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.
The Distributed Switch Architecture subsystem was primarily designed to support Marvell Ethernet switches (MV88E6xxx, a.k.a. Link Street 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 typically comprises multiple front-panel ports and one or more CPU or management ports. 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-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 creates 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 frame it receives 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 supports many vendor-specific tagging protocols, one software-defined
tagging protocol, and a tag-less mode as well (
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
All tagging protocols are in
net/dsa/tag_*.c files and implement the
methods of the
struct dsa_device_ops structure, which are detailed below.
Tagging protocols generally fall in one of three categories:
The switch-specific frame header is located before the Ethernet header, shifting to the right (from the perspective of the DSA master's frame parser) the MAC DA, MAC SA, EtherType and the entire L2 payload.
The switch-specific frame header is located before the EtherType, keeping the MAC DA and MAC SA in place from the DSA master's perspective, but shifting the 'real' EtherType and L2 payload to the right.
The switch-specific frame header is located at the tail of the packet, keeping all frame headers in place and not altering the view of the packet that the DSA master's frame parser has.
A tagging protocol may tag all packets with switch tags of the same length, or
the tag length might vary (for example packets with PTP timestamps might
require an extended switch tag, or there might be one tag length on TX and a
different one on RX). Either way, the tagging protocol driver must populate the
struct dsa_device_ops::needed_headroom and/or
with the length in octets of the longest switch frame header/trailer. The DSA
framework will automatically adjust the MTU of the master interface to
accommodate for this extra size in order for DSA user ports to support the
standard MTU (L2 payload length) of 1500 octets. The
needed_tailroom properties are also used to request from the network stack,
on a best-effort basis, the allocation of packets with enough extra space such
that the act of pushing the switch tag on transmission of a packet does not
cause it to reallocate due to lack of memory.
Even though applications are not expected to parse DSA-specific frame headers,
the format on the wire of the tagging protocol represents an Application Binary
Interface exposed by the kernel towards user space, for decoders such as
libpcap. The tagging protocol driver must populate the
proto member of
struct dsa_device_ops with a value that uniquely describes the
characteristics of the interaction required between the switch hardware and the
data path driver: the offset of each bit field within the frame header and any
stateful processing required to deal with the frames (as may be required for
From the perspective of the network stack, all switches within the same DSA switch tree use the same tagging protocol. In case of a packet transiting a fabric with more than one switch, the switch-specific frame header is inserted by the first switch in the fabric that the packet was received on. This header typically contains information regarding its type (whether it is a control frame that must be trapped to the CPU, or a data frame to be forwarded). Control frames should be decapsulated only by the software data path, whereas data frames might also be autonomously forwarded towards other user ports of other switches from the same fabric, and in this case, the outermost switch ports must decapsulate the packet.
Note that in certain cases, it might be the case that the tagging format used
by a leaf switch (not connected directly to the CPU) is not the same as what
the network stack sees. This can be seen with Marvell switch trees, where the
CPU port can be configured to use either the DSA or the Ethertype DSA (EDSA)
format, but the DSA links are configured to use the shorter (without Ethertype)
DSA frame header, in order to reduce the autonomous packet forwarding overhead.
It still remains the case that, if the DSA switch tree is configured for the
EDSA tagging protocol, the operating system sees EDSA-tagged packets from the
leaf switches that tagged them with the shorter DSA header. This can be done
because the Marvell switch connected directly to the CPU is configured to
perform tag translation between DSA and EDSA (which is simply the operation of
adding or removing the
ETH_P_EDSA EtherType and some padding octets).
It is possible to construct cascaded setups of DSA switches even if their tagging protocols are not compatible with one another. In this case, there are no DSA links in this fabric, and each switch constitutes a disjoint DSA switch tree. The DSA links are viewed as simply a pair of a DSA master (the out-facing port of the upstream DSA switch) and a CPU port (the in-facing port of the downstream DSA switch).
The tagging protocol of the attached DSA switch tree can be viewed through the
dsa/tagging sysfs attribute of the DSA master:
If the hardware and driver are capable, the tagging protocol of the DSA switch tree can be changed at runtime. This is done by writing the new tagging protocol name to the same sysfs device attribute as above (the DSA master and all attached switch ports must be down while doing this).
