602 lines
27 KiB
Plaintext
Executable file
602 lines
27 KiB
Plaintext
Executable file
Distributed Switch Architecture
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===============================
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Introduction
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============
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This document describes the Distributed Switch Architecture (DSA) subsystem
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design principles, limitations, interactions with other subsystems, and how to
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develop drivers for this subsystem as well as a TODO for developers interested
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in joining the effort.
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Design principles
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=================
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The Distributed Switch Architecture is a subsystem which was primarily designed
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to support Marvell Ethernet switches (MV88E6xxx, a.k.a Linkstreet product line)
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using Linux, but has since evolved to support other vendors as well.
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The original philosophy behind this design was to be able to use unmodified
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Linux tools such as bridge, iproute2, ifconfig to work transparently whether
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they configured/queried a switch port network device or a regular network
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device.
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An Ethernet switch is typically comprised of multiple front-panel ports, and one
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or more CPU or management port. The DSA subsystem currently relies on the
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presence of a management port connected to an Ethernet controller capable of
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receiving Ethernet frames from the switch. This is a very common setup for all
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kinds of Ethernet switches found in Small Home and Office products: routers,
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gateways, or even top-of-the rack switches. This host Ethernet controller will
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be later referred to as "master" and "cpu" in DSA terminology and code.
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The D in DSA stands for Distributed, because the subsystem has been designed
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with the ability to configure and manage cascaded switches on top of each other
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using upstream and downstream Ethernet links between switches. These specific
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ports are referred to as "dsa" ports in DSA terminology and code. A collection
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of multiple switches connected to each other is called a "switch tree".
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For each front-panel port, DSA will create specialized network devices which are
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used as controlling and data-flowing endpoints for use by the Linux networking
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stack. These specialized network interfaces are referred to as "slave" network
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interfaces in DSA terminology and code.
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The ideal case for using DSA is when an Ethernet switch supports a "switch tag"
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which is a hardware feature making the switch insert a specific tag for each
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Ethernet frames it received to/from specific ports to help the management
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interface figure out:
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- what port is this frame coming from
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- what was the reason why this frame got forwarded
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- how to send CPU originated traffic to specific ports
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The subsystem does support switches not capable of inserting/stripping tags, but
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the features might be slightly limited in that case (traffic separation relies
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on Port-based VLAN IDs).
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Note that DSA does not currently create network interfaces for the "cpu" and
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"dsa" ports because:
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- the "cpu" port is the Ethernet switch facing side of the management
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controller, and as such, would create a duplication of feature, since you
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would get two interfaces for the same conduit: master netdev, and "cpu" netdev
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- the "dsa" port(s) are just conduits between two or more switches, and as such
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cannot really be used as proper network interfaces either, only the
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downstream, or the top-most upstream interface makes sense with that model
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Switch tagging protocols
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------------------------
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DSA currently supports 5 different tagging protocols, and a tag-less mode as
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well. The different protocols are implemented in:
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net/dsa/tag_trailer.c: Marvell's 4 trailer tag mode (legacy)
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net/dsa/tag_dsa.c: Marvell's original DSA tag
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net/dsa/tag_edsa.c: Marvell's enhanced DSA tag
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net/dsa/tag_brcm.c: Broadcom's 4 bytes tag
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net/dsa/tag_qca.c: Qualcomm's 2 bytes tag
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The exact format of the tag protocol is vendor specific, but in general, they
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all contain something which:
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- identifies which port the Ethernet frame came from/should be sent to
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- provides a reason why this frame was forwarded to the management interface
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Master network devices
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----------------------
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Master network devices are regular, unmodified Linux network device drivers for
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the CPU/management Ethernet interface. Such a driver might occasionally need to
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know whether DSA is enabled (e.g.: to enable/disable specific offload features),
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but the DSA subsystem has been proven to work with industry standard drivers:
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e1000e, mv643xx_eth etc. without having to introduce modifications to these
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drivers. Such network devices are also often referred to as conduit network
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devices since they act as a pipe between the host processor and the hardware
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Ethernet switch.
