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Based on kernel version 4.9. Page generated on 2016-12-21 14:36 EST.

1	Distributed Switch Architecture
2	===============================
3	
4	Introduction
5	============
6	
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.
11	
12	Design principles
13	=================
14	
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.
18	
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.
23	
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.
31	
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".
37	
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.
42	
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:
47	
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
51	
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).
55	
56	Note that DSA does not currently create network interfaces for the "cpu" and
57	"dsa" ports because:
58	
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
62	
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
66	
67	Switch tagging protocols
68	------------------------
69	
70	DSA currently supports 5 different tagging protocols, and a tag-less mode as
71	well. The different protocols are implemented in:
72	
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
78	
79	The exact format of the tag protocol is vendor specific, but in general, they
80	all contain something which:
81	
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
84	
85	Master network devices
86	----------------------
87	
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.
96	
97	Networking stack hooks
98	----------------------
99	
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:
106	
107	Master network device (e.g.: e1000e):
108	
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
114	
115	net/ethernet/eth.c:
116	
117	eth_type_trans(skb, dev)
118		if (dev->dsa_ptr != NULL)
119			-> skb->protocol = ETH_P_XDSA
120	
121	drivers/net/ethernet/*:
122	
123	netif_receive_skb(skb)
124		-> iterate over registered packet_type
125			-> invoke handler for ETH_P_XDSA, calls dsa_switch_rcv()
126	
127	net/dsa/dsa.c:
128		-> dsa_switch_rcv()
129			-> invoke switch tag specific protocol handler in
130			   net/dsa/tag_*.c
131	
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()
137	
138	Past this point, the DSA slave network devices get delivered regular Ethernet
139	frames that can be processed by the networking stack.
140	
141	Slave network devices
142	---------------------
143	
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:
148	
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.
154	
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.
158	
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.
163	
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.
168	
169	Graphical representation
170	------------------------
171	
172	Summarized, this is basically how DSA looks like from a network device
173	perspective:
174	
175	
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			|-------|  |-------|  |-------|
190	
191	Slave MDIO bus
192	--------------
193	
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..
201	
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.
206	
207	Data structures
208	---------------
209	
210	DSA data structures are defined in include/net/dsa.h as well as
211	net/dsa/dsa_priv.h.
212	
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)
217	
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
221	
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.
228	
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
232	
233	dsa_switch_ops: structure referencing function pointers, see below for a full
234	description.
235	
236	Design limitations
237	==================
238	
239	DSA is a platform device driver
240	-------------------------------
241	
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:
246	
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
251	
252	Limits on the number of devices and ports
253	-----------------------------------------
254	
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.
259	
260	Lack of CPU/DSA network devices
261	-------------------------------
262	
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:
265	
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
268	
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/
271	
272	- inability to configure specific VLAN IDs / trunking VLANs between switches
273	  when using a cascaded setup
274	
275	Common pitfalls using DSA setups
276	--------------------------------
277	
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.
285	
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.
289	
290	Interactions with other subsystems
291	==================================
292	
293	DSA currently leverages the following subsystems:
294	
295	- MDIO/PHY library: drivers/net/phy/phy.c, mdio_bus.c
296	- Switchdev: net/switchdev/*
297	- Device Tree for various of_* functions
298	- HWMON: drivers/hwmon/*
299	
300	MDIO/PHY library
301	----------------
302	
303	Slave network devices exposed by DSA may or may not be interfacing with PHY
304	devices (struct phy_device as defined in include/linux/phy.h), but the DSA
305	subsystem deals with all possible combinations:
306	
307	- internal PHY devices, built into the Ethernet switch hardware
308	- external PHY devices, connected via an internal or external MDIO bus
309	- internal PHY devices, connected via an internal MDIO bus
310	- special, non-autonegotiated or non MDIO-managed PHY devices: SFPs, MoCA; a.k.a
311	  fixed PHYs
312	
313	The PHY configuration is done by the dsa_slave_phy_setup() function and the
314	logic basically looks like this:
315	
316	- if Device Tree is used, the PHY device is looked up using the standard
317	  "phy-handle" property, if found, this PHY device is created and registered
318	  using of_phy_connect()
319	
320	- if Device Tree is used, and the PHY device is "fixed", that is, conforms to
321	  the definition of a non-MDIO managed PHY as defined in
322	  Documentation/devicetree/bindings/net/fixed-link.txt, the PHY is registered
323	  and connected transparently using the special fixed MDIO bus driver
324	
325	- finally, if the PHY is built into the switch, as is very common with
326	  standalone switch packages, the PHY is probed using the slave MII bus created
327	  by DSA
328	
329	
330	SWITCHDEV
331	---------
332	
333	DSA directly utilizes SWITCHDEV when interfacing with the bridge layer, and
334	more specifically with its VLAN filtering portion when configuring VLANs on top
335	of per-port slave network devices. Since DSA primarily deals with
336	MDIO-connected switches, although not exclusively, SWITCHDEV's
337	prepare/abort/commit phases are often simplified into a prepare phase which
338	checks whether the operation is supported by the DSA switch driver, and a commit
339	phase which applies the changes.
