--- /dev/null
+Distributed Switch Architecture
+===============================
+
+Introduction
+============
+
+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 4 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
+
+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 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):
+
+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
+
+net/ethernet/eth.c:
+
+eth_type_trans(skb, dev)
+ if (dev->dsa_ptr != NULL)
+ -> skb->protocol = ETH_P_XDSA
+
+drivers/net/ethernet/*:
+
+netif_receive_skb(skb)
+ -> iterate over registered packet_type
+ -> invoke handler for ETH_P_XDSA, calls dsa_switch_rcv()
+
+net/dsa/dsa.c:
+ -> dsa_switch_rcv()
+ -> invoke switch tag specific protocol handler in
+ net/dsa/tag_*.c
+
+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_driver
+
+dsa_switch_driver: structure referencing function pointers, see below for a full
+description.
+
+Design limitations
+==================
+
+DSA is a platform device driver
+-------------------------------
+
+DSA is implemented as a DSA platform device driver which is convenient because
+it will register the entire DSA switch tree attached to a master network device
+in one-shot, facilitating the device creation and simplifying the device driver
+model a bit, this comes however with a number of limitations:
+
+- building DSA and its switch drivers as modules is currently not working
+- the device driver parenting does not necessarily reflect the original
+ bus/device the switch can be created from
+- supporting non-MDIO and non-MMIO (platform) switches is not possible
+
+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
+- HWMON: drivers/hwmon/*
+
+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 supporte 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..
+
+HWMON
+-----
+
+Some switch drivers feature internal temperature sensors which are exposed as
+regular HWMON devices in /sys/class/hwmon/.
+
+Driver development
+==================
+
+DSA switch drivers need to implement a dsa_switch_driver structure which will
+contain the various members described below.
+
+register_switch_driver() registers this dsa_switch_driver 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
+--------------------
+
+- priv_size: additional size needed by the switch driver for its private context
+
+- 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_driver 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.
+
+- set_addr: Some switches require the programming of the management interface's
+ Ethernet MAC address, switch drivers can also disable ageing of MAC addresses
+ on the management interface and "hardcode"/"force" this MAC address for the
+ CPU/management interface as an optimization
+
+PHY devices and link management
+-------------------------------
+
+- get_phy_flags: Some switches are interfaced to various kinds of Ethernet PHYs,
+ if the PHY library PHY driver needs to know about information it cannot obtain
+ on its own (e.g.: coming from switch memory mapped registers), this function
+ should return a 32-bits bitmask of "flags", that is private between the switch
+ driver and the Ethernet PHY driver in drivers/net/phy/*.
+
+- phy_read: Function invoked by the DSA slave MDIO bus when attempting to read
+ the switch port MDIO registers. If unavailable, return 0xffff for each read.
+ For builtin switch Ethernet PHYs, this function should allow reading the link
+ status, auto-negotiation results, link partner pages etc..
+
+- phy_write: Function invoked by the DSA slave MDIO bus when attempting to write
+ to the switch port MDIO registers. If unavailable return a negative error
+ code.
+
+- poll_link: Function invoked by DSA to query the link state of the switch
+ builtin Ethernet PHYs, per port. This function is responsible for calling
+ netif_carrier_{on,off} when appropriate, and can be used to poll all ports in a
+ single call. Executes from workqueue context.
+
+- adjust_link: Function invoked by the PHY library when a slave network device
+ is attached to a PHY device. This function is responsible for appropriately
+ configuring the switch port link parameters: speed, duplex, pause based on
+ what the phy_device is providing.
+
+- fixed_link_update: Function invoked by the PHY library, and specifically by
+ the fixed PHY driver asking the switch driver for link parameters that could
+ not be auto-negotiated, or obtained by reading the PHY registers through MDIO.
+ This is particularly useful for specific kinds of hardware such as QSGMII,
+ MoCA or other kinds of non-MDIO managed PHYs where out of band link
+ information is obtained
+
+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
+
+Hardware monitoring
+-------------------
+
+These callbacks are only available if CONFIG_NET_DSA_HWMON is enabled:
+
+- get_temp: this function queries the given switch for its temperature
+
+- get_temp_limit: this function returns the switch current maximum temperature
+ limit
+
+- set_temp_limit: this function configures the maximum temperature limit allowed
+
+- get_temp_alarm: this function returns the critical temperature threshold
+ returning an alarm notification
+
+See Documentation/hwmon/sysfs-interface for details.
+
+Bridge layer
+------------
+
+- port_join_bridge: 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. DSA
+ does nothing but calculate a bitmask of switch ports currently members of the
+ specified bridge being requested the join
+
+- port_leave_bridge: 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. DSA calculates the bitmask of ports still members of the bridge
+ being left
+
+- port_stp_update: 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_pvid_get: bridge layer function invoked when a Port-based VLAN ID is
+ queried for the given switch port
+
+- port_pvid_set: bridge layer function invoked when a Port-based VLAN ID needs
+ to be configured on the given switch port
+
+- 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
+
+- vlan_getnext: bridge layer function invoked to query the next configured VLAN
+ in the switch, i.e. returns the bitmaps of members and untagged ports
+
+- 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
+
+Note: VLAN ID 0 corresponds to the port private database, which, in the context
+of DSA, would be the 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
+
+TODO
+====
+
+The platform device problem
+---------------------------
+DSA is currently implemented as a platform device driver which is far from ideal
+as was discussed in this thread:
+
+http://permalink.gmane.org/gmane.linux.network/329848
+
+This basically prevents the device driver model to be properly used and applied,
+and support non-MDIO, non-MMIO Ethernet connected switches.
+
+Another problem with the platform device driver approach is that it prevents the
+use of a modular switch drivers build due to a circular dependency, illustrated
+here:
+
+http://comments.gmane.org/gmane.linux.network/345803
+
+Attempts of reworking this has been done here:
+
+https://lwn.net/Articles/643149/
+
+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
+--------------------
+
+- making the number of ports fully dynamic and not dependent on DSA_MAX_PORTS
+- allowing more than one CPU/management interface:
+ http://comments.gmane.org/gmane.linux.network/365657
+- porting more drivers from other vendors:
+ http://comments.gmane.org/gmane.linux.network/365510
projects / GitHub / LineageOS / android_kernel_motorola_exynos9610.git / commitdiff