Documentation: networking: dsa: Add Broadcom SF2 document

Broadcom Starfighter 2 Ethernet switch driver

=============================================

Broadcom's Starfighter 2 Ethernet switch hardware block is commonly found and

deployed in the following products:

- xDSL gateways such as BCM63138

- streaming/multimedia Set Top Box such as BCM7445

- Cable Modem/residential gateways such as BCM7145/BCM3390

The switch is typically deployed in a configuration involving between 5 to 13

ports, offering a range of built-in and customizable interfaces:

- single integrated Gigabit PHY

- quad integrated Gigabit PHY

- quad external Gigabit PHY w/ MDIO multiplexer

- integrated MoCA PHY

- several external MII/RevMII/GMII/RGMII interfaces

The switch also supports specific congestion control features which allow MoCA

fail-over not to lose packets during a MoCA role re-election, as well as out of

band back-pressure to the host CPU network interface when downstream interfaces

are connected at a lower speed.

The switch hardware block is typically interfaced using MMIO accesses and

contains a bunch of sub-blocks/registers:

* SWITCH_CORE: common switch registers

* SWITCH_REG: external interfaces switch register

* SWITCH_MDIO: external MDIO bus controller (there is another one in SWITCH_CORE,

  which is used for indirect PHY accesses)

* SWITCH_INDIR_RW: 64-bits wide register helper block

* SWITCH_INTRL2_0/1: Level-2 interrupt controllers

* SWITCH_ACB: Admission control block

* SWITCH_FCB: Fail-over control block

Implementation details

======================

The driver is located in drivers/net/dsa/bcm_sf2.c and is implemented as a DSA

driver; see Documentation/networking/dsa/dsa.txt for details on the subsytem

and what it provides.

The SF2 switch is configured to enable a Broadcom specific 4-bytes switch tag

which gets inserted by the switch for every packet forwarded to the CPU

interface, conversely, the CPU network interface should insert a similar tag for

packets entering the CPU port. The tag format is described in

net/dsa/tag_brcm.c.

Overall, the SF2 driver is a fairly regular DSA driver; there are a few

specifics covered below.

Device Tree probing

-------------------

The DSA platform device driver is probed using a specific compatible string

provided in net/dsa/dsa.c. The reason for that is because the DSA subsystem gets

registered as a platform device driver currently. DSA will provide the needed

device_node pointers which are then accessible by the switch driver setup

function to setup resources such as register ranges and interrupts. This

currently works very well because none of the of_* functions utilized by the

driver require a struct device to be bound to a struct device_node, but things

may change in the future.

MDIO indirect accesses

----------------------

Due to a limitation in how Broadcom switches have been designed, external

Broadcom switches connected to a SF2 require the use of the DSA slave MDIO bus

in order to properly configure them. By default, the SF2 pseudo-PHY address, and

an external switch pseudo-PHY address will both be snooping for incoming MDIO

transactions, since they are at the same address (30), resulting in some kind of

"double" programming. Using DSA, and setting ds->phys_mii_mask accordingly, we

selectively divert reads and writes towards external Broadcom switches

pseudo-PHY addresses. Newer revisions of the SF2 hardware have introduced a

configurable pseudo-PHY address which circumvents the initial design limitation.

Multimedia over CoAxial (MoCA) interfaces

-----------------------------------------

MoCA interfaces are fairly specific and require the use of a firmware blob which

gets loaded onto the MoCA processor(s) for packet processing. The switch

hardware contains logic which will assert/de-assert link states accordingly for

the MoCA interface whenever the MoCA coaxial cable gets disconnected or the

firmware gets reloaded. The SF2 driver relies on such events to properly set its

MoCA interface carrier state and properly report this to the networking stack.

The MoCA interfaces are supported using the PHY library's fixed PHY/emulated PHY

device and the switch driver registers a fixed_link_update callback for such

PHYs which reflects the link state obtained from the interrupt handler.

Power Management

----------------

Whenever possible, the SF2 driver tries to minimize the overall switch power

consumption by applying a combination of:

- turning off internal buffers/memories

- disabling packet processing logic

- putting integrated PHYs in IDDQ/low-power

- reducing the switch core clock based on the active port count

- enabling and advertising EEE

- turning off RGMII data processing logic when the link goes down

Wake-on-LAN

-----------

Wake-on-LAN is currently implemented by utilizing the host processor Ethernet

MAC controller wake-on logic. Whenever Wake-on-LAN is requested, an intersection

between the user request and the supported host Ethernet interface WoL

capabilities is done and the intersection result gets configured. During

system-wide suspend/resume, only ports not participating in Wake-on-LAN are

disabled.