riscv: add backtrace support

When debugging, it is useful to have a backtrace to find
out what is in the call stack as the previous function (RA)
may not have been the culprit.

Since this adds size to the build, do not add it by default
and avoid putting it in the SPL build if not needed.

Signed-off-by: Ben Dooks <[email protected]>
Tested-by: Leo Yu-Chi Liang <[email protected]>

4 files changed, 60 insertions, 0 deletions

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 6c26f91f166..1819db1e256 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -119,6 +119,26 @@ config ARCH_RV64I
 
 endchoice
 
+config FRAMEPOINTER
+	bool "Build with frame pointer for stack unwinding"
+	help
+	  Choose this option to use the frame pointer so the stack can be
+	  unwound if needed. This is useful for tracing where faults came
+	  from as the source may be several functions back
+
+	  If you say Y here, then the code size will be increased due to
+	  having to store the fp.
+
+config SPL_FRAMEPOINTER
+	bool "Build SPL with frame pointer for stack unwinding"
+	help
+	  Choose this option to use the frame pointer so the stack can be
+	  unwound if needed. This is useful for tracing where faults came
+	  from as the source may be several functions back
+
+	  If you say Y here, then the code size will be increased due to
+	  having to store the fp.
+
 choice
 	prompt "Code Model"
 	default CMODEL_MEDLOW
diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
index b3ef87078b5..c36a8533e0f 100644
--- a/arch/riscv/Makefile
+++ b/arch/riscv/Makefile
@@ -48,6 +48,10 @@ endif
 ARCH_FLAGS = -march=$(RISCV_MARCH) -mabi=$(ABI) \
 	     -mcmodel=$(CMODEL)
 
+ifeq ($(CONFIG_$(SPL_)FRAMEPOINTER),y)
+	ARCH_FLAGS += -fno-omit-frame-pointer
+endif
+
 PLATFORM_CPPFLAGS	+= $(ARCH_FLAGS)
 CFLAGS_EFI		+= $(ARCH_FLAGS)
 
diff --git a/arch/riscv/cpu/start.S b/arch/riscv/cpu/start.S
index 6cecadfac56..a9e19356928 100644
--- a/arch/riscv/cpu/start.S
+++ b/arch/riscv/cpu/start.S
@@ -418,6 +418,7 @@ call_board_init_r:
  */
 	mv	a0, s3			/* gd_t */
 	mv	a1, s4			/* dest_addr */
+	mv	s0, zero		/* fp == NULL */
 
 /*
  * jump to it ...
diff --git a/arch/riscv/lib/interrupts.c b/arch/riscv/lib/interrupts.c
index a26ccc721fd..7350e2ced85 100644
--- a/arch/riscv/lib/interrupts.c
+++ b/arch/riscv/lib/interrupts.c
@@ -60,6 +60,40 @@ static void show_regs(struct pt_regs *regs)
 #endif
 }
 
+#if defined(CONFIG_FRAMEPOINTER) || defined(CONFIG_SPL_FRAMEPOINTER)
+static void show_backtrace(struct pt_regs *regs)
+{
+	uintptr_t *fp = (uintptr_t *)regs->s0;
+	unsigned count = 0;
+	ulong ra;
+
+	printf("backtrace:\n");
+
+	/* there are a few entry points where the s0 register is
+	 * set to gd, so to avoid changing those, just abort if
+	 * the value is the same */
+	while (fp != NULL && fp != (uintptr_t *)gd) {
+		ra = fp[-1];
+		printf("backtrace %2d: FP: " REG_FMT " RA: " REG_FMT,
+		       count, (ulong)fp, ra);
+
+		if (gd && gd->flags & GD_FLG_RELOC)
+			printf(" - RA: " REG_FMT " reloc adjusted\n",
+			ra - gd->reloc_off);
+		else
+			printf("\n");
+
+		fp = (uintptr_t *)fp[-2];
+		count++;
+	}
+}
+#else
+static void show_backtrace(struct pt_regs *regs)
+{
+	printf("No backtrace support enabled\n");
+}
+#endif
+
 /**
  * instr_len() - get instruction length
  *
@@ -131,6 +165,7 @@ static void _exit_trap(ulong code, ulong epc, ulong tval, struct pt_regs *regs)
 		       epc - gd->reloc_off, regs->ra - gd->reloc_off);
 
 	show_regs(regs);
+	show_backtrace(regs);
 	show_code(epc);
 	show_efi_loaded_images(epc);
 	panic("\n");