tty/serial: Add RISC-V SBI earlycon support

In RISC-V, the M-mode runtime firmware provide SBI calls for
debug prints. This patch adds earlycon support using RISC-V
SBI console calls. To enable it, just pass "earlycon=sbi" in
kernel parameters.
Signed-off-by: Anup Patel <anup@brainfault.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>

drivers/tty/serial/Kconfig

@@ -85,6 +85,18 @@ config SERIAL_EARLYCON_ARM_SEMIHOST
 	  with "earlycon=smh" on the kernel command line. The console is
 	  enabled when early_param is processed.
 
+config SERIAL_EARLYCON_RISCV_SBI
+	bool "Early console using RISC-V SBI"
+	depends on RISCV
+	select SERIAL_CORE
+	select SERIAL_CORE_CONSOLE
+	select SERIAL_EARLYCON
+	help
+	  Support for early debug console using RISC-V SBI. This enables
+	  the console before standard serial driver is probed. This is enabled
+	  with "earlycon=sbi" on the kernel command line. The console is
+	  enabled when early_param is processed.
+
 config SERIAL_SB1250_DUART
 	tristate "BCM1xxx on-chip DUART serial support"
 	depends on SIBYTE_SB1xxx_SOC=y

drivers/tty/serial/Makefile

@@ -7,6 +7,7 @@ obj-$(CONFIG_SERIAL_CORE) += serial_core.o
 
 obj-$(CONFIG_SERIAL_EARLYCON) += earlycon.o
 obj-$(CONFIG_SERIAL_EARLYCON_ARM_SEMIHOST) += earlycon-arm-semihost.o
+obj-$(CONFIG_SERIAL_EARLYCON_RISCV_SBI) += earlycon-riscv-sbi.o
 
 # These Sparc drivers have to appear before others such as 8250
 # which share ttySx minor node space.  Otherwise console device

drivers/tty/serial/earlycon-riscv-sbi.c

+// SPDX-License-Identifier: GPL-2.0
+/*
+ * RISC-V SBI based earlycon
+ *
+ * Copyright (C) 2018 Anup Patel <anup@brainfault.org>
+ */
+#include <linux/kernel.h>
+#include <linux/console.h>
+#include <linux/init.h>
+#include <linux/serial_core.h>
+#include <asm/sbi.h>
+
+static void sbi_console_write(struct console *con,
+			      const char *s, unsigned int n)
+{
+	int i;
+
+	for (i = 0; i < n; ++i)
+		sbi_console_putchar(s[i]);
+}
+
+static int __init early_sbi_setup(struct earlycon_device *device,
+				  const char *opt)
+{
+	device->con->write = sbi_console_write;
+	return 0;
+}
+EARLYCON_DECLARE(sbi, early_sbi_setup);