It is desirable that all tagging protocols are testable with the
mockup driver, which can be attached to any network interface. The goal is that
any network interface should be capable of transmitting the same packet in the
same way, and the tagger should decode the same received packet in the same way
regardless of the driver used for the switch control path, and the driver used
for the DSA master.
The transmission of a packet goes through the tagger's
struct sk_buff *skb has
skb->data pointing at
skb_mac_header(skb), i.e. at the destination MAC address, and the passed
struct net_device *dev represents the virtual DSA user network interface
whose hardware counterpart the packet must be steered to (i.e.
The job of this method is to prepare the skb in a way that the switch will
understand what egress port the packet is for (and not deliver it towards other
ports). Typically this is fulfilled by pushing a frame header. Checking for
insufficient size in the skb headroom or tailroom is unnecessary provided that
needed_tailroom properties were filled out
properly, because DSA ensures there is enough space before calling this method.
The reception of a packet goes through the tagger's
rcv function. The
struct sk_buff *skb has
skb->data pointing at
skb_mac_header(skb) + ETH_ALEN octets, i.e. to where the first octet after
the EtherType would have been, were this frame not tagged. The role of this
method is to consume the frame header, adjust
skb->data to really point at
the first octet after the EtherType, and to change
skb->dev to point to the
virtual DSA user network interface corresponding to the physical front-facing
switch port that the packet was received on.
Since tagging protocols in category 1 and 2 break software (and most often also
hardware) packet dissection on the DSA master, features such as RPS (Receive
Packet Steering) on the DSA master would be broken. The DSA framework deals
with this by hooking into the flow dissector and shifting the offset at which
the IP header is to be found in the tagged frame as seen by the DSA master.
This behavior is automatic based on the
overhead value of the tagging
protocol. If not all packets are of equal size, the tagger can implement the
flow_dissect method of the
struct dsa_device_ops and override this
default behavior by specifying the correct offset incurred by each individual
RX packet. Tail taggers do not cause issues to the flow dissector.
Checksum offload should work with category 1 and 2 taggers when the DSA master driver declares NETIF_F_HW_CSUM in vlan_features and looks at csum_start and csum_offset. For those cases, DSA will shift the checksum start and offset by the tag size. If the DSA master driver still uses the legacy NETIF_F_IP_CSUM or NETIF_F_IPV6_CSUM in vlan_features, the offload might only work if the offload hardware already expects that specific tag (perhaps due to matching vendors). DSA slaves inherit those flags from the master port, and it is up to the driver to correctly fall back to software checksum when the IP header is not where the hardware expects. If that check is ineffective, the packets might go to the network without a proper checksum (the checksum field will have the pseudo IP header sum). For category 3, when the offload hardware does not already expect the switch tag in use, the checksum must be calculated before any tag is inserted (i.e. inside the tagger). Otherwise, the DSA master would include the tail tag in the (software or hardware) checksum calculation. Then, when the tag gets stripped by the switch during transmission, it will leave an incorrect IP checksum in place.
Due to various reasons (most common being category 1 taggers being associated
with DSA-unaware masters, mangling what the master perceives as MAC DA), the
tagging protocol may require the DSA master to operate in promiscuous mode, to
receive all frames regardless of the value of the MAC DA. This can be done by
promisc_on_master property of the
Note that this assumes a DSA-unaware master driver, which is the norm.
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:
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
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
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(skb, dev) if (dev->dsa_ptr != NULL) -> skb->protocol = ETH_P_XDSA
netif_receive_skb(skb) -> iterate over registered packet_type -> invoke handler for ETH_P_XDSA, calls dsa_switch_rcv()
-> dsa_switch_rcv() -> invoke switch tag specific protocol handler in 'net/dsa/tag_*.c'
inspect and strip switch tag protocol to determine originating port
locate per-port network device
eth_type_trans()with the DSA slave network device
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...
manage external/internal PHY: 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 deliver them to the physical switch port.
