network interfaces are referred to as “slave” network interfaces in DSA terminology and code.

The ideal case for using DSA is when an Ethernet switch supports a “switch tag” which is a hardware feature making the switch insert a specific tag for each Ethernet frames it received to/from specific ports to help the management interface figure out:

  • what port is this frame coming from
  • what was the reason why this frame got forwarded
  • how to send CPU originated traffic to specific ports

The subsystem does support switches not capable of inserting/stripping tags, but the features might be slightly limited in that case (traffic separation relies on Port-based VLAN IDs).

Note that DSA does not currently create network interfaces for the “cpu” and “dsa” ports because:

  • the “cpu” port is the Ethernet switch facing side of the management controller, and as such, would create a duplication of feature, since you would get two interfaces for the same conduit: master netdev, and “cpu” netdev
  • the “dsa” port(s) are just conduits between two or more switches, and as such cannot really be used as proper network interfaces either, only the downstream, or the top-most upstream interface makes sense with that model

Switch tagging protocols

DSA currently supports 5 different tagging protocols, and a tag-less mode as well. The different protocols are implemented in:

  • net/dsa/tag_trailer.c: Marvell’s 4 trailer tag mode (legacy)
  • net/dsa/tag_dsa.c: Marvell’s original DSA tag
  • net/dsa/tag_edsa.c: Marvell’s enhanced DSA tag
  • net/dsa/tag_brcm.c: Broadcom’s 4 bytes tag
  • net/dsa/tag_qca.c: Qualcomm’s 2 bytes tag

The exact format of the tag protocol is vendor specific, but in general, they all contain something which:

  • identifies which port the Ethernet frame came from/should be sent to
  • provides a reason why this frame was forwarded to the management interface

Master network devices

Master network devices are regular, unmodified Linux network device drivers for the CPU/management Ethernet interface. Such a driver might occasionally need to know whether DSA is enabled (e.g.: to enable/disable specific offload features), but the DSA subsystem has been proven to work with industry standard drivers: e1000e, mv643xx_eth etc. without having to introduce modifications to these drivers. Such network devices are also often referred to as conduit network devices since they act as a pipe between the host processor and the hardware Ethernet switch.

Networking stack hooks

When a master netdev is used with DSA, a small hook is placed in the networking stack is in order to have the DSA subsystem process the Ethernet switch specific tagging protocol. DSA accomplishes this by registering a specific (and fake) Ethernet type (later becoming skb->protocol) with the networking stack, this is also known as a ptype or packet_type. A typical Ethernet Frame receive sequence looks like this:

Master network device (e.g.: e1000e):

  1. Receive interrupt fires:

    • receive function is invoked
    • basic packet processing is done: getting length, status etc.
    • packet is prepared to be processed by the Ethernet layer by calling eth_type_trans

  2. net/ethernet/eth.c:

    eth_type_trans(skb, dev)
        if (dev->dsa_ptr != NULL)
                -> skb->protocol = ETH_P_XDSA

  3. drivers/net/ethernet/*:

    netif_receive_skb(skb)
        -> iterate over registered packet_type
                -> invoke handler for ETH_P_XDSA, calls dsa_switch_rcv()

  4. net/dsa/dsa.c:

    -> dsa_switch_rcv()
        -> invoke switch tag specific protocol handler in 'net/dsa/tag_*.c'

  5. net/dsa/tag_*.c:

    • inspect and strip switch tag protocol to determine originating port
    • locate per-port network device
    • invoke eth_type_trans() with the DSA slave network device
    • invoked netif_receive_skb()

Past this point, the DSA slave network devices get delivered regular Ethernet frames that can be processed by the networking stack.

Slave network devices

Slave network devices created by DSA are stacked on top of their master network device, each of these network interfaces will be responsible for being a controlling and data-flowing end-point for each front-panel port of the switch. These interfaces are specialized in order to:

  • insert/remove the switch tag protocol (if it exists) when sending traffic to/from specific switch ports
  • query the switch for ethtool operations: statistics, link state, Wake-on-LAN, register dumps…
  • external/internal PHY management: link, auto-negotiation etc.

These slave network devices have custom net_device_ops and ethtool_ops function pointers which allow DSA to introduce a level of layering between the networking stack/ethtool, and the switch driver implementation.

Upon frame transmission from these slave network devices, DSA will look up which switch tagging protocol is currently registered with these network devices, and invoke a specific transmit routine which takes care of adding the relevant switch tag in the Ethernet frames.

These frames are then queued for transmission using the master network device ndo_start_xmit() function, since they contain the appropriate switch tag, the Ethernet switch will be able to process these incoming frames from the management interface and delivers these frames to the physical switch port.