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Networking stack hooks
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----------------------
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When a master netdev is used with DSA, a small hook is placed in in the
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networking stack is in order to have the DSA subsystem process the Ethernet
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switch specific tagging protocol. DSA accomplishes this by registering a
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specific (and fake) Ethernet type (later becoming skb->protocol) with the
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networking stack, this is also known as a ptype or packet_type. A typical
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Ethernet Frame receive sequence looks like this:
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Master network device (e.g.: e1000e):
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Receive interrupt fires:
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- receive function is invoked
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- basic packet processing is done: getting length, status etc.
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- packet is prepared to be processed by the Ethernet layer by calling
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eth_type_trans
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net/ethernet/eth.c:
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eth_type_trans(skb, dev)
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if (dev->dsa_ptr != NULL)
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-> skb->protocol = ETH_P_XDSA
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drivers/net/ethernet/*:
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netif_receive_skb(skb)
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-> iterate over registered packet_type
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-> invoke handler for ETH_P_XDSA, calls dsa_switch_rcv()
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net/dsa/dsa.c:
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-> dsa_switch_rcv()
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-> invoke switch tag specific protocol handler in
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net/dsa/tag_*.c
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net/dsa/tag_*.c:
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-> inspect and strip switch tag protocol to determine originating port
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-> locate per-port network device
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-> invoke eth_type_trans() with the DSA slave network device
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-> invoked netif_receive_skb()
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Past this point, the DSA slave network devices get delivered regular Ethernet
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frames that can be processed by the networking stack.
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Slave network devices
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---------------------
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Slave network devices created by DSA are stacked on top of their master network
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device, each of these network interfaces will be responsible for being a
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controlling and data-flowing end-point for each front-panel port of the switch.
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These interfaces are specialized in order to:
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- insert/remove the switch tag protocol (if it exists) when sending traffic
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to/from specific switch ports
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- query the switch for ethtool operations: statistics, link state,
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Wake-on-LAN, register dumps...
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- external/internal PHY management: link, auto-negotiation etc.
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These slave network devices have custom net_device_ops and ethtool_ops function
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pointers which allow DSA to introduce a level of layering between the networking
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stack/ethtool, and the switch driver implementation.
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Upon frame transmission from these slave network devices, DSA will look up which
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switch tagging protocol is currently registered with these network devices, and
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invoke a specific transmit routine which takes care of adding the relevant
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switch tag in the Ethernet frames.
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These frames are then queued for transmission using the master network device
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ndo_start_xmit() function, since they contain the appropriate switch tag, the
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Ethernet switch will be able to process these incoming frames from the
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management interface and delivers these frames to the physical switch port.
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Graphical representation
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------------------------
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Summarized, this is basically how DSA looks like from a network device
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perspective:
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|---------------------------
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| CPU network device (eth0)|
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----------------------------
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| <tag added by switch |
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| tag added by CPU> |
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|--------------------------------------------|
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| Switch driver |
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|--------------------------------------------|
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|| || ||
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|-------| |-------| |-------|
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| sw0p0 | | sw0p1 | | sw0p2 |
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|-------| |-------| |-------|
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Slave MDIO bus
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--------------
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In order to be able to read to/from a switch PHY built into it, DSA creates a
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slave MDIO bus which allows a specific switch driver to divert and intercept
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MDIO reads/writes towards specific PHY addresses. In most MDIO-connected
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switches, these functions would utilize direct or indirect PHY addressing mode
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to return standard MII registers from the switch builtin PHYs, allowing the PHY
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library and/or to return link status, link partner pages, auto-negotiation
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results etc..
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For Ethernet switches which have both external and internal MDIO busses, the
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slave MII bus can be utilized to mux/demux MDIO reads and writes towards either
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internal or external MDIO devices this switch might be connected to: internal
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PHYs, external PHYs, or even external switches.
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Data structures
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---------------
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DSA data structures are defined in include/net/dsa.h as well as
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net/dsa/dsa_priv.h.