When using multiple CPU ports, it is possible to stack a LAG (bonding/team)
device between the DSA slave devices and the physical DSA masters. The LAG
device is thus also a DSA master, but the LAG slave devices continue to be DSA
masters as well (just with no user port assigned to them; this is needed for
recovery in case the LAG DSA master disappears). Thus, the data path of the LAG
DSA master is used asymmetrically. On RX, the
ETH_P_XDSA handler, which
dsa_switch_rcv(), is invoked early (on the physical DSA master;
LAG slave). Therefore, the RX data path of the LAG DSA master is not used.
On the other hand, TX takes place linearly:
dsa_enqueue_skb, which calls
dev_queue_xmit towards the LAG DSA master.
The latter calls
dev_queue_xmit towards one physical DSA master or the
other, and in both cases, the packet exits the system through a hardware path
towards the switch.
Summarized, this is basically how DSA looks like from a network device perspective:
Unaware application opens and binds socket | ^ | | +-----------v--|--------------------+ |+------+ +------+ +------+ +------+| || swp0 | | swp1 | | swp2 | | swp3 || |+------+-+------+-+------+-+------+| | DSA switch driver | +-----------------------------------+ | ^ Tag added by | | Tag consumed by switch driver | | switch driver v | +-----------------------------------+ | Unmodified host interface driver | Software --------+-----------------------------------+------------ | Host interface (eth0) | Hardware +-----------------------------------+ | ^ Tag consumed by | | Tag added by switch hardware | | switch hardware v | +-----------------------------------+ | Switch | |+------+ +------+ +------+ +------+| || swp0 | | swp1 | | swp2 | | swp3 || ++------+-+------+-+------+-+------++
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 buses, 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.
DSA data structures are defined in
include/net/dsa.h as well as
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 structures if multiple 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_treeas 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.
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.
Interactions with other subsystems¶
DSA currently leverages the following subsystems:
Device Tree for various of_* functions
Slave network devices exposed by DSA may or may not be interfacing with PHY
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
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
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. As of today, the only SWITCHDEV objects supported by DSA are the FDB and VLAN objects.
DSA features a standardized binding which is documented in
Documentation/devicetree/bindings/net/dsa/dsa.txt. PHY/MDIO library helper
functions such as
of_phy_connect() are also used to query
per-port PHY specific details: interface connection, MDIO bus location, etc.
DSA switch drivers need to implement a
dsa_switch_ops structure which will
contain the various members described below.
Probing, registration and device lifetime¶
DSA switches are regular
device structures on buses (be they platform, SPI,
I2C, MDIO or otherwise). The DSA framework is not involved in their probing
with the device core.
Switch registration from the perspective of a driver means passing a valid
struct dsa_switch pointer to
dsa_register_switch(), usually from the
switch driver's probing function. The following members must be valid in the
ds->dev: will be used to parse the switch's OF node or platform data.
ds->num_ports: will be used to create the port list for this switch, and to validate the port indices provided in the OF node.
ds->ops: a pointer to the
dsa_switch_opsstructure holding the DSA method implementations.
ds->priv: backpointer to a driver-private data structure which can be retrieved in all further DSA method callbacks.
In addition, the following flags in the
dsa_switch structure may optionally
be configured to obtain driver-specific behavior from the DSA core. Their
behavior when set is documented through comments in
Internally, DSA keeps an array of switch trees (group of switches) global to
the kernel, and attaches a
dsa_switch structure to a tree on registration.
The tree ID to which the switch is attached is determined by the first u32
number of the
dsa,member property of the switch's OF node (0 if missing).
The switch ID within the tree is determined by the second u32 number of the
same OF property (0 if missing). Registering multiple switches with the same
switch ID and tree ID is illegal and will cause an error. Using platform data,
a single switch and a single switch tree is permitted.
In case of a tree with multiple switches, probing takes place asymmetrically.
The first N-1 callers of
dsa_register_switch() only add their ports to the
port list of the tree (
dst->ports), each port having a backpointer to its
associated switch (
dp->ds). Then, these switches exit their
dsa_register_switch() call early, because
has determined that the tree is not yet complete (not all ports referenced by
DSA links are present in the tree's port list). The tree becomes complete when
the last switch calls
dsa_register_switch(), and this triggers the effective
continuation of initialization (including the call to
all switches within that tree, all as part of the calling context of the last
switch's probe function.