Graphical representation

Summarized, this is basically how DSA looks like from a network device perspective:

        |---------------------------
        | CPU network device (eth0)|
        ----------------------------
        | <tag added by switch     |
        |                          |
        |                          |
        |        tag added by CPU> |
|--------------------------------------------|
|            Switch driver                   |
|--------------------------------------------|
          ||        ||         ||
      |-------|  |-------|  |-------|
      | sw0p0 |  | sw0p1 |  | sw0p2 |
      |-------|  |-------|  |-------|

Slave MDIO bus

In order to be able to read to/from a switch PHY built into it, DSA creates a slave MDIO bus which allows a specific switch driver to divert and intercept MDIO reads/writes towards specific PHY addresses. In most MDIO-connected switches, these functions would utilize direct or indirect PHY addressing mode to return standard MII registers from the switch builtin PHYs, allowing the PHY library and/or to return link status, link partner pages, auto-negotiation results etc..

For Ethernet switches which have both external and internal MDIO busses, the slave MII bus can be utilized to mux/demux MDIO reads and writes towards either internal or external MDIO devices this switch might be connected to: internal PHYs, external PHYs, or even external switches.

Data structures

DSA data structures are defined in include/net/dsa.h as well as net/dsa/dsa_priv.h:

  • dsa_chip_data: platform data configuration for a given switch device, this structure describes a switch device’s parent device, its address, as well as various properties of its ports: names/labels, and finally a routing table indication (when cascading switches)
  • dsa_platform_data: platform device configuration data which can reference a collection of dsa_chip_data structure if multiples switches are cascaded, the master network device this switch tree is attached to needs to be referenced
  • dsa_switch_tree: structure assigned to the master network device under dsa_ptr, this structure references a dsa_platform_data structure as well as the tagging protocol supported by the switch tree, and which receive/transmit function hooks should be invoked, information about the directly attached switch is also provided: CPU port. Finally, a collection of dsa_switch are referenced to address individual switches in the tree.
  • dsa_switch: structure describing a switch device in the tree, referencing a dsa_switch_tree as a backpointer, slave network devices, master network device, and a reference to the backing``dsa_switch_ops``
  • dsa_switch_ops: structure referencing function pointers, see below for a full description.

Design limitations

Limits on the number of devices and ports

DSA currently limits the number of maximum switches within a tree to 4 (DSA_MAX_SWITCHES), and the number of ports per switch to 12 (DSA_MAX_PORTS). These limits could be extended to support larger configurations would this need arise.

Lack of CPU/DSA network devices

DSA does not currently create slave network devices for the CPU or DSA ports, as described before. This might be an issue in the following cases:

  • inability to fetch switch CPU port statistics counters using ethtool, which can make it harder to debug MDIO switch connected using xMII interfaces
  • inability to configure the CPU port link parameters based on the Ethernet controller capabilities attached to it: http://patchwork.ozlabs.org/patch/509806/
  • inability to configure specific VLAN IDs / trunking VLANs between switches when using a cascaded setup

Common pitfalls using DSA setups

Once a master network device is configured to use DSA (dev->dsa_ptr becomes non-NULL), and the switch behind it expects a tagging protocol, this network interface can only exclusively be used as a conduit interface. Sending packets directly through this interface (e.g.: opening a socket using this interface) will not make us go through the switch tagging protocol transmit function, so the Ethernet switch on the other end, expecting a tag will typically drop this frame.

Slave network devices check that the master network device is UP before allowing you to administratively bring UP these slave network devices. A common configuration mistake is forgetting to bring UP the master network device first.

Interactions with other subsystems

DSA currently leverages the following subsystems:

  • MDIO/PHY library: drivers/net/phy/phy.c, mdio_bus.c
  • Switchdev:net/switchdev/*
  • Device Tree for various of_* functions

MDIO/PHY library

Slave network devices exposed by DSA may or may not be interfacing with PHY devices (struct phy_device as defined in include/linux/phy.h), but the DSA subsystem deals with all possible combinations:

  • internal PHY devices, built into the Ethernet switch hardware
  • external PHY devices, connected via an internal or external MDIO bus
  • internal PHY devices, connected via an internal MDIO bus
  • special, non-autonegotiated or non MDIO-managed PHY devices: SFPs, MoCA; a.k.a fixed PHYs

The PHY configuration is done by the dsa_slave_phy_setup() function and the logic basically looks like this:

