bus: Add support for Moxtet bus

On the Turris Mox router different modules can be connected to the main
CPU board: currently a module with a SFP cage, a module with MiniPCIe
connector, a PCIe pass-through MiniPCIe connector module, a 4-port
switch module, an 8-port switch module, and a 4-port USB3 module.

For example:
  [CPU]-[PCIe-pass-through]-[PCIe]-[8-port switch]-[8-port switch]-[SFP]

Each of this modules has an input and output shift register, and these
are connected via SPI to the CPU board.

Via SPI we are able to discover which modules are connected, in which
order, and we can also read some information about the modules (eg.
their interrupt status), and configure them.
From each module 8 bits can be read (of which low 4 bits identify the
module) and 8 bits can be written.

For example from the module with a SFP cage we can read the LOS,
TX-FAULT and MOD-DEF0 signals, while we can write TX-DISABLE and
RATE-SELECT signals.

This driver creates a new bus type, called "moxtet". For each Mox module
it finds via SPI, it creates a new device on the moxtet bus so that
drivers can be written for them.

It also implements a virtual interrupt controller for the modules which
send their interrupt status over the SPI shift register. These modules
do this in addition to sending their interrupt status via the shared
interrupt line. When the shared interrupt is triggered, we read from the
shift register and handle IRQs for all devices which are in interrupt.

The topology of how Mox modules are connected can then be read by
listing /sys/bus/moxtet/devices.

Link: https://lore.kernel.org/r/20190812161118.21476-2-marek.behun@nic.cz
Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

MAINTAINERS

@@ -1626,6 +1626,13 @@ F:	drivers/clocksource/timer-atlas7.c
 N:	[^a-z]sirf
 X:	drivers/gnss
 
+ARM/CZ.NIC TURRIS MOX SUPPORT
+M:	Marek Behun <marek.behun@nic.cz>
+W:	http://mox.turris.cz
+S:	Maintained
+F:	include/linux/moxtet.h
+F:	drivers/bus/moxtet.c
+
 ARM/EBSA110 MACHINE SUPPORT
 M:	Russell King <linux@armlinux.org.uk>
 L:	linux-arm-kernel@lists.infradead.org (moderated for non-subscribers)

drivers/bus/Kconfig

@@ -29,6 +29,16 @@ config BRCMSTB_GISB_ARB
 	  arbiter. This driver provides timeout and target abort error handling
 	  and internal bus master decoding.
 
+config MOXTET
+	tristate "CZ.NIC Turris Mox module configuration bus"
+	depends on SPI_MASTER && OF
+	help
+	  Say yes here to add support for the module configuration bus found
+	  on CZ.NIC's Turris Mox. This is needed for the ability to discover
+	  the order in which the modules are connected and to get/set some of
+	  their settings. For example the GPIOs on Mox SFP module are
+	  configured through this bus.
+
 config HISILICON_LPC
 	bool "Support for ISA I/O space on HiSilicon Hip06/7"
 	depends on ARM64 && (ARCH_HISI || COMPILE_TEST)

drivers/bus/Makefile

@@ -8,6 +8,7 @@ obj-$(CONFIG_ARM_CCI)		+= arm-cci.o
 
 obj-$(CONFIG_HISILICON_LPC)	+= hisi_lpc.o
 obj-$(CONFIG_BRCMSTB_GISB_ARB)	+= brcmstb_gisb.o
+obj-$(CONFIG_MOXTET)		+= moxtet.o
 
 # DPAA2 fsl-mc bus
 obj-$(CONFIG_FSL_MC_BUS)	+= fsl-mc/