The opposite of registration takes place when calling
which removes a switch's ports from the port list of the tree. The entire tree
is torn down when the first switch unregisters.
It is mandatory for DSA switch drivers to implement the
of their respective bus, and call
dsa_switch_shutdown() from it (a minimal
version of the full teardown performed by
The reason is that DSA keeps a reference on the master net device, and if the
driver for the master device decides to unbind on shutdown, DSA's reference
will block that operation from finalizing.
dsa_unregister_switch() must be called,
but not both, and the device driver model permits the bus'
to be called even if
shutdown() was already called. Therefore, drivers are
expected to implement a mutual exclusion method between
shutdown() by setting their drvdata to NULL after any of these has run, and
checking whether the drvdata is NULL before proceeding to take any action.
dsa_unregister_switch() was called, no
further callbacks via the provided
dsa_switch_ops may take place, and the
driver may free the data structures associated with the
get_tag_protocol: this is to indicate what kind of tagging protocol is supported, should be a valid value from the
dsa_tag_protocolenum. The returned information does not have to be static; the driver is passed the CPU port number, as well as the tagging protocol of a possibly stacked upstream switch, in case there are hardware limitations in terms of supported tag formats.
change_tag_protocol: when the default tagging protocol has compatibility problems with the master or other issues, the driver may support changing it at runtime, either through a device tree property or through sysfs. In that case, further calls to
get_tag_protocolshould report the protocol in current use.
setup: setup function for the switch, this function is responsible for setting up the
dsa_switch_opsprivate 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. The method responsible for undoing any applicable allocations or operations done here is
port_teardown: methods for initialization and destruction of per-port data structures. It is mandatory for some operations such as registering and unregistering devlink port regions to be done from these methods, otherwise they are optional. A port will be torn down only if it has been previously set up. It is possible for a port to be set up during probing only to be torn down immediately afterwards, for example in case its PHY cannot be found. In this case, probing of the DSA switch continues without that particular port.
port_change_master: method through which the affinity (association used for traffic termination purposes) between a user port and a CPU port can be changed. By default all user ports from a tree are assigned to the first available CPU port that makes sense for them (most of the times this means the user ports of a tree are all assigned to the same CPU port, except for H topologies as described in commit 2c0b03258b8b). The
portargument represents the index of the user port, and the
masterargument represents the new DSA master
net_device. The CPU port associated with the new master can be retrieved by looking at
struct dsa_port *cpu_dp = master->dsa_ptr. Additionally, the master can also be a LAG device where all the slave devices are physical DSA masters. LAG DSA masters also have a valid
master->dsa_ptrpointer, however this is not unique, but rather a duplicate of the first physical DSA master's (LAG slave)
dsa_ptr. In case of a LAG DSA master, a further call to
port_lag_joinwill be emitted separately for the physical CPU ports associated with the physical DSA masters, requesting them to create a hardware LAG associated with the LAG interface.
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
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 fully enable a given switch port. DSA takes care of marking the port with
BR_STATE_BLOCKINGif the port is a bridge member, or
BR_STATE_FORWARDINGif 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 fully disable a given switch port. DSA takes care of marking the port with
BR_STATE_DISABLEDand propagating changes to the hardware if this port is disabled while being a bridge member
Switching hardware is expected to have a table for FDB entries, however not all of them are active at the same time. An address database is the subset (partition) of FDB entries that is active (can be matched by address learning on RX, or FDB lookup on TX) depending on the state of the port. An address database may occasionally be called "FID" (Filtering ID) in this document, although the underlying implementation may choose whatever is available to the hardware.
For example, all ports that belong to a VLAN-unaware bridge (which is currently VLAN-unaware) are expected to learn source addresses in the database associated by the driver with that bridge (and not with other VLAN-unaware bridges). During forwarding and FDB lookup, a packet received on a VLAN-unaware bridge port should be able to find a VLAN-unaware FDB entry having the same MAC DA as the packet, which is present on another port member of the same bridge. At the same time, the FDB lookup process must be able to not find an FDB entry having the same MAC DA as the packet, if that entry points towards a port which is a member of a different VLAN-unaware bridge (and is therefore associated with a different address database).