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dsa_chip_data: platform data configuration for a given switch device, this
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structure describes a switch device's parent device, its address, as well as
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various properties of its ports: names/labels, and finally a routing table
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indication (when cascading switches)
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dsa_platform_data: platform device configuration data which can reference a
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collection of dsa_chip_data structure if multiples switches are cascaded, the
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master network device this switch tree is attached to needs to be referenced
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dsa_switch_tree: structure assigned to the master network device under
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"dsa_ptr", this structure references a dsa_platform_data structure as well as
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the tagging protocol supported by the switch tree, and which receive/transmit
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function hooks should be invoked, information about the directly attached switch
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is also provided: CPU port. Finally, a collection of dsa_switch are referenced
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to address individual switches in the tree.
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dsa_switch: structure describing a switch device in the tree, referencing a
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dsa_switch_tree as a backpointer, slave network devices, master network device,
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and a reference to the backing dsa_switch_ops
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dsa_switch_ops: structure referencing function pointers, see below for a full
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description.
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Design limitations
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==================
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DSA is a platform device driver
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-------------------------------
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DSA is implemented as a DSA platform device driver which is convenient because
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it will register the entire DSA switch tree attached to a master network device
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in one-shot, facilitating the device creation and simplifying the device driver
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model a bit, this comes however with a number of limitations:
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- building DSA and its switch drivers as modules is currently not working
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- the device driver parenting does not necessarily reflect the original
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bus/device the switch can be created from
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- supporting non-MDIO and non-MMIO (platform) switches is not possible
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Limits on the number of devices and ports
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-----------------------------------------
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DSA currently limits the number of maximum switches within a tree to 4
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(DSA_MAX_SWITCHES), and the number of ports per switch to 12 (DSA_MAX_PORTS).
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These limits could be extended to support larger configurations would this need
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arise.
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Lack of CPU/DSA network devices
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-------------------------------
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DSA does not currently create slave network devices for the CPU or DSA ports, as
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described before. This might be an issue in the following cases:
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- inability to fetch switch CPU port statistics counters using ethtool, which
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can make it harder to debug MDIO switch connected using xMII interfaces
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- inability to configure the CPU port link parameters based on the Ethernet
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controller capabilities attached to it: http://patchwork.ozlabs.org/patch/509806/
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- inability to configure specific VLAN IDs / trunking VLANs between switches
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when using a cascaded setup
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Common pitfalls using DSA setups
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--------------------------------
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Once a master network device is configured to use DSA (dev->dsa_ptr becomes
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non-NULL), and the switch behind it expects a tagging protocol, this network
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interface can only exclusively be used as a conduit interface. Sending packets
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directly through this interface (e.g.: opening a socket using this interface)
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will not make us go through the switch tagging protocol transmit function, so
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the Ethernet switch on the other end, expecting a tag will typically drop this
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frame.
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Slave network devices check that the master network device is UP before allowing
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you to administratively bring UP these slave network devices. A common
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configuration mistake is forgetting to bring UP the master network device first.
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Interactions with other subsystems
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==================================
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DSA currently leverages the following subsystems:
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- MDIO/PHY library: drivers/net/phy/phy.c, mdio_bus.c
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- Switchdev: net/switchdev/*
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- Device Tree for various of_* functions
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MDIO/PHY library
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----------------
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Slave network devices exposed by DSA may or may not be interfacing with PHY
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devices (struct phy_device as defined in include/linux/phy.h), but the DSA
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subsystem deals with all possible combinations:
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- internal PHY devices, built into the Ethernet switch hardware
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- external PHY devices, connected via an internal or external MDIO bus
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- internal PHY devices, connected via an internal MDIO bus
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- special, non-autonegotiated or non MDIO-managed PHY devices: SFPs, MoCA; a.k.a
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fixed PHYs
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The PHY configuration is done by the dsa_slave_phy_setup() function and the
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logic basically looks like this:
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- if Device Tree is used, the PHY device is looked up using the standard
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"phy-handle" property, if found, this PHY device is created and registered
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using of_phy_connect()
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- if Device Tree is used, and the PHY device is "fixed", that is, conforms to
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the definition of a non-MDIO managed PHY as defined in
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Documentation/devicetree/bindings/net/fixed-link.txt, the PHY is registered
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and connected transparently using the special fixed MDIO bus driver
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- finally, if the PHY is built into the switch, as is very common with
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standalone switch packages, the PHY is probed using the slave MII bus created
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by DSA
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SWITCHDEV
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---------
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DSA directly utilizes SWITCHDEV when interfacing with the bridge layer, and
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more specifically with its VLAN filtering portion when configuring VLANs on top
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of per-port slave network devices. Since DSA primarily deals with
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MDIO-connected switches, although not exclusively, SWITCHDEV's
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prepare/abort/commit phases are often simplified into a prepare phase which
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checks whether the operation is supported by the DSA switch driver, and a commit
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phase which applies the changes.
