RISC-V: add stacktrace support via frame pointer walking

Make debugging more convenient by implementing stack_dump() and changing
exception handlers to print stack trace along with the register dump.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Link: https://lore.barebox.org/20220108171524.587144-8-a.fatoum@pengutronix.de
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

7 files changed, 106 insertions, 6 deletions

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 8c66697185..c5b373e3af 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -83,6 +83,16 @@ config RISCV_EXCEPTIONS
 	default y
 	select ARCH_HAS_DATA_ABORT_MASK
 
+config RISCV_UNWIND
+	bool "enable stack unwinding support"
+	select ARCH_HAS_STACK_DUMP
+	select ARCH_WANT_FRAME_POINTERS
+	help
+	  This option enables stack unwinding support in barebox
+	  using the information automatically generated by the
+	  compiler. The resulting kernel image is slightly bigger but
+	  the performance is not affected.
+
 config HAS_NMON
 	bool
 
diff --git a/arch/riscv/cpu/interrupts.c b/arch/riscv/cpu/interrupts.c
index 1f2d7b8857..0bb56d441d 100644
--- a/arch/riscv/cpu/interrupts.c
+++ b/arch/riscv/cpu/interrupts.c
@@ -14,6 +14,7 @@
 #include <asm/ptrace.h>
 #include <asm/irq.h>
 #include <asm/csr.h>
+#include <asm/unwind.h>
 #include <abort.h>
 #include <pbl.h>
 
@@ -81,6 +82,8 @@ static void report_trap(const struct pt_regs *regs)
 	       regs->epc, regs->ra, regs->badaddr);
 
 	show_regs(regs);
+
+	unwind_backtrace(regs);
 }
 
 
diff --git a/arch/riscv/include/asm/barebox-riscv-head.h b/arch/riscv/include/asm/barebox-riscv-head.h
index a4c33472cd..4d62c20d1b 100644
--- a/arch/riscv/include/asm/barebox-riscv-head.h
+++ b/arch/riscv/include/asm/barebox-riscv-head.h
@@ -23,6 +23,7 @@
 		".ascii \"" magic2 "\"\n"      /* magic 2 */                          \
 		".word 0\n"                    /* reserved (PE-COFF offset) */        \
 		"1:\n"                                                                \
+		"li fp, 0\n"                                                          \
 	)
 
 #define __barebox_riscv_header(instr, load_offset, version, magic1, magic2) \
diff --git a/arch/riscv/include/asm/ptrace.h b/arch/riscv/include/asm/ptrace.h
index b5e792f666..319c50f946 100644
--- a/arch/riscv/include/asm/ptrace.h
+++ b/arch/riscv/include/asm/ptrace.h
@@ -61,7 +61,7 @@ struct pt_regs {
 #define MAX_REG_OFFSET offsetof(struct pt_regs, cause)
 
 /* Helpers for working with the instruction pointer */
-static inline unsigned long instruction_pointer(struct pt_regs *regs)
+static inline unsigned long instruction_pointer(const struct pt_regs *regs)
 {
 	return regs->epc;
 }
@@ -74,7 +74,7 @@ static inline void instruction_pointer_set(struct pt_regs *regs,
 #define profile_pc(regs) instruction_pointer(regs)
 
 /* Helpers for working with the user stack pointer */
-static inline unsigned long user_stack_pointer(struct pt_regs *regs)
+static inline unsigned long user_stack_pointer(const struct pt_regs *regs)
 {
 	return regs->sp;
 }
@@ -85,13 +85,13 @@ static inline void user_stack_pointer_set(