  • if Device Tree is used, the PHY device is looked up using the standard “phy-handle” property, if found, this PHY device is created and registered using of_phy_connect()
  • if Device Tree is used, and the PHY device is “fixed”, that is, conforms to the definition of a non-MDIO managed PHY as defined in Documentation/devicetree/bindings/net/fixed-link.txt, the PHY is registered and connected transparently using the special fixed MDIO bus driver
  • finally, if the PHY is built into the switch, as is very common with standalone switch packages, the PHY is probed using the slave MII bus created by DSA

SWITCHDEV

DSA directly utilizes SWITCHDEV when interfacing with the bridge layer, and more specifically with its VLAN filtering portion when configuring VLANs on top of per-port slave network devices. Since DSA primarily deals with MDIO-connected switches, although not exclusively, SWITCHDEV’s prepare/abort/commit phases are often simplified into a prepare phase which checks whether the operation is supported by the DSA switch driver, and a commit phase which applies the changes.

As of today, the only SWITCHDEV objects supported by DSA are the FDB and VLAN objects.

Device Tree

DSA features a standardized binding which is documented in Documentation/devicetree/bindings/net/dsa/dsa.txt. PHY/MDIO library helper functions such as of_get_phy_mode(), of_phy_connect() are also used to query per-port PHY specific details: interface connection, MDIO bus location etc..

Driver development

DSA switch drivers need to implement a dsa_switch_ops structure which will contain the various members described below.

register_switch_driver() registers this dsa_switch_ops in its internal list of drivers to probe for. unregister_switch_driver() does the exact opposite.

Unless requested differently by setting the priv_size member accordingly, DSA does not allocate any driver private context space.

Switch configuration

  • tag_protocol: this is to indicate what kind of tagging protocol is supported, should be a valid value from the dsa_tag_protocol enum
  • probe: probe routine which will be invoked by the DSA platform device upon registration to test for the presence/absence of a switch device. For MDIO devices, it is recommended to issue a read towards internal registers using the switch pseudo-PHY and return whether this is a supported device. For other buses, return a non-NULL string
  • setup: setup function for the switch, this function is responsible for setting up the dsa_switch_ops private structure with all it needs: register maps, interrupts, mutexes, locks etc.. This function is also expected to properly configure the switch to separate all network interfaces from each other, that is, they should be isolated by the switch hardware itself, typically by creating a Port-based VLAN ID for each port and allowing only the CPU port and the specific port to be in the forwarding vector. Ports that are unused by the platform should be disabled. Past this function, the switch is expected to be fully configured and ready to serve any kind of request. It is recommended to issue a software reset of the switch during this setup function in order to avoid relying on what a previous software agent such as a bootloader/firmware may have previously configured.

Ethtool operations

  • get_strings: ethtool function used to query the driver’s strings, will typically return statistics strings, private flags strings etc.
  • get_ethtool_stats: ethtool function used to query per-port statistics and return their values. DSA overlays slave network devices general statistics: RX/TX counters from the network device, with switch driver specific statistics per port
  • get_sset_count: ethtool function used to query the number of statistics items
  • get_wol: ethtool function used to obtain Wake-on-LAN settings per-port, this function may, for certain implementations also query the master network device Wake-on-LAN settings if this interface needs to participate in Wake-on-LAN
  • set_wol: ethtool function used to configure Wake-on-LAN settings per-port, direct counterpart to set_wol with similar restrictions
  • set_eee: ethtool function which is used to configure a switch port EEE (Green Ethernet) settings, can optionally invoke the PHY library to enable EEE at the PHY level if relevant. This function should enable EEE at the switch port MAC controller and data-processing logic
  • get_eee: ethtool function which is used to query a switch port EEE settings, this function should return the EEE state of the switch port MAC controller and data-processing logic as well as query the PHY for its currently configured EEE settings
  • get_eeprom_len: ethtool function returning for a given switch the EEPROM length/size in bytes
  • get_eeprom: ethtool function returning for a given switch the EEPROM contents
  • set_eeprom: ethtool function writing specified data to a given switch EEPROM
  • get_regs_len: ethtool function returning the register length for a given switch
  • get_regs: ethtool function returning the Ethernet switch internal register contents. This function might require user-land code in ethtool to pretty-print register values and registers

Power management

  • suspend: function invoked by the DSA platform device when the system goes to suspend, should quiesce all Ethernet switch activities, but keep ports participating in Wake-on-LAN active as well as additional wake-up logic if supported
  • resume: function invoked by the DSA platform device when the system resumes, should resume all Ethernet switch activities and re-configure the switch to be in a fully active state
  • port_enable: function invoked by the DSA slave network device ndo_open function when a port is administratively brought up, this function should be fully enabling a given switch port. DSA takes care of marking the port with BR_STATE_BLOCKING if the port is a bridge member, or BR_STATE_FORWARDING if it was not, and propagating these changes down to the hardware
  • port_disable: function invoked by the DSA slave network device ndo_close function when a port is administratively brought down, this function should be fully disabling a given switch port. DSA takes care of marking the port with BR_STATE_DISABLED and propagating changes to the hardware if this port is disabled while being a bridge member