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As of today, the only SWITCHDEV objects supported by DSA are the FDB and VLAN
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objects.
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Device Tree
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-----------
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DSA features a standardized binding which is documented in
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Documentation/devicetree/bindings/net/dsa/dsa.txt. PHY/MDIO library helper
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functions such as of_get_phy_mode(), of_phy_connect() are also used to query
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per-port PHY specific details: interface connection, MDIO bus location etc..
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Driver development
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==================
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DSA switch drivers need to implement a dsa_switch_ops structure which will
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contain the various members described below.
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register_switch_driver() registers this dsa_switch_ops in its internal list
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of drivers to probe for. unregister_switch_driver() does the exact opposite.
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Unless requested differently by setting the priv_size member accordingly, DSA
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does not allocate any driver private context space.
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Switch configuration
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--------------------
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- tag_protocol: this is to indicate what kind of tagging protocol is supported,
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should be a valid value from the dsa_tag_protocol enum
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- probe: probe routine which will be invoked by the DSA platform device upon
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registration to test for the presence/absence of a switch device. For MDIO
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devices, it is recommended to issue a read towards internal registers using
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the switch pseudo-PHY and return whether this is a supported device. For other
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buses, return a non-NULL string
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- setup: setup function for the switch, this function is responsible for setting
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up the dsa_switch_ops private structure with all it needs: register maps,
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interrupts, mutexes, locks etc.. This function is also expected to properly
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configure the switch to separate all network interfaces from each other, that
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is, they should be isolated by the switch hardware itself, typically by creating
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a Port-based VLAN ID for each port and allowing only the CPU port and the
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specific port to be in the forwarding vector. Ports that are unused by the
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platform should be disabled. Past this function, the switch is expected to be
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fully configured and ready to serve any kind of request. It is recommended
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to issue a software reset of the switch during this setup function in order to
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avoid relying on what a previous software agent such as a bootloader/firmware
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may have previously configured.
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PHY devices and link management
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-------------------------------
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- get_phy_flags: Some switches are interfaced to various kinds of Ethernet PHYs,
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if the PHY library PHY driver needs to know about information it cannot obtain
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on its own (e.g.: coming from switch memory mapped registers), this function
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should return a 32-bits bitmask of "flags", that is private between the switch
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driver and the Ethernet PHY driver in drivers/net/phy/*.
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- phy_read: Function invoked by the DSA slave MDIO bus when attempting to read
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the switch port MDIO registers. If unavailable, return 0xffff for each read.
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For builtin switch Ethernet PHYs, this function should allow reading the link
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status, auto-negotiation results, link partner pages etc..
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- phy_write: Function invoked by the DSA slave MDIO bus when attempting to write
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to the switch port MDIO registers. If unavailable return a negative error
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code.
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- adjust_link: Function invoked by the PHY library when a slave network device
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is attached to a PHY device. This function is responsible for appropriately
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configuring the switch port link parameters: speed, duplex, pause based on
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what the phy_device is providing.
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- fixed_link_update: Function invoked by the PHY library, and specifically by
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the fixed PHY driver asking the switch driver for link parameters that could
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not be auto-negotiated, or obtained by reading the PHY registers through MDIO.
