soc: aspeed: Add UART routing support

Add driver support for the UART routing control. Users can perform
runtime configuration of the RX muxes among the UART controllers and
the UART IO pins.

The sysfs interface is also exported for the convenience of routing paths
check and update.

Signed-off-by: Oskar Senft <[email protected]>
Signed-off-by: Chia-Wei Wang <[email protected]>
Signed-off-by: Joel Stanley <[email protected]>
Tested-by: Lei YU <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Link: https://lore.kernel.org/r/[email protected]'
Signed-off-by: Arnd Bergmann <[email protected]>

Author: Chia-Wei Wang

Documentation/ABI/testing/sysfs-driver-aspeed-uart-routing

@@ -0,0 +1,27 @@
+What:		/sys/bus/platform/drivers/aspeed-uart-routing/*/uart*
+Date:		September 2021
+Contact:	Oskar Senft <[email protected]>
+		Chia-Wei Wang <[email protected]>
+Description:	Selects the RX source of the UARTx device.
+
+		When read, each file shows the list of available options with currently
+		selected option marked by brackets "[]". The list of available options
+		depends on the selected file.
+
+		e.g.
+		cat /sys/bus/platform/drivers/aspeed-uart-routing/*.uart_routing/uart1
+		[io1] io2 io3 io4 uart2 uart3 uart4 io6
+
+		In this case, UART1 gets its input from IO1 (physical serial port 1).
+
+Users:		OpenBMC.  Proposed changes should be mailed to
+		[email protected]
+
+What:		/sys/bus/platform/drivers/aspeed-uart-routing/*/io*
+Date:		September 2021
+Contact:	Oskar Senft <[email protected]>
+		Chia-Wei Wang <[email protected]>
+Description:	Selects the RX source of IOx serial port. The current selection
+		will be marked by brackets "[]".
+Users:		OpenBMC.  Proposed changes should be mailed to
+		[email protected]

drivers/soc/aspeed/Kconfig

@@ -24,6 +24,16 @@ config ASPEED_LPC_SNOOP
 	  allows the BMC to listen on and save the data written by
 	  the host to an arbitrary LPC I/O port.
 
+config ASPEED_UART_ROUTING
+	tristate "ASPEED uart routing control"
+	select REGMAP
+	select MFD_SYSCON
+	default ARCH_ASPEED
+	help
+	  Provides a driver to control the UART routing paths, allowing
+	  users to perform runtime configuration of the RX muxes among
+	  the UART controllers and I/O pins.
+
 config ASPEED_P2A_CTRL
 	tristate "ASPEED P2A (VGA MMIO to BMC) bridge control"
 	select REGMAP

drivers/soc/aspeed/Makefile

@@ -1,5 +1,6 @@
 # SPDX-License-Identifier: GPL-2.0-only
-obj-$(CONFIG_ASPEED_LPC_CTRL)	+= aspeed-lpc-ctrl.o
-obj-$(CONFIG_ASPEED_LPC_SNOOP)	+= aspeed-lpc-snoop.o
-obj-$(CONFIG_ASPEED_P2A_CTRL)	+= aspeed-p2a-ctrl.o
-obj-$(CONFIG_ASPEED_SOCINFO)	+= aspeed-socinfo.o
+obj-$(CONFIG_ASPEED_LPC_CTRL)		+= aspeed-lpc-ctrl.o
+obj-$(CONFIG_ASPEED_LPC_SNOOP)		+= aspeed-lpc-snoop.o
+obj-$(CONFIG_ASPEED_UART_ROUTING)	+= aspeed-uart-routing.o
+obj-$(CONFIG_ASPEED_P2A_CTRL)		+= aspeed-p2a-ctrl.o
+obj-$(CONFIG_ASPEED_SOCINFO)		+= aspeed-socinfo.o