340	
341	As of today, the only SWITCHDEV objects supported by DSA are the FDB and VLAN
342	objects.
343	
344	Device Tree
345	-----------
346	
347	DSA features a standardized binding which is documented in
348	Documentation/devicetree/bindings/net/dsa/dsa.txt. PHY/MDIO library helper
349	functions such as of_get_phy_mode(), of_phy_connect() are also used to query
350	per-port PHY specific details: interface connection, MDIO bus location etc..
351	
352	HWMON
353	-----
354	
355	Some switch drivers feature internal temperature sensors which are exposed as
356	regular HWMON devices in /sys/class/hwmon/.
357	
358	Driver development
359	==================
360	
361	DSA switch drivers need to implement a dsa_switch_ops structure which will
362	contain the various members described below.
363	
364	register_switch_driver() registers this dsa_switch_ops in its internal list
365	of drivers to probe for. unregister_switch_driver() does the exact opposite.
366	
367	Unless requested differently by setting the priv_size member accordingly, DSA
368	does not allocate any driver private context space.
369	
370	Switch configuration
371	--------------------
372	
373	- tag_protocol: this is to indicate what kind of tagging protocol is supported,
374	  should be a valid value from the dsa_tag_protocol enum
375	
376	- probe: probe routine which will be invoked by the DSA platform device upon
377	  registration to test for the presence/absence of a switch device. For MDIO
378	  devices, it is recommended to issue a read towards internal registers using
379	  the switch pseudo-PHY and return whether this is a supported device. For other
380	  buses, return a non-NULL string
381	
382	- setup: setup function for the switch, this function is responsible for setting
383	  up the dsa_switch_ops private structure with all it needs: register maps,
384	  interrupts, mutexes, locks etc.. This function is also expected to properly
385	  configure the switch to separate all network interfaces from each other, that
386	  is, they should be isolated by the switch hardware itself, typically by creating
387	  a Port-based VLAN ID for each port and allowing only the CPU port and the
388	  specific port to be in the forwarding vector. Ports that are unused by the
389	  platform should be disabled. Past this function, the switch is expected to be
390	  fully configured and ready to serve any kind of request. It is recommended
391	  to issue a software reset of the switch during this setup function in order to
392	  avoid relying on what a previous software agent such as a bootloader/firmware
393	  may have previously configured.
394	
395	- set_addr: Some switches require the programming of the management interface's
396	  Ethernet MAC address, switch drivers can also disable ageing of MAC addresses
397	  on the management interface and "hardcode"/"force" this MAC address for the
398	  CPU/management interface as an optimization
399	
400	PHY devices and link management
401	-------------------------------
402	
403	- get_phy_flags: Some switches are interfaced to various kinds of Ethernet PHYs,
404	  if the PHY library PHY driver needs to know about information it cannot obtain
405	  on its own (e.g.: coming from switch memory mapped registers), this function
406	  should return a 32-bits bitmask of "flags", that is private between the switch
407	  driver and the Ethernet PHY driver in drivers/net/phy/*.
408	
409	- phy_read: Function invoked by the DSA slave MDIO bus when attempting to read
410	  the switch port MDIO registers. If unavailable, return 0xffff for each read.
411	  For builtin switch Ethernet PHYs, this function should allow reading the link
412	  status, auto-negotiation results, link partner pages etc..
413	
414	- phy_write: Function invoked by the DSA slave MDIO bus when attempting to write
415	  to the switch port MDIO registers. If unavailable return a negative error
416	  code.
417	
418	- adjust_link: Function invoked by the PHY library when a slave network device
419	  is attached to a PHY device. This function is responsible for appropriately
420	  configuring the switch port link parameters: speed, duplex, pause based on
421	  what the phy_device is providing.
422	
423	- fixed_link_update: Function invoked by the PHY library, and specifically by
424	  the fixed PHY driver asking the switch driver for link parameters that could
425	  not be auto-negotiated, or obtained by reading the PHY registers through MDIO.
