[PATCH v3 2/3] riscv: Implement semihost.h for earlycon semihost driver

[Bin Meng]

Per RISC-V semihosting spec [1], implement semihost.h for the existing
Arm semihosting earlycon driver to work on RISC-V.

Link: https://github.com/riscv/riscv-semihosting-spec/blob/main/riscv-semihosting-spec.adoc [1]

Signed-off-by: Bin Meng <bmeng@xxxxxxxxxxx>

---

Changes in v3:
- add #ifdef in the header to prevent from multiple inclusion
- add forward-declare struct uart_port
- add a Link tag in the commit message

Changes in v2:
- Move the RISC-V implementation to semihost.h

 arch/riscv/include/asm/semihost.h | 26 ++++++++++++++++++++++++++
 drivers/tty/serial/Kconfig        |  2 +-
 2 files changed, 27 insertions(+), 1 deletion(-)
 create mode 100644 arch/riscv/include/asm/semihost.h

diff --git a/arch/riscv/include/asm/semihost.h b/arch/riscv/include/asm/semihost.h
new file mode 100644
index 000000000000..557a34938193
--- /dev/null
+++ b/arch/riscv/include/asm/semihost.h
@@ -0,0 +1,26 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (C) 2022 tinylab.org
+ * Author: Bin Meng <bmeng@xxxxxxxxxxx>
+ */
+
+#ifndef _RISCV_SEMIHOST_H_
+#define _RISCV_SEMIHOST_H_
+
+struct uart_port;
+
+static inline void smh_putc(struct uart_port *port, unsigned char c)
+{
+	asm volatile("addi    a1, %0, 0\n"
+		     "addi    a0, zero, 3\n"
+		     ".balign 16\n"
+		     ".option push\n"
+		     ".option norvc\n"
+		     "slli    zero, zero, 0x1f\n"
+		     "ebreak\n"
+		     "srai    zero, zero, 0x7\n"
+		     ".option pop\n"
+		     : : "r" (&c) : "a0", "a1", "memory");
+}
+
+#endif /* _RISCV_SEMIHOST_H_ */
diff --git a/drivers/tty/serial/Kconfig b/drivers/tty/serial/Kconfig
index 434f83168546..e94d1265151c 100644
--- a/drivers/tty/serial/Kconfig
+++ b/drivers/tty/serial/Kconfig
@@ -75,7 +75,7 @@ config SERIAL_AMBA_PL011_CONSOLE
 
 config SERIAL_EARLYCON_ARM_SEMIHOST
 	bool "Early console using ARM semihosting"
-	depends on ARM64 || ARM
+	depends on ARM64 || ARM || RISCV
 	select SERIAL_CORE
 	select SERIAL_CORE_CONSOLE
 	select SERIAL_EARLYCON
-- 
2.34.1