Message-ID: <20150825162413.GA18741@lunn.ch>
Date:	Tue, 25 Aug 2015 18:24:13 +0200
From:	Andrew Lunn <andrew@...n.ch>
To:	Florian Fainelli <f.fainelli@...il.com>
Cc:	netdev@...r.kernel.org, vivien.didelot@...oirfairelinux.com,
	sfeldma@...il.com, linux@...ck-us.net, davem@...emloft.net
Subject: Re: [PATCH net-next 1/2] Documentation: networking: add a DSA
 document

Hi Florian

Nice document. A few typos below, but i did not get all the way to the
bottom, so another email will follow.

Also, maybe it would be good to mention the philosophy, that features
should be configured in the usual Linux way. We use the hardware to
accelerate features which Linux already supports, and fall back to
software when the hardware cannot accelerate it.

Acked-by: Andrew Lunn <andrew@...n.ch>

	  Andrew


On Tue, Aug 25, 2015 at 06:50:41AM -0700, Florian Fainelli wrote:
> Describe how the DSA subsystem works, its design principles,
> limitations, and describe in details how to implement a DSA switch
> driver.
> 
> Signed-off-by: Florian Fainelli <f.fainelli@...il.com>
> ---
>  Documentation/networking/dsa/dsa.txt | 584 +++++++++++++++++++++++++++++++++++
>  1 file changed, 584 insertions(+)
>  create mode 100644 Documentation/networking/dsa/dsa.txt
> 
> diff --git a/Documentation/networking/dsa/dsa.txt b/Documentation/networking/dsa/dsa.txt
> new file mode 100644
> index 0000000..2873309
> --- /dev/null
> +++ b/Documentation/networking/dsa/dsa.txt
> @@ -0,0 +1,584 @@
> +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, aka Linkstreet product line)
> +using Linux, but has since evolved to support other vendors as well.
> +
> +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 subsytem has been designed with

                                                    subsystem

> +the abibility to configure and manage cascaded switches on top of each other

      ability 
   
> +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 current 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.
> +
> +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 bytes trailer mode
> +net/dsa/tag_dsa.c: Marvell's 8 bytes original DSA tag

This is 4 bytes, but if there is already a VLAN tag, no additional
bytes are needed.

> +net/dsa/tag_edsa.c: Marvell's 4 bytes enhanced DSA tag

EDSA is 8 bytes, but if the frame already have a VLAN tag, only 4
additional bytes need adding.

> +net/dsa/tag_brcm.c: Broadcom's 4 byte 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 forwared to the management interface

                                          forwarded 
> +
> +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 device is also often refered to as a conduit network

                                              referred 
also, "Such network devices are ..."


> +device since it acts 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 in the networking stack is

 a small hook is placed in the network stack in order

> +placed 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 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 specliazed in order to:

                        specialized or specialised. 
> +
> +- 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..
> +
> +This customization is achieved by inserting an additional layer and overriding,
> +where specific the slave network device's ethtool_ops and net_device_ops

This does not parse.

> +functions.
> +
> +Upon frame transmission from these slave network devices, DSA will look up with

                                                                   which, not with.

> +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 PHYs built into it, DSA creates a
                               to/from the PHYs built into the switch,  

> +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 builtint PHYs, allowing the PHY

                                                    builtin

> +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 sif multiples switches are cascaded, the

                                         if 

> +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 invididual switches in the tree.

              individual 

> +
> +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 simplyfing the device driver

                                                    simplifying 

> +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
> +- supportning non-MDIO and non-MMIO switches is currently not supported

     supporting 

> +
> +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 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 vian an internal MDIO bus

                                     via

> +- special, non-autonegotiated or non MDIO-managed PHY devices: SFPs, MoCA; aka
> +  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/binding/net/fixed-link.txt, the PHY is the registered

                                                                   the not required

> +  and connected to using the special fixed MDIO bus driver transparently

                no to and transparently goes after is.

> +
> +- 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
> +switch, although not exclusively, SWITCHDEV's commit/prepare/ phase
> +
> +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 request differently by setting the priv_size member accordingly, DSA does

          requested 

> +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 service 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 aging of MAC addresses

                                                           ageing 

> +  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 built-in 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
> +  built-int Ethernet PHYs, per port. This function is reponsible for calling

                                                        responsible 

> +  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 activity, 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 activity 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;s 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 aging 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
> +  (taggedd or untagged) for the given switch port

      tagged 
> +
> +- 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
> +  on the given switch port
> +
> +- 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
> +
> +- 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
> +
> +TODO
> +====
> +
> +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 module 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/
> +
> +Other hanging fruits include:
> +
> +- 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
> -- 
> 2.1.4
> 
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