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This is particularly useful for specific kinds of hardware such as QSGMII,
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MoCA or other kinds of non-MDIO managed PHYs where out of band link
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information is obtained
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Ethtool operations
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------------------
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- get_strings: ethtool function used to query the driver's strings, will
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typically return statistics strings, private flags strings etc.
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- get_ethtool_stats: ethtool function used to query per-port statistics and
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return their values. DSA overlays slave network devices general statistics:
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RX/TX counters from the network device, with switch driver specific statistics
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per port
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- get_sset_count: ethtool function used to query the number of statistics items
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- get_wol: ethtool function used to obtain Wake-on-LAN settings per-port, this
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function may, for certain implementations also query the master network device
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Wake-on-LAN settings if this interface needs to participate in Wake-on-LAN
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- set_wol: ethtool function used to configure Wake-on-LAN settings per-port,
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direct counterpart to set_wol with similar restrictions
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- set_eee: ethtool function which is used to configure a switch port EEE (Green
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Ethernet) settings, can optionally invoke the PHY library to enable EEE at the
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PHY level if relevant. This function should enable EEE at the switch port MAC
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controller and data-processing logic
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- get_eee: ethtool function which is used to query a switch port EEE settings,
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this function should return the EEE state of the switch port MAC controller
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and data-processing logic as well as query the PHY for its currently configured
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EEE settings
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- get_eeprom_len: ethtool function returning for a given switch the EEPROM
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length/size in bytes
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- get_eeprom: ethtool function returning for a given switch the EEPROM contents
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- set_eeprom: ethtool function writing specified data to a given switch EEPROM
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- get_regs_len: ethtool function returning the register length for a given
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switch
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- get_regs: ethtool function returning the Ethernet switch internal register
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contents. This function might require user-land code in ethtool to
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pretty-print register values and registers
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Power management
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----------------
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- suspend: function invoked by the DSA platform device when the system goes to
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suspend, should quiesce all Ethernet switch activities, but keep ports
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participating in Wake-on-LAN active as well as additional wake-up logic if
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supported
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- resume: function invoked by the DSA platform device when the system resumes,
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should resume all Ethernet switch activities and re-configure the switch to be
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in a fully active state
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- port_enable: function invoked by the DSA slave network device ndo_open
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function when a port is administratively brought up, this function should be
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fully enabling a given switch port. DSA takes care of marking the port with
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BR_STATE_BLOCKING if the port is a bridge member, or BR_STATE_FORWARDING if it
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was not, and propagating these changes down to the hardware
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- port_disable: function invoked by the DSA slave network device ndo_close
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function when a port is administratively brought down, this function should be
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fully disabling a given switch port. DSA takes care of marking the port with
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BR_STATE_DISABLED and propagating changes to the hardware if this port is
|
|
disabled while being a bridge member
|
|
|
|
Bridge layer
|
|
------------
|
|
|
|
- port_bridge_join: bridge layer function invoked when a given switch port is
|
|
added to a bridge, this function should be doing the necessary at the switch
|
|
level to permit the joining port from being added to the relevant logical
|
|
domain for it to ingress/egress traffic with other members of the bridge.
|
|
|
|
- port_bridge_leave: bridge layer function invoked when a given switch port is
|
|
removed from a bridge, this function should be doing the necessary at the
|
|
switch level to deny the leaving port from ingress/egress traffic from the
|
|
remaining bridge members. When the port leaves the bridge, it should be aged
|
|
out at the switch hardware for the switch to (re) learn MAC addresses behind
|
|
this port.
|
|
|
|
- port_stp_state_set: bridge layer function invoked when a given switch port STP
|
|
state is computed by the bridge layer and should be propagated to switch
|
|
hardware to forward/block/learn traffic. The switch driver is responsible for
|
|
computing a STP state change based on current and asked parameters and perform
|
|
the relevant ageing based on the intersection results
|
|
|
|
Bridge VLAN filtering
|
|
---------------------
|
|
|
|
- port_vlan_filtering: bridge layer function invoked when the bridge gets
|
|
configured for turning on or off VLAN filtering. If nothing specific needs to
|
|
be done at the hardware level, this callback does not need to be implemented.