426	  This is particularly useful for specific kinds of hardware such as QSGMII,
427	  MoCA or other kinds of non-MDIO managed PHYs where out of band link
428	  information is obtained
429	
430	Ethtool operations
431	------------------
432	
433	- get_strings: ethtool function used to query the driver's strings, will
434	  typically return statistics strings, private flags strings etc.
435	
436	- get_ethtool_stats: ethtool function used to query per-port statistics and
437	  return their values. DSA overlays slave network devices general statistics:
438	  RX/TX counters from the network device, with switch driver specific statistics
439	  per port
440	
441	- get_sset_count: ethtool function used to query the number of statistics items
442	
443	- get_wol: ethtool function used to obtain Wake-on-LAN settings per-port, this
444	  function may, for certain implementations also query the master network device
445	  Wake-on-LAN settings if this interface needs to participate in Wake-on-LAN
446	
447	- set_wol: ethtool function used to configure Wake-on-LAN settings per-port,
448	  direct counterpart to set_wol with similar restrictions
449	
450	- set_eee: ethtool function which is used to configure a switch port EEE (Green
451	  Ethernet) settings, can optionally invoke the PHY library to enable EEE at the
452	  PHY level if relevant. This function should enable EEE at the switch port MAC
453	  controller and data-processing logic
454	
455	- get_eee: ethtool function which is used to query a switch port EEE settings,
456	  this function should return the EEE state of the switch port MAC controller
457	  and data-processing logic as well as query the PHY for its currently configured
458	  EEE settings
459	
460	- get_eeprom_len: ethtool function returning for a given switch the EEPROM
461	  length/size in bytes
462	
463	- get_eeprom: ethtool function returning for a given switch the EEPROM contents
464	
465	- set_eeprom: ethtool function writing specified data to a given switch EEPROM
466	
467	- get_regs_len: ethtool function returning the register length for a given
468	  switch
469	
470	- get_regs: ethtool function returning the Ethernet switch internal register
471	  contents. This function might require user-land code in ethtool to
472	  pretty-print register values and registers
473	
474	Power management
475	----------------
476	
477	- suspend: function invoked by the DSA platform device when the system goes to
478	  suspend, should quiesce all Ethernet switch activities, but keep ports
479	  participating in Wake-on-LAN active as well as additional wake-up logic if
480	  supported
481	
482	- resume: function invoked by the DSA platform device when the system resumes,
483	  should resume all Ethernet switch activities and re-configure the switch to be
484	  in a fully active state
485	
486	- port_enable: function invoked by the DSA slave network device ndo_open
487	  function when a port is administratively brought up, this function should be
488	  fully enabling a given switch port. DSA takes care of marking the port with
489	  BR_STATE_BLOCKING if the port is a bridge member, or BR_STATE_FORWARDING if it
490	  was not, and propagating these changes down to the hardware
491	
492	- port_disable: function invoked by the DSA slave network device ndo_close
493	  function when a port is administratively brought down, this function should be
494	  fully disabling a given switch port. DSA takes care of marking the port with
495	  BR_STATE_DISABLED and propagating changes to the hardware if this port is
496	  disabled while being a bridge member
497	
498	Hardware monitoring
499	-------------------
500	
501	These callbacks are only available if CONFIG_NET_DSA_HWMON is enabled:
502	
503	- get_temp: this function queries the given switch for its temperature
504	
505	- get_temp_limit: this function returns the switch current maximum temperature
506	  limit
507	
508	- set_temp_limit: this function configures the maximum temperature limit allowed
509	
510	- get_temp_alarm: this function returns the critical temperature threshold
511	  returning an alarm notification
512	
513	See Documentation/hwmon/sysfs-interface for details.
514	
515	Bridge layer
516	------------
517	
518	- port_bridge_join: bridge layer function invoked when a given switch port is
519	  added to a bridge, this function should be doing the necessary at the switch
520	  level to permit the joining port from being added to the relevant logical
521	  domain for it to ingress/egress traffic with other members of the bridge.
522	
523	- port_bridge_leave: bridge layer function invoked when a given switch port is
524	  removed from a bridge, this function should be doing the necessary at the
525	  switch level to deny the leaving port from ingress/egress traffic from the
526	  remaining bridge members. When the port leaves the bridge, it should be aged
527	  out at the switch hardware for the switch to (re) learn MAC addresses behind
528	  this port.
529	
530	- port_stp_state_set: bridge layer function invoked when a given switch port STP
531	  state is computed by the bridge layer and should be propagated to switch
532	  hardware to forward/block/learn traffic. The switch driver is responsible for
533	  computing a STP state change based on current and asked parameters and perform
534	  the relevant ageing based on the intersection results
535	
536	Bridge VLAN filtering
537	---------------------
538	
539	- port_vlan_filtering: bridge layer function invoked when the bridge gets
540	  configured for turning on or off VLAN filtering. If nothing specific needs to
541	  be done at the hardware level, this callback does not need to be implemented.