Similarly, each VLAN of each offloaded VLAN-aware bridge should have an associated address database, which is shared by all ports which are members of that VLAN, but not shared by ports belonging to different bridges that are members of the same VID.
In this context, a VLAN-unaware database means that all packets are expected to match on it irrespective of VLAN ID (only MAC address lookup), whereas a VLAN-aware database means that packets are supposed to match based on the VLAN ID from the classified 802.1Q header (or the pvid if untagged).
At the bridge layer, VLAN-unaware FDB entries have the special VID value of 0, whereas VLAN-aware FDB entries have non-zero VID values. Note that a VLAN-unaware bridge may have VLAN-aware (non-zero VID) FDB entries, and a VLAN-aware bridge may have VLAN-unaware FDB entries. As in hardware, the software bridge keeps separate address databases, and offloads to hardware the FDB entries belonging to these databases, through switchdev, asynchronously relative to the moment when the databases become active or inactive.
When a user port operates in standalone mode, its driver should configure it to use a separate database called a port private database. This is different from the databases described above, and should impede operation as standalone port (packet in, packet out to the CPU port) as little as possible. For example, on ingress, it should not attempt to learn the MAC SA of ingress traffic, since learning is a bridging layer service and this is a standalone port, therefore it would consume useless space. With no address learning, the port private database should be empty in a naive implementation, and in this case, all received packets should be trivially flooded to the CPU port.
DSA (cascade) and CPU ports are also called "shared" ports because they service multiple address databases, and the database that a packet should be associated to is usually embedded in the DSA tag. This means that the CPU port may simultaneously transport packets coming from a standalone port (which were classified by hardware in one address database), and from a bridge port (which were classified to a different address database).
Switch drivers which satisfy certain criteria are able to optimize the naive configuration by removing the CPU port from the flooding domain of the switch, and just program the hardware with FDB entries pointing towards the CPU port for which it is known that software is interested in those MAC addresses. Packets which do not match a known FDB entry will not be delivered to the CPU, which will save CPU cycles required for creating an skb just to drop it.
DSA is able to perform host address filtering for the following kinds of addresses:
Primary unicast MAC addresses of ports (
dev->dev_addr). These are associated with the port private database of the respective user port, and the driver is notified to install them through
port_fdb_addtowards the CPU port.
Secondary unicast and multicast MAC addresses of ports (addresses added through
dev_mc_add()). These are also associated with the port private database of the respective user port.
Local/permanent bridge FDB entries (
BR_FDB_LOCAL). These are the MAC addresses of the bridge ports, for which packets must be terminated locally and not forwarded. They are associated with the address database for that bridge.
Static bridge FDB entries installed towards foreign (non-DSA) interfaces present in the same bridge as some DSA switch ports. These are also associated with the address database for that bridge.
Dynamically learned FDB entries on foreign interfaces present in the same bridge as some DSA switch ports, only if
ds->assisted_learning_on_cpu_portis set to true by the driver. These are associated with the address database for that bridge.
For various operations detailed below, DSA provides a
which can be of the following types:
DSA_DB_PORT: the FDB (or MDB) entry to be installed or deleted belongs to the port private database of user port
DSA_DB_BRIDGE: the entry belongs to one of the address databases of bridge
db->bridge. Separation between the VLAN-unaware database and the per-VID databases of this bridge is expected to be done by the driver.
DSA_DB_LAG: the entry belongs to the address database of LAG
DSA_DB_LAGis currently unused and may be removed in the future.
The drivers which act upon the
dsa_db argument in
port_mdb_add etc should declare
ds->fdb_isolation as true.
DSA associates each offloaded bridge and each offloaded LAG with a one-based ID
struct dsa_bridge :: num,
struct dsa_lag :: id) for the purposes of
refcounting addresses on shared ports. Drivers may piggyback on DSA's numbering
scheme (the ID is readable through
db->lag.id or may
implement their own.
Only the drivers which declare support for FDB isolation are notified of FDB
entries on the CPU port belonging to
For compatibility/legacy reasons,
DSA_DB_BRIDGE addresses are notified to
drivers even if they do not support FDB isolation. However,
db->lag.id are always set to 0 in that case (to denote the lack of
isolation, for refcounting purposes).
