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Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.

1	Distributed Switch Architecture
2	===============================
4	Introduction
5	============
7	This document describes the Distributed Switch Architecture (DSA) subsystem
8	design principles, limitations, interactions with other subsystems, and how to
9	develop drivers for this subsystem as well as a TODO for developers interested
10	in joining the effort.
12	Design principles
13	=================
15	The Distributed Switch Architecture is a subsystem which was primarily designed
16	to support Marvell Ethernet switches (MV88E6xxx, a.k.a Linkstreet product line)
17	using Linux, but has since evolved to support other vendors as well.
19	The original philosophy behind this design was to be able to use unmodified
20	Linux tools such as bridge, iproute2, ifconfig to work transparently whether
21	they configured/queried a switch port network device or a regular network
22	device.
24	An Ethernet switch is typically comprised of multiple front-panel ports, and one
25	or more CPU or management port. The DSA subsystem currently relies on the
26	presence of a management port connected to an Ethernet controller capable of
27	receiving Ethernet frames from the switch. This is a very common setup for all
28	kinds of Ethernet switches found in Small Home and Office products: routers,
29	gateways, or even top-of-the rack switches. This host Ethernet controller will
30	be later referred to as "master" and "cpu" in DSA terminology and code.
32	The D in DSA stands for Distributed, because the subsystem has been designed
33	with the ability to configure and manage cascaded switches on top of each other
34	using upstream and downstream Ethernet links between switches. These specific
35	ports are referred to as "dsa" ports in DSA terminology and code. A collection
36	of multiple switches connected to each other is called a "switch tree".
38	For each front-panel port, DSA will create specialized network devices which are
39	used as controlling and data-flowing endpoints for use by the Linux networking
40	stack. These specialized network interfaces are referred to as "slave" network
41	interfaces in DSA terminology and code.
43	The ideal case for using DSA is when an Ethernet switch supports a "switch tag"
44	which is a hardware feature making the switch insert a specific tag for each
45	Ethernet frames it received to/from specific ports to help the management
46	interface figure out:
48	- what port is this frame coming from
49	- what was the reason why this frame got forwarded
50	- how to send CPU originated traffic to specific ports
52	The subsystem does support switches not capable of inserting/stripping tags, but
53	the features might be slightly limited in that case (traffic separation relies
54	on Port-based VLAN IDs).
56	Note that DSA does not currently create network interfaces for the "cpu" and
57	"dsa" ports because:
59	- the "cpu" port is the Ethernet switch facing side of the management
60	  controller, and as such, would create a duplication of feature, since you
61	  would get two interfaces for the same conduit: master netdev, and "cpu" netdev
63	- the "dsa" port(s) are just conduits between two or more switches, and as such
64	  cannot really be used as proper network interfaces either, only the
65	  downstream, or the top-most upstream interface makes sense with that model
67	Switch tagging protocols
68	------------------------
70	DSA currently supports 5 different tagging protocols, and a tag-less mode as
71	well. The different protocols are implemented in:
73	net/dsa/tag_trailer.c: Marvell's 4 trailer tag mode (legacy)
74	net/dsa/tag_dsa.c: Marvell's original DSA tag
75	net/dsa/tag_edsa.c: Marvell's enhanced DSA tag
76	net/dsa/tag_brcm.c: Broadcom's 4 bytes tag
77	net/dsa/tag_qca.c: Qualcomm's 2 bytes tag
79	The exact format of the tag protocol is vendor specific, but in general, they
80	all contain something which:
82	- identifies which port the Ethernet frame came from/should be sent to
83	- provides a reason why this frame was forwarded to the management interface
85	Master network devices
86	----------------------
88	Master network devices are regular, unmodified Linux network device drivers for
89	the CPU/management Ethernet interface. Such a driver might occasionally need to
90	know whether DSA is enabled (e.g.: to enable/disable specific offload features),
91	but the DSA subsystem has been proven to work with industry standard drivers:
92	e1000e, mv643xx_eth etc. without having to introduce modifications to these
93	drivers. Such network devices are also often referred to as conduit network
94	devices since they act as a pipe between the host processor and the hardware
95	Ethernet switch.
97	Networking stack hooks
98	----------------------
100	When a master netdev is used with DSA, a small hook is placed in in the
101	networking stack is in order to have the DSA subsystem process the Ethernet
102	switch specific tagging protocol. DSA accomplishes this by registering a
103	specific (and fake) Ethernet type (later becoming skb->protocol) with the
104	networking stack, this is also known as a ptype or packet_type. A typical
105	Ethernet Frame receive sequence looks like this:
107	Master network device (e.g.: e1000e):
109	Receive interrupt fires:
110	- receive function is invoked
111	- basic packet processing is done: getting length, status etc.