|
|
When VLAN filtering is turned on, the hardware must be programmed with
|
|
rejecting 802.1Q frames which have VLAN IDs outside of the programmed allowed
|
|
VLAN ID map/rules. If there is no PVID programmed into the switch port,
|
|
untagged frames must be rejected as well. When turned off the switch must
|
|
accept any 802.1Q frames irrespective of their VLAN ID, and untagged frames are
|
|
allowed.
|
|
|
|
- port_vlan_prepare: bridge layer function invoked when the bridge prepares the
|
|
configuration of a VLAN on the given port. If the operation is not supported
|
|
by the hardware, this function should return -EOPNOTSUPP to inform the bridge
|
|
code to fallback to a software implementation. No hardware setup must be done
|
|
in this function. See port_vlan_add for this and details.
|
|
|
|
- port_vlan_add: bridge layer function invoked when a VLAN is configured
|
|
(tagged or untagged) for the given switch port
|
|
|
|
- port_vlan_del: bridge layer function invoked when a VLAN is removed from the
|
|
given switch port
|
|
|
|
- port_vlan_dump: bridge layer function invoked with a switchdev callback
|
|
function that the driver has to call for each VLAN the given port is a member
|
|
of. A switchdev object is used to carry the VID and bridge flags.
|
|
|
|
- port_fdb_prepare: bridge layer function invoked when the bridge prepares the
|
|
installation of a Forwarding Database entry. If the operation is not
|
|
supported, this function should return -EOPNOTSUPP to inform the bridge code
|
|
to fallback to a software implementation. No hardware setup must be done in
|
|
this function. See port_fdb_add for this and details.
|
|
|
|
- port_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
|
|
|
|
- port_fdb_dump: bridge layer function invoked with a switchdev callback
|
|
function that the driver has to call for each MAC address known to be behind
|
|
the given port. A switchdev object is used to carry the VID and FDB info.
|
|
|
|
- port_mdb_prepare: bridge layer function invoked when the bridge prepares the
|
|
installation of a multicast database entry. If the operation is not supported,
|
|
this function should return -EOPNOTSUPP to inform the bridge code to fallback
|
|
to a software implementation. No hardware setup must be done in this function.
|
|
See port_fdb_add for this and details.
|
|
|
|
- port_mdb_add: bridge layer function invoked when the bridge wants to install
|
|
a multicast database entry, the switch hardware should be programmed with the
|
|
specified address in the specified VLAN ID in the forwarding database
|
|
associated with this VLAN ID.
|
|
|
|
Note: VLAN ID 0 corresponds to the port private database, which, in the context
|
|
of DSA, would be the its port-based VLAN, used by the associated bridge device.
|
|
|
|
- port_mdb_del: bridge layer function invoked when the bridge wants to remove a
|
|
multicast database entry, the switch hardware should be programmed to delete
|
|
the specified MAC address from the specified VLAN ID if it was mapped into
|
|
this port forwarding database.
|
|
|
|
- port_mdb_dump: bridge layer function invoked with a switchdev callback
|
|
function that the driver has to call for each MAC address known to be behind
|
|
the given port. A switchdev object is used to carry the VID and MDB info.
|
|
|
|
TODO
|
|
====
|
|
|
|
Making SWITCHDEV and DSA converge towards an unified codebase
|
|
-------------------------------------------------------------
|
|
|
|
SWITCHDEV properly takes care of abstracting the networking stack with offload
|
|
capable hardware, but does not enforce a strict switch device driver model. On
|
|
the other DSA enforces a fairly strict device driver model, and deals with most
|
|
of the switch specific. At some point we should envision a merger between these
|
|
two subsystems and get the best of both worlds.
|
|
|
|
Other hanging fruits
|
|
--------------------
|
|
|
|
- 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
|