Subject: Re: [PATCH net-next v2 2/2] Documentation: networking: dsa: Add Broadcom SF2 document

From: Randy Dunlap <[email-masked]>
Date: Tue, 25 Aug 2015 11:47:18 -0700

On 08/25/15 10:40, Florian Fainelli wrote:
> Add a document describing the Broadcom Starfigther 2 switch hardware,
> its specifics, and how the driver is implemented and its specifics.
> 
> Signed-off-by: Florian Fainelli <[email-masked]>
> ---
>  Documentation/networking/dsa/bcm_sf2.txt | 113 +++++++++++++++++++++++++++++++
>  1 file changed, 113 insertions(+)
>  create mode 100644 Documentation/networking/dsa/bcm_sf2.txt
> 
> diff --git a/Documentation/networking/dsa/bcm_sf2.txt b/Documentation/networking/dsa/bcm_sf2.txt
> new file mode 100644
> index 000000000000..7b1502bb707d
> --- /dev/null
> +++ b/Documentation/networking/dsa/bcm_sf2.txt
> @@ -0,0 +1,113 @@
> +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 gatewasy 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

what is MoCA?  Maybe answer in "MoCA interfaces" section.

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

                    lose

> +band back-pressure to the host CPU network interface when downstream interfaces
> +are connected at a lower speed.
> +
> +The switch hardware block is typically interfaces using MMIO accesses and

                                          interfaced
or drop the "is"

> +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_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

s/,/;/

> +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

                         s/,/;/ here.....................^

> +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.
> +
> +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.

-- 
~Randy