[PATCH v2 2/3] arch/riscv: add semihosting support for RISC-V

[Kautuk Consul <kconsul@ventanamicro.com>]

We add RISC-V semihosting based serial console for JTAG based early
debugging.

The RISC-V semihosting specification is available at:
https://github.com/riscv/riscv-semihosting-spec/blob/main/riscv-semihosting-spec.adoc

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Signed-off-by: Kautuk Consul <kconsul@ventanamicro.com>
---
 arch/riscv/Kconfig                   | 47 ++++++++++++++++++++++++++++
 arch/riscv/include/asm/semihosting.h | 11 +++++++
 arch/riscv/include/asm/spl.h         |  1 +
 arch/riscv/lib/Makefile              |  2 ++
 arch/riscv/lib/interrupts.c          | 11 +++++++
 arch/riscv/lib/semihosting.c         | 24 ++++++++++++++
 6 files changed, 96 insertions(+)
 create mode 100644 arch/riscv/include/asm/semihosting.h
 create mode 100644 arch/riscv/lib/semihosting.c

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 78e964db12..b15d9028bd 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -371,4 +371,51 @@ config TPL_USE_ARCH_MEMSET
 
 endmenu
 
+config SEMIHOSTING
+	bool "Support RISCV semihosting"
+	select LIB_SEMIHOSTING
+	help
+	  Semihosting is a method for a target to communicate with a host
+	  debugger. It uses special instructions which the debugger will trap
+	  on and interpret. This allows U-Boot to read/write files, print to
+	  the console, and execute arbitrary commands on the host system.
+
+	  Enabling this option will add support for reading and writing files
+	  on the host system. If you don't have a debugger attached then trying
+	  to do this will likely cause U-Boot to hang. Say 'n' if you are unsure.
+
+config SEMIHOSTING_FALLBACK
+	bool "Recover gracefully when semihosting fails"
+	depends on SEMIHOSTING && RISCV
+	default y
+	help
+	  Normally, if U-Boot makes a semihosting call and no debugger is
+	  attached, then it will panic due to a synchronous abort
+	  exception. This config adds an exception handler which will allow
+	  U-Boot to recover. Say 'y' if unsure.
+
+config SPL_SEMIHOSTING
+	bool "Support RISCV semihosting in SPL"
+	depends on SPL
+	select LIB_SEMIHOSTING
+	help
+	  Semihosting is a method for a target to communicate with a host
+	  debugger. It uses special instructions which the debugger will trap
+	  on and interpret. This allows U-Boot to read/write files, print to
+	  the console, and execute arbitrary commands on the host system.
+
+	  Enabling this option will add support for reading and writing files
+	  on the host system. If you don't have a debugger attached then trying
+	  to do this will likely cause U-Boot to hang. Say 'n' if you are unsure.
+
+config SPL_SEMIHOSTING_FALLBACK
+	bool "Recover gracefully when semihosting fails in SPL"
+	depends on SPL_SEMIHOSTING && RISCV
+	default y
+	help
+	  Normally, if U-Boot makes a semihosting call and no debugger is
+	  attached, then it will panic due to a synchronous abort
+	  exception. This config adds an exception handler which will allow
+	  U-Boot to recover. Say 'y' if unsure.
+
 endmenu
diff --git a/arch/riscv/include/asm/semihosting.h b/arch/riscv/include/asm/semihosting.h
new file mode 100644
index 0000000000..7042821e00
--- /dev/null
+++ b/arch/riscv/include/asm/semihosting.h
@@ -0,0 +1,11 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * Copyright (C) 2022 Ventana Micro Systems Inc.
+ */
+
+#ifndef __ASM_RISCV_SEMIHOSTING_H
+#define __ASM_RISCV_SEMIHOSTING_H
+
+long smh_trap(int sysnum, void *addr);
+
+#endif /* __ASM_RISCV_SEMIHOSTING_H */
diff --git a/arch/riscv/include/asm/spl.h b/arch/riscv/include/asm/spl.h
index e8a94fcb1f..2898a770ee 100644
--- a/arch/riscv/include/asm/spl.h
+++ b/arch/riscv/include/asm/spl.h
@@ -25,6 +25,7 @@ enum {
 	BOOT_DEVICE_DFU,
 	BOOT_DEVICE_XIP,
 	BOOT_DEVICE_BOOTROM,
+	BOOT_DEVICE_SMH,
 	BOOT_DEVICE_NONE
 };
 
diff --git a/arch/riscv/lib/Makefile b/arch/riscv/lib/Makefile
index 06020fcc2a..64e29804c1 100644
--- a/arch/riscv/lib/Makefile
+++ b/arch/riscv/lib/Makefile
@@ -42,3 +42,5 @@ extra-$(CONFIG_EFI) += $(EFI_CRT0) $(EFI_RELOC)
 obj-$(CONFIG_$(SPL_TPL_)USE_ARCH_MEMSET) += memset.o
 obj-$(CONFIG_$(SPL_TPL_)USE_ARCH_MEMMOVE) += memmove.o
 obj-$(CONFIG_$(SPL_TPL_)USE_ARCH_MEMCPY) += memcpy.o
+
+obj-$(CONFIG_$(SPL_TPL_)SEMIHOSTING) += semihosting.o
diff --git a/arch/riscv/lib/interrupts.c b/arch/riscv/lib/interrupts.c
index 100be2e966..bd7cd772b8 100644
--- a/arch/riscv/lib/interrupts.c
+++ b/arch/riscv/lib/interrupts.c
@@ -17,6 +17,7 @@
 #include <asm/ptrace.h>
 #include <asm/system.h>
 #include <asm/encoding.h>
+#include <semihosting.h>
 
 DECLARE_GLOBAL_DATA_PTR;
 
@@ -149,6 +150,16 @@ ulong handle_trap(ulong cause, ulong epc, ulong tval, struct pt_regs *regs)
 	/* An UEFI application may have changed gd. Restore U-Boot's gd. */
 	efi_restore_gd();
 
+	if (cause == CAUSE_BREAKPOINT &&
+	    CONFIG_IS_ENABLED(SEMIHOSTING_FALLBACK)) {
+		/* For semihosting fallback we simply skip the ebreak
+		 * instruction.
+		 */
+		disable_semihosting();
+		epc += 4;
+		return epc;
+	}
+
 	is_irq = (cause & MCAUSE_INT);
 	irq = (cause & ~MCAUSE_INT);
 
diff --git a/arch/riscv/lib/semihosting.c b/arch/riscv/lib/semihosting.c
new file mode 100644
index 0000000000..72ea9521dc
--- /dev/null
+++ b/arch/riscv/lib/semihosting.c
@@ -0,0 +1,24 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Copyright (C) 2022 Ventana Micro Systems Inc.
+ */
+
+#include <common.h>
+
+long smh_trap(int sysnum, void *addr)
+{
+	register int ret asm ("a0") = sysnum;
+	register void *param0 asm ("a1") = addr;
+
+	asm volatile ("\t.option push\n"
+		"\t.option norvc\n"
+		"\tj 1f\n"
+		"\t.align 4\n"
+		"\t1: slli zero, zero, 0x1f\n"
+		"\tebreak\n"
+		"\tsrai zero, zero, 7\n"
+		"\t.option pop\n"
+		: "+r" (ret) : "r" (param0) : "memory");
+
+	return ret;
+}
-- 
2.34.1