542	  When VLAN filtering is turned on, the hardware must be programmed with
543	  rejecting 802.1Q frames which have VLAN IDs outside of the programmed allowed
544	  VLAN ID map/rules.  If there is no PVID programmed into the switch port,
545	  untagged frames must be rejected as well. When turned off the switch must
546	  accept any 802.1Q frames irrespective of their VLAN ID, and untagged frames are
547	  allowed.
548	
549	- port_vlan_prepare: bridge layer function invoked when the bridge prepares the
550	  configuration of a VLAN on the given port. If the operation is not supported
551	  by the hardware, this function should return -EOPNOTSUPP to inform the bridge
552	  code to fallback to a software implementation. No hardware setup must be done
553	  in this function. See port_vlan_add for this and details.
554	
555	- port_vlan_add: bridge layer function invoked when a VLAN is configured
556	  (tagged or untagged) for the given switch port
557	
558	- port_vlan_del: bridge layer function invoked when a VLAN is removed from the
559	  given switch port
560	
561	- port_vlan_dump: bridge layer function invoked with a switchdev callback
562	  function that the driver has to call for each VLAN the given port is a member
563	  of. A switchdev object is used to carry the VID and bridge flags.
564	
565	- port_fdb_prepare: bridge layer function invoked when the bridge prepares the
566	  installation of a Forwarding Database entry. If the operation is not
567	  supported, this function should return -EOPNOTSUPP to inform the bridge code
568	  to fallback to a software implementation. No hardware setup must be done in
569	  this function. See port_fdb_add for this and details.
570	
571	- port_fdb_add: bridge layer function invoked when the bridge wants to install a
572	  Forwarding Database entry, the switch hardware should be programmed with the
573	  specified address in the specified VLAN Id in the forwarding database
574	  associated with this VLAN ID
575	
576	Note: VLAN ID 0 corresponds to the port private database, which, in the context
577	of DSA, would be the its port-based VLAN, used by the associated bridge device.
578	
579	- port_fdb_del: bridge layer function invoked when the bridge wants to remove a
580	  Forwarding Database entry, the switch hardware should be programmed to delete
581	  the specified MAC address from the specified VLAN ID if it was mapped into
582	  this port forwarding database
583	
584	- port_fdb_dump: bridge layer function invoked with a switchdev callback
585	  function that the driver has to call for each MAC address known to be behind
586	  the given port. A switchdev object is used to carry the VID and FDB info.
587	
588	- port_mdb_prepare: bridge layer function invoked when the bridge prepares the
589	  installation of a multicast database entry. If the operation is not supported,
590	  this function should return -EOPNOTSUPP to inform the bridge code to fallback
591	  to a software implementation. No hardware setup must be done in this function.
592	  See port_fdb_add for this and details.
593	
594	- port_mdb_add: bridge layer function invoked when the bridge wants to install
595	  a multicast database entry, the switch hardware should be programmed with the
596	  specified address in the specified VLAN ID in the forwarding database
597	  associated with this VLAN ID.
598	
599	Note: VLAN ID 0 corresponds to the port private database, which, in the context
600	of DSA, would be the its port-based VLAN, used by the associated bridge device.
601	
602	- port_mdb_del: bridge layer function invoked when the bridge wants to remove a
603	  multicast database entry, the switch hardware should be programmed to delete
604	  the specified MAC address from the specified VLAN ID if it was mapped into
605	  this port forwarding database.
606	
607	- port_mdb_dump: bridge layer function invoked with a switchdev callback
608	  function that the driver has to call for each MAC address known to be behind
609	  the given port. A switchdev object is used to carry the VID and MDB info.
610	
611	TODO
612	====
613	
614	Making SWITCHDEV and DSA converge towards an unified codebase
615	-------------------------------------------------------------
616	
617	SWITCHDEV properly takes care of abstracting the networking stack with offload
618	capable hardware, but does not enforce a strict switch device driver model. On
619	the other DSA enforces a fairly strict device driver model, and deals with most
620	of the switch specific. At some point we should envision a merger between these
621	two subsystems and get the best of both worlds.
622	
623	Other hanging fruits
624	--------------------
625	
626	- making the number of ports fully dynamic and not dependent on DSA_MAX_PORTS
627	- allowing more than one CPU/management interface:
628	  http://comments.gmane.org/gmane.linux.network/365657
629	- porting more drivers from other vendors:
630	  http://comments.gmane.org/gmane.linux.network/365510
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