Note that it is not mandatory for a switch driver to implement physically
separate address databases for each standalone user port. Since FDB entries in
the port private databases will always point to the CPU port, there is no risk
for incorrect forwarding decisions. In this case, all standalone ports may
share the same database, but the reference counting of host-filtered addresses
(not deleting the FDB entry for a port's MAC address if it's still in use by
another port) becomes the responsibility of the driver, because DSA is unaware
that the port databases are in fact shared. This can be achieved by calling
The down side is that the RX filtering lists of each user port are in fact
shared, which means that user port A may accept a packet with a MAC DA it
shouldn't have, only because that MAC address was in the RX filtering list of
user port B. These packets will still be dropped in software, however.
Offloading the bridge forwarding plane is optional and handled by the methods
below. They may be absent, return -EOPNOTSUPP, or
be non-zero and exceeded, and in this case, joining a bridge port is still
possible, but the packet forwarding will take place in software, and the ports
under a software bridge must remain configured in the same way as for
standalone operation, i.e. have all bridging service functions (address
learning etc) disabled, and send all received packets to the CPU port only.
Concretely, a port starts offloading the forwarding plane of a bridge once it
returns success to the
port_bridge_join method, and stops doing so after
port_bridge_leave has been called. Offloading the bridge means autonomously
learning FDB entries in accordance with the software bridge port's state, and
autonomously forwarding (or flooding) received packets without CPU intervention.
This is optional even when offloading a bridge port. Tagging protocol drivers
are expected to call
dsa_default_offload_fwd_mark(skb) for packets which
have already been autonomously forwarded in the forwarding domain of the
ingress switch port. DSA, through
dsa_port_devlink_setup(), considers all
switch ports part of the same tree ID to be part of the same bridge forwarding
domain (capable of autonomous forwarding to each other).
Offloading the TX forwarding process of a bridge is a distinct concept from simply offloading its forwarding plane, and refers to the ability of certain driver and tag protocol combinations to transmit a single skb coming from the bridge device's transmit function to potentially multiple egress ports (and thereby avoid its cloning in software).
Packets for which the bridge requests this behavior are called data plane
packets and have
skb->offload_fwd_mark set to true in the tag protocol
xmit function. Data plane packets are subject to FDB lookup,
hardware learning on the CPU port, and do not override the port STP state.
Additionally, replication of data plane packets (multicast, flooding) is
handled in hardware and the bridge driver will transmit a single skb for each
packet that may or may not need replication.
When the TX forwarding offload is enabled, the tag protocol driver is responsible to inject packets into the data plane of the hardware towards the correct bridging domain (FID) that the port is a part of. The port may be VLAN-unaware, and in this case the FID must be equal to the FID used by the driver for its VLAN-unaware address database associated with that bridge. Alternatively, the bridge may be VLAN-aware, and in that case, it is guaranteed that the packet is also VLAN-tagged with the VLAN ID that the bridge processed this packet in. It is the responsibility of the hardware to untag the VID on the egress-untagged ports, or keep the tag on the egress-tagged ones.
port_bridge_join: bridge layer function invoked when a given switch port is added to a bridge, this function should do what's necessary at the switch level to permit the joining port to be added to the relevant logical domain for it to ingress/egress traffic with other members of the bridge. By setting the
tx_fwd_offloadargument to true, the TX forwarding process of this bridge is also offloaded.
port_bridge_leave: bridge layer function invoked when a given switch port is removed from a bridge, this function should do what's necessary at the switch level to deny the leaving port from ingress/egress traffic from the remaining bridge members.