112	- packet is prepared to be processed by the Ethernet layer by calling
113	  eth_type_trans
115	net/ethernet/eth.c:
117	eth_type_trans(skb, dev)
118		if (dev->dsa_ptr != NULL)
119			-> skb->protocol = ETH_P_XDSA
121	drivers/net/ethernet/*:
123	netif_receive_skb(skb)
124		-> iterate over registered packet_type
125			-> invoke handler for ETH_P_XDSA, calls dsa_switch_rcv()
127	net/dsa/dsa.c:
128		-> dsa_switch_rcv()
129			-> invoke switch tag specific protocol handler in
130			   net/dsa/tag_*.c
132	net/dsa/tag_*.c:
133		-> inspect and strip switch tag protocol to determine originating port
134		-> locate per-port network device
135		-> invoke eth_type_trans() with the DSA slave network device
136		-> invoked netif_receive_skb()
138	Past this point, the DSA slave network devices get delivered regular Ethernet
139	frames that can be processed by the networking stack.
141	Slave network devices
142	---------------------
144	Slave network devices created by DSA are stacked on top of their master network
145	device, each of these network interfaces will be responsible for being a
146	controlling and data-flowing end-point for each front-panel port of the switch.
147	These interfaces are specialized in order to:
149	- insert/remove the switch tag protocol (if it exists) when sending traffic
150	  to/from specific switch ports
151	- query the switch for ethtool operations: statistics, link state,
152	  Wake-on-LAN, register dumps...
153	- external/internal PHY management: link, auto-negotiation etc.
155	These slave network devices have custom net_device_ops and ethtool_ops function
156	pointers which allow DSA to introduce a level of layering between the networking
157	stack/ethtool, and the switch driver implementation.
159	Upon frame transmission from these slave network devices, DSA will look up which
160	switch tagging protocol is currently registered with these network devices, and
161	invoke a specific transmit routine which takes care of adding the relevant
162	switch tag in the Ethernet frames.
164	These frames are then queued for transmission using the master network device
165	ndo_start_xmit() function, since they contain the appropriate switch tag, the
166	Ethernet switch will be able to process these incoming frames from the
167	management interface and delivers these frames to the physical switch port.
169	Graphical representation
170	------------------------
172	Summarized, this is basically how DSA looks like from a network device
173	perspective:
176				|---------------------------
177				| CPU network device (eth0)|
178				----------------------------
179				| <tag added by switch     |
180				|                          |
181				|                          |
182				|        tag added by CPU> |
183			|--------------------------------------------|
184			| Switch driver				     |
185			|--------------------------------------------|
186	                    ||        ||         ||
187			|-------|  |-------|  |-------|
188			| sw0p0 |  | sw0p1 |  | sw0p2 |
189			|-------|  |-------|  |-------|
191	Slave MDIO bus
192	--------------
194	In order to be able to read to/from a switch PHY built into it, DSA creates a
195	slave MDIO bus which allows a specific switch driver to divert and intercept
196	MDIO reads/writes towards specific PHY addresses. In most MDIO-connected
197	switches, these functions would utilize direct or indirect PHY addressing mode
198	to return standard MII registers from the switch builtin PHYs, allowing the PHY
199	library and/or to return link status, link partner pages, auto-negotiation
200	results etc..
202	For Ethernet switches which have both external and internal MDIO busses, the
203	slave MII bus can be utilized to mux/demux MDIO reads and writes towards either
204	internal or external MDIO devices this switch might be connected to: internal
205	PHYs, external PHYs, or even external switches.
207	Data structures
208	---------------
210	DSA data structures are defined in include/net/dsa.h as well as
211	net/dsa/dsa_priv.h.
