Date:	Tue, 25 Aug 2015 06:50:42 -0700
From:	Florian Fainelli <f.fainelli@...il.com>
To:	netdev@...r.kernel.org
Cc:	andrew@...n.ch, vivien.didelot@...oirfairelinux.com,
	sfeldma@...il.com, linux@...ck-us.net, davem@...emloft.net,
	Florian Fainelli <f.fainelli@...il.com>
Subject: [PATCH net-next 2/2] Documentation: networking: dsa: Add Broadcom SF2 document

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 <f.fainelli@...il.com>
---
 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 0000000..7b1502b
--- /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
+fail-over not to loose 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 interfaces 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_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.
+
+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.
-- 
2.1.4

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