Bridge layer

  • port_bridge_join: bridge layer function invoked when a given switch port is added to a bridge, this function should be doing the necessary at the switch level to permit the joining port from being added to the relevant logical domain for it to ingress/egress traffic with other members of the bridge.
  • port_bridge_leave: bridge layer function invoked when a given switch port is removed from a bridge, this function should be doing the necessary at the switch level to deny the leaving port from ingress/egress traffic from the remaining bridge members. When the port leaves the bridge, it should be aged out at the switch hardware for the switch to (re) learn MAC addresses behind this port.
  • port_stp_state_set: bridge layer function invoked when a given switch port STP state is computed by the bridge layer and should be propagated to switch hardware to forward/block/learn traffic. The switch driver is responsible for computing a STP state change based on current and asked parameters and perform the relevant ageing based on the intersection results

Bridge VLAN filtering

  • port_vlan_filtering: bridge layer function invoked when the bridge gets configured for turning on or off VLAN filtering. If nothing specific needs to be done at the hardware level, this callback does not need to be implemented. When VLAN filtering is turned on, the hardware must be programmed with rejecting 802.1Q frames which have VLAN IDs outside of the programmed allowed VLAN ID map/rules. If there is no PVID programmed into the switch port, untagged frames must be rejected as well. When turned off the switch must accept any 802.1Q frames irrespective of their VLAN ID, and untagged frames are allowed.
  • port_vlan_prepare: bridge layer function invoked when the bridge prepares the configuration of a VLAN on the given port. If the operation is not supported by the hardware, this function should return -EOPNOTSUPP to inform the bridge code to fallback to a software implementation. No hardware setup must be done in this function. See port_vlan_add for this and details.
  • port_vlan_add: bridge layer function invoked when a VLAN is configured (tagged or untagged) for the given switch port
  • port_vlan_del: bridge layer function invoked when a VLAN is removed from the given switch port
  • port_vlan_dump: bridge layer function invoked with a switchdev callback function that the driver has to call for each VLAN the given port is a member of. A switchdev object is used to carry the VID and bridge flags.
  • port_fdb_add: bridge layer function invoked when the bridge wants to install a Forwarding Database entry, the switch hardware should be programmed with the specified address in the specified VLAN Id in the forwarding database associated with this VLAN ID. If the operation is not supported, this function should return -EOPNOTSUPP to inform the bridge code to fallback to a software implementation.

Note

VLAN ID 0 corresponds to the port private database, which, in the context of DSA, would be its port-based VLAN, used by the associated bridge device.

  • port_fdb_del: bridge layer function invoked when the bridge wants to remove a Forwarding Database entry, the switch hardware should be programmed to delete the specified MAC address from the specified VLAN ID if it was mapped into this port forwarding database
  • port_fdb_dump: bridge layer function invoked with a switchdev callback function that the driver has to call for each MAC address known to be behind the given port. A switchdev object is used to carry the VID and FDB info.
  • port_mdb_prepare: bridge layer function invoked when the bridge prepares the installation of a multicast database entry. If the operation is not supported, this function should return -EOPNOTSUPP to inform the bridge code to fallback to a software implementation. No hardware setup must be done in this function. See port_fdb_add for this and details.
  • port_mdb_add: bridge layer function invoked when the bridge wants to install a multicast database entry, the switch hardware should be programmed with the specified address in the specified VLAN ID in the forwarding database associated with this VLAN ID.

Note

VLAN ID 0 corresponds to the port private database, which, in the context of DSA, would be its port-based VLAN, used by the associated bridge device.

  • port_mdb_del: bridge layer function invoked when the bridge wants to remove a multicast database entry, the switch hardware should be programmed to delete the specified MAC address from the specified VLAN ID if it was mapped into this port forwarding database.
  • port_mdb_dump: bridge layer function invoked with a switchdev callback function that the driver has to call for each MAC address known to be behind the given port. A switchdev object is used to carry the VID and MDB info.

TODO

Making SWITCHDEV and DSA converge towards an unified codebase

SWITCHDEV properly takes care of abstracting the networking stack with offload capable hardware, but does not enforce a strict switch device driver model. On the other DSA enforces a fairly strict device driver model, and deals with most of the switch specific. At some point we should envision a merger between these two subsystems and get the best of both worlds.

Other hanging fruits