tty: serial: add driver for the SiFive UART

Add a serial driver for the SiFive UART, found on SiFive FU540 devices
(among others).

The underlying serial IP block is relatively basic, and currently does
not support serial break detection.  Further information on the IP
block can be found in the documentation and Chisel sources:

https://static.dev.sifive.com/FU540-C000-v1.0.pdf

https://github.com/sifive/sifive-blocks/tree/master/src/main/scala/devices/uart

This driver was written in collaboration with Wesley Terpstra
<wesley@sifive.com>.

Boot-tested on a SiFive HiFive Unleashed A00 board.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.com>
Cc: Palmer Dabbelt <palmer@sifive.com>
Cc: Wesley Terpstra <wesley@sifive.com>
Cc: linux-serial@vger.kernel.org
Cc: linux-riscv@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Cc: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>

drivers/tty/serial/Kconfig

@@ -1095,6 +1095,30 @@ config SERIAL_OMAP_CONSOLE
 	  your boot loader about how to pass options to the kernel at
 	  boot time.)
 
+config SERIAL_SIFIVE
+	tristate "SiFive UART support"
+	depends on OF
+	select SERIAL_CORE
+	help
+	  Select this option if you are building a kernel for a device that
+	  contains a SiFive UART IP block.  This type of UART is present on
+	  SiFive FU540 SoCs, among others.
+
+config SERIAL_SIFIVE_CONSOLE
+	bool "Console on SiFive UART"
+	depends on SERIAL_SIFIVE=y
+	select SERIAL_CORE_CONSOLE
+	help
+	  Select this option if you would like to use a SiFive UART as the
+	  system console.
+
+	  Even if you say Y here, the currently visible virtual console
+	  (/dev/tty0) will still be used as the system console by default, but
+	  you can alter that using a kernel command line option such as
+	  "console=ttySIFx". (Try "man bootparam" or see the documentation of
+	  your boot loader about how to pass options to the kernel at
+	  boot time.)
+
 config SERIAL_LANTIQ
 	bool "Lantiq serial driver"
 	depends on LANTIQ

drivers/tty/serial/Makefile

@@ -92,6 +92,7 @@ obj-$(CONFIG_SERIAL_PIC32)	+= pic32_uart.o
 obj-$(CONFIG_SERIAL_MPS2_UART)	+= mps2-uart.o
 obj-$(CONFIG_SERIAL_OWL)	+= owl-uart.o
 obj-$(CONFIG_SERIAL_RDA)	+= rda-uart.o
+obj-$(CONFIG_SERIAL_SIFIVE)	+= sifive.o
 
 # GPIOLIB helpers for modem control lines
 obj-$(CONFIG_SERIAL_MCTRL_GPIO)	+= serial_mctrl_gpio.o

include/uapi/linux/serial_core.h

@@ -287,4 +287,7 @@
 /* RDA UART */
 #define PORT_RDA	118
 
+/* SiFive UART */
+#define PORT_SIFIVE_V0	118
+
 #endif /* _UAPILINUX_SERIAL_CORE_H */