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.
port_bridge_flags: bridge layer function invoked when a port must configure its settings for e.g. flooding of unknown traffic or source address learning. The switch driver is responsible for initial setup of the standalone ports with address learning disabled and egress flooding of all types of traffic, then the DSA core notifies of any change to the bridge port flags when the port joins and leaves a bridge. DSA does not currently manage the bridge port flags for the CPU port. The assumption is that address learning should be statically enabled (if supported by the hardware) on the CPU port, and flooding towards the CPU port should also be enabled, due to a lack of an explicit address filtering mechanism in the DSA core.
port_fast_age: bridge layer function invoked when flushing the dynamically learned FDB entries on the port is necessary. This is called when transitioning from an STP state where learning should take place to an STP state where it shouldn't, or when leaving a bridge, or when address learning is turned off via
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_add: bridge layer function invoked when a VLAN is configured (tagged or untagged) for the given switch port. The CPU port becomes a member of a VLAN only if a foreign bridge port is also a member of it (and forwarding needs to take place in software), or the VLAN is installed to the VLAN group of the bridge device itself, for termination purposes (
bridge vlan add dev br0 vid 100 self). VLANs on shared ports are reference counted and removed when there is no user left. Drivers do not need to manually install a VLAN on the CPU port.
port_vlan_del: bridge layer function invoked when a VLAN is removed from 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 in the forwarding database associated with this 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 if it was mapped into this port forwarding database
port_fdb_dump: bridge bypass function invoked by
ndo_fdb_dumpon the physical DSA port interfaces. Since DSA does not attempt to keep in sync its hardware FDB entries with the software bridge, this method is implemented as a means to view the entries visible on user ports in the hardware database. The entries reported by this function have the
selfflag in the output of the
bridge fdb showcommand.
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.
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.
IEC 62439-2 (MRP)¶
The Media Redundancy Protocol is a topology management protocol optimized for fast fault recovery time for ring networks, which has some components implemented as a function of the bridge driver. MRP uses management PDUs (Test, Topology, LinkDown/Up, Option) sent at a multicast destination MAC address range of 01:15:4e:00:00:0x and with an EtherType of 0x88e3. Depending on the node's role in the ring (MRM: Media Redundancy Manager, MRC: Media Redundancy Client, MRA: Media Redundancy Automanager), certain MRP PDUs might need to be terminated locally and others might need to be forwarded. An MRM might also benefit from offloading to hardware the creation and transmission of certain MRP PDUs (Test).
Normally an MRP instance can be created on top of any network interface, however in the case of a device with an offloaded data path such as DSA, it is necessary for the hardware, even if it is not MRP-aware, to be able to extract the MRP PDUs from the fabric before the driver can proceed with the software implementation. DSA today has no driver which is MRP-aware, therefore it only listens for the bare minimum switchdev objects required for the software assist to work properly. The operations are detailed below.
port_mrp_del: notifies driver when an MRP instance with a certain ring ID, priority, primary port and secondary port is created/deleted.
port_mrp_del_ring_role: function invoked when an MRP instance changes ring roles between MRM or MRC. This affects which MRP PDUs should be trapped to software and which should be autonomously forwarded.
IEC 62439-3 (HSR/PRP)¶
The Parallel Redundancy Protocol (PRP) is a network redundancy protocol which works by duplicating and sequence numbering packets through two independent L2 networks (which are unaware of the PRP tail tags carried in the packets), and eliminating the duplicates at the receiver. The High-availability Seamless Redundancy (HSR) protocol is similar in concept, except all nodes that carry the redundant traffic are aware of the fact that it is HSR-tagged (because HSR uses a header with an EtherType of 0x892f) and are physically connected in a ring topology. Both HSR and PRP use supervision frames for monitoring the health of the network and for discovery of other nodes.
In Linux, both HSR and PRP are implemented in the hsr driver, which instantiates a virtual, stackable network interface with two member ports. The driver only implements the basic roles of DANH (Doubly Attached Node implementing HSR) and DANP (Doubly Attached Node implementing PRP); the roles of RedBox and QuadBox are not implemented (therefore, bridging a hsr network interface with a physical switch port does not produce the expected result).
A driver which is able of offloading certain functions of a DANP or DANH should
declare the corresponding netdev features as indicated by the documentation at
Documentation/networking/netdev-features.rst. Additionally, the following
methods must be implemented:
port_hsr_join: function invoked when a given switch port is added to a DANP/DANH. The driver may return
-EOPNOTSUPPand in this case, DSA will fall back to a software implementation where all traffic from this port is sent to the CPU.
port_hsr_leave: function invoked when a given switch port leaves a DANP/DANH and returns to normal operation as a standalone port.
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.