213	dsa_chip_data: platform data configuration for a given switch device, this
214	structure describes a switch device's parent device, its address, as well as
215	various properties of its ports: names/labels, and finally a routing table
216	indication (when cascading switches)
218	dsa_platform_data: platform device configuration data which can reference a
219	collection of dsa_chip_data structure if multiples switches are cascaded, the
220	master network device this switch tree is attached to needs to be referenced
222	dsa_switch_tree: structure assigned to the master network device under
223	"dsa_ptr", this structure references a dsa_platform_data structure as well as
224	the tagging protocol supported by the switch tree, and which receive/transmit
225	function hooks should be invoked, information about the directly attached switch
226	is also provided: CPU port. Finally, a collection of dsa_switch are referenced
227	to address individual switches in the tree.
229	dsa_switch: structure describing a switch device in the tree, referencing a
230	dsa_switch_tree as a backpointer, slave network devices, master network device,
231	and a reference to the backing dsa_switch_ops
233	dsa_switch_ops: structure referencing function pointers, see below for a full
234	description.
236	Design limitations
237	==================
239	DSA is a platform device driver
240	-------------------------------
242	DSA is implemented as a DSA platform device driver which is convenient because
243	it will register the entire DSA switch tree attached to a master network device
244	in one-shot, facilitating the device creation and simplifying the device driver
245	model a bit, this comes however with a number of limitations:
247	- building DSA and its switch drivers as modules is currently not working
248	- the device driver parenting does not necessarily reflect the original
249	  bus/device the switch can be created from
250	- supporting non-MDIO and non-MMIO (platform) switches is not possible
252	Limits on the number of devices and ports
253	-----------------------------------------
255	DSA currently limits the number of maximum switches within a tree to 4
256	(DSA_MAX_SWITCHES), and the number of ports per switch to 12 (DSA_MAX_PORTS).
257	These limits could be extended to support larger configurations would this need
258	arise.
260	Lack of CPU/DSA network devices
261	-------------------------------
263	DSA does not currently create slave network devices for the CPU or DSA ports, as
264	described before. This might be an issue in the following cases:
266	- inability to fetch switch CPU port statistics counters using ethtool, which
267	  can make it harder to debug MDIO switch connected using xMII interfaces
269	- inability to configure the CPU port link parameters based on the Ethernet
270	  controller capabilities attached to it: http://patchwork.ozlabs.org/patch/509806/
272	- inability to configure specific VLAN IDs / trunking VLANs between switches
273	  when using a cascaded setup
275	Common pitfalls using DSA setups
276	--------------------------------
278	Once a master network device is configured to use DSA (dev->dsa_ptr becomes
279	non-NULL), and the switch behind it expects a tagging protocol, this network
280	interface can only exclusively be used as a conduit interface. Sending packets
281	directly through this interface (e.g.: opening a socket using this interface)
282	will not make us go through the switch tagging protocol transmit function, so
283	the Ethernet switch on the other end, expecting a tag will typically drop this
284	frame.
286	Slave network devices check that the master network device is UP before allowing
287	you to administratively bring UP these slave network devices. A common
288	configuration mistake is forgetting to bring UP the master network device first.
290	Interactions with other subsystems
291	==================================
293	DSA currently leverages the following subsystems:
295	- MDIO/PHY library: drivers/net/phy/phy.c, mdio_bus.c
296	- Switchdev: net/switchdev/*
297	- Device Tree for various of_* functions
299	MDIO/PHY library
300	----------------
302	Slave network devices exposed by DSA may or may not be interfacing with PHY
303	devices (struct phy_device as defined in include/linux/phy.h), but the DSA
304	subsystem deals with all possible combinations:
306	- internal PHY devices, built into the Ethernet switch hardware
307	- external PHY devices, connected via an internal or external MDIO bus
308	- internal PHY devices, connected via an internal MDIO bus
309	- special, non-autonegotiated or non MDIO-managed PHY devices: SFPs, MoCA; a.k.a
310	  fixed PHYs
312	The PHY configuration is done by the dsa_slave_phy_setup() function and the
313	logic basically looks like this:
315	- if Device Tree is used, the PHY device is looked up using the standard
316	  "phy-handle" property, if found, this PHY device is created and registered
317	  using of_phy_connect()
319	- if Device Tree is used, and the PHY device is "fixed", that is, conforms to
320	  the definition of a non-MDIO managed PHY as defined in
321	  Documentation/devicetree/bindings/net/fixed-link.txt, the PHY is registered
322	  and connected transparently using the special fixed MDIO bus driver
324	- finally, if the PHY is built into the switch, as is very common with
325	  standalone switch packages, the PHY is probed using the slave MII bus created
326	  by DSA
330	---------
332	DSA directly utilizes SWITCHDEV when interfacing with the bridge layer, and
333	more specifically with its VLAN filtering portion when configuring VLANs on top
334	of per-port slave network devices. Since DSA primarily deals with
335	MDIO-connected switches, although not exclusively, SWITCHDEV's
336	prepare/abort/commit phases are often simplified into a prepare phase which
337	checks whether the operation is supported by the DSA switch driver, and a commit
338	phase which applies the changes.
340	As of today, the only SWITCHDEV objects supported by DSA are the FDB and VLAN
341	objects.
343	Device Tree
344	-----------
346	DSA features a standardized binding which is documented in
347	Documentation/devicetree/bindings/net/dsa/dsa.txt. PHY/MDIO library helper
348	functions such as of_get_phy_mode(), of_phy_connect() are also used to query
349	per-port PHY specific details: interface connection, MDIO bus location etc..
351	Driver development
352	==================
354	DSA switch drivers need to implement a dsa_switch_ops structure which will
355	contain the various members described below.
357	register_switch_driver() registers this dsa_switch_ops in its internal list
358	of drivers to probe for. unregister_switch_driver() does the exact opposite.
360	Unless requested differently by setting the priv_size member accordingly, DSA
361	does not allocate any driver private context space.
363	Switch configuration
364	--------------------
366	- tag_protocol: this is to indicate what kind of tagging protocol is supported,
367	  should be a valid value from the dsa_tag_protocol enum
369	- probe: probe routine which will be invoked by the DSA platform device upon
370	  registration to test for the presence/absence of a switch device. For MDIO
371	  devices, it is recommended to issue a read towards internal registers using
372	  the switch pseudo-PHY and return whether this is a supported device. For other
373	  buses, return a non-NULL string
375	- setup: setup function for the switch, this function is responsible for setting
376	  up the dsa_switch_ops private structure with all it needs: register maps,
377	  interrupts, mutexes, locks etc.. This function is also expected to properly
378	  configure the switch to separate all network interfaces from each other, that
379	  is, they should be isolated by the switch hardware itself, typically by creating
380	  a Port-based VLAN ID for each port and allowing only the CPU port and the
381	  specific port to be in the forwarding vector. Ports that are unused by the
382	  platform should be disabled. Past this function, the switch is expected to be
383	  fully configured and ready to serve any kind of request. It is recommended
384	  to issue a software reset of the switch during this setup function in order to
385	  avoid relying on what a previous software agent such as a bootloader/firmware
386	  may have previously configured.
388	PHY devices and link management
389	-------------------------------
391	- get_phy_flags: Some switches are interfaced to various kinds of Ethernet PHYs,
392	  if the PHY library PHY driver needs to know about information it cannot obtain
393	  on its own (e.g.: coming from switch memory mapped registers), this function
394	  should return a 32-bits bitmask of "flags", that is private between the switch
395	  driver and the Ethernet PHY driver in drivers/net/phy/*.
397	- phy_read: Function invoked by the DSA slave MDIO bus when attempting to read
398	  the switch port MDIO registers. If unavailable, return 0xffff for each read.
399	  For builtin switch Ethernet PHYs, this function should allow reading the link
400	  status, auto-negotiation results, link partner pages etc..
402	- phy_write: Function invoked by the DSA slave MDIO bus when attempting to write
403	  to the switch port MDIO registers. If unavailable return a negative error
404	  code.
406	- adjust_link: Function invoked by the PHY library when a slave network device
407	  is attached to a PHY device. This function is responsible for appropriately
408	  configuring the switch port link parameters: speed, duplex, pause based on
409	  what the phy_device is providing.
411	- fixed_link_update: Function invoked by the PHY library, and specifically by
412	  the fixed PHY driver asking the switch driver for link parameters that could
413	  not be auto-negotiated, or obtained by reading the PHY registers through MDIO.
414	  This is particularly useful for specific kinds of hardware such as QSGMII,
415	  MoCA or other kinds of non-MDIO managed PHYs where out of band link
416	  information is obtained
418	Ethtool operations
419	------------------
421	- get_strings: ethtool function used to query the driver's strings, will
422	  typically return statistics strings, private flags strings etc.
424	- get_ethtool_stats: ethtool function used to query per-port statistics and
425	  return their values. DSA overlays slave network devices general statistics:
426	  RX/TX counters from the network device, with switch driver specific statistics
427	  per port
429	- get_sset_count: ethtool function used to query the number of statistics items
431	- get_wol: ethtool function used to obtain Wake-on-LAN settings per-port, this
432	  function may, for certain implementations also query the master network device
433	  Wake-on-LAN settings if this interface needs to participate in Wake-on-LAN
435	- set_wol: ethtool function used to configure Wake-on-LAN settings per-port,
436	  direct counterpart to set_wol with similar restrictions
438	- set_eee: ethtool function which is used to configure a switch port EEE (Green
439	  Ethernet) settings, can optionally invoke the PHY library to enable EEE at the
440	  PHY level if relevant. This function should enable EEE at the switch port MAC
441	  controller and data-processing logic
443	- get_eee: ethtool function which is used to query a switch port EEE settings,
444	  this function should return the EEE state of the switch port MAC controller
445	  and data-processing logic as well as query the PHY for its currently configured
446	  EEE settings
448	- get_eeprom_len: ethtool function returning for a given switch the EEPROM
449	  length/size in bytes
451	- get_eeprom: ethtool function returning for a given switch the EEPROM contents
453	- set_eeprom: ethtool function writing specified data to a given switch EEPROM
455	- get_regs_len: ethtool function returning the register length for a given
456	  switch
458	- get_regs: ethtool function returning the Ethernet switch internal register
459	  contents. This function might require user-land code in ethtool to
460	  pretty-print register values and registers
462	Power management
463	----------------
465	- suspend: function invoked by the DSA platform device when the system goes to
466	  suspend, should quiesce all Ethernet switch activities, but keep ports
467	  participating in Wake-on-LAN active as well as additional wake-up logic if
468	  supported
470	- resume: function invoked by the DSA platform device when the system resumes,
471	  should resume all Ethernet switch activities and re-configure the switch to be
472	  in a fully active state
474	- port_enable: function invoked by the DSA slave network device ndo_open
475	  function when a port is administratively brought up, this function should be
476	  fully enabling a given switch port. DSA takes care of marking the port with
477	  BR_STATE_BLOCKING if the port is a bridge member, or BR_STATE_FORWARDING if it
478	  was not, and propagating these changes down to the hardware
480	- port_disable: function invoked by the DSA slave network device ndo_close
481	  function when a port is administratively brought down, this function should be
482	  fully disabling a given switch port. DSA takes care of marking the port with
483	  BR_STATE_DISABLED and propagating changes to the hardware if this port is
484	  disabled while being a bridge member
486	Bridge layer
487	------------
489	- port_bridge_join: bridge layer function invoked when a given switch port is
490	  added to a bridge, this function should be doing the necessary at the switch
491	  level to permit the joining port from being added to the relevant logical
492	  domain for it to ingress/egress traffic with other members of the bridge.
494	- port_bridge_leave: bridge layer function invoked when a given switch port is
495	  removed from a bridge, this function should be doing the necessary at the
496	  switch level to deny the leaving port from ingress/egress traffic from the
497	  remaining bridge members. When the port leaves the bridge, it should be aged
498	  out at the switch hardware for the switch to (re) learn MAC addresses behind
499	  this port.
501	- port_stp_state_set: bridge layer function invoked when a given switch port STP
502	  state is computed by the bridge layer and should be propagated to switch
503	  hardware to forward/block/learn traffic. The switch driver is responsible for
504	  computing a STP state change based on current and asked parameters and perform
505	  the relevant ageing based on the intersection results
507	Bridge VLAN filtering
508	---------------------
510	- port_vlan_filtering: bridge layer function invoked when the bridge gets
511	  configured for turning on or off VLAN filtering. If nothing specific needs to
512	  be done at the hardware level, this callback does not need to be implemented.
513	  When VLAN filtering is turned on, the hardware must be programmed with
514	  rejecting 802.1Q frames which have VLAN IDs outside of the programmed allowed
515	  VLAN ID map/rules.  If there is no PVID programmed into the switch port,
516	  untagged frames must be rejected as well. When turned off the switch must
517	  accept any 802.1Q frames irrespective of their VLAN ID, and untagged frames are
518	  allowed.
520	- port_vlan_prepare: bridge layer function invoked when the bridge prepares the
521	  configuration of a VLAN on the given port. If the operation is not supported
522	  by the hardware, this function should return -EOPNOTSUPP to inform the bridge
523	  code to fallback to a software implementation. No hardware setup must be done
524	  in this function. See port_vlan_add for this and details.
526	- port_vlan_add: bridge layer function invoked when a VLAN is configured
527	  (tagged or untagged) for the given switch port
529	- port_vlan_del: bridge layer function invoked when a VLAN is removed from the
530	  given switch port
532	- port_vlan_dump: bridge layer function invoked with a switchdev callback
533	  function that the driver has to call for each VLAN the given port is a member
534	  of. A switchdev object is used to carry the VID and bridge flags.
536	- port_fdb_prepare: bridge layer function invoked when the bridge prepares the
537	  installation of a Forwarding Database entry. If the operation is not
538	  supported, this function should return -EOPNOTSUPP to inform the bridge code
539	  to fallback to a software implementation. No hardware setup must be done in
540	  this function. See port_fdb_add for this and details.
542	- port_fdb_add: bridge layer function invoked when the bridge wants to install a
543	  Forwarding Database entry, the switch hardware should be programmed with the
544	  specified address in the specified VLAN Id in the forwarding database
545	  associated with this VLAN ID
547	Note: VLAN ID 0 corresponds to the port private database, which, in the context
548	of DSA, would be the its port-based VLAN, used by the associated bridge device.
550	- port_fdb_del: bridge layer function invoked when the bridge wants to remove a
551	  Forwarding Database entry, the switch hardware should be programmed to delete
552	  the specified MAC address from the specified VLAN ID if it was mapped into
553	  this port forwarding database
555	- port_fdb_dump: bridge layer function invoked with a switchdev callback
556	  function that the driver has to call for each MAC address known to be behind
557	  the given port. A switchdev object is used to carry the VID and FDB info.
559	- port_mdb_prepare: bridge layer function invoked when the bridge prepares the
560	  installation of a multicast database entry. If the operation is not supported,
561	  this function should return -EOPNOTSUPP to inform the bridge code to fallback
562	  to a software implementation. No hardware setup must be done in this function.
563	  See port_fdb_add for this and details.
565	- port_mdb_add: bridge layer function invoked when the bridge wants to install
566	  a multicast database entry, the switch hardware should be programmed with the
567	  specified address in the specified VLAN ID in the forwarding database
568	  associated with this VLAN ID.
570	Note: VLAN ID 0 corresponds to the port private database, which, in the context
571	of DSA, would be the its port-based VLAN, used by the associated bridge device.
573	- port_mdb_del: bridge layer function invoked when the bridge wants to remove a
574	  multicast database entry, the switch hardware should be programmed to delete
575	  the specified MAC address from the specified VLAN ID if it was mapped into
576	  this port forwarding database.
578	- port_mdb_dump: bridge layer function invoked with a switchdev callback
579	  function that the driver has to call for each MAC address known to be behind
580	  the given port. A switchdev object is used to carry the VID and MDB info.
582	TODO
583	====
585	Making SWITCHDEV and DSA converge towards an unified codebase
586	-------------------------------------------------------------
588	SWITCHDEV properly takes care of abstracting the networking stack with offload
589	capable hardware, but does not enforce a strict switch device driver model. On
590	the other DSA enforces a fairly strict device driver model, and deals with most
591	of the switch specific. At some point we should envision a merger between these
592	two subsystems and get the best of both worlds.
594	Other hanging fruits
595	--------------------
597	- making the number of ports fully dynamic and not dependent on DSA_MAX_PORTS
598	- allowing more than one CPU/management interface:
599	  http://comments.gmane.org/gmane.linux.network/365657
600	- porting more drivers from other vendors:
601	  http://comments.gmane.org/gmane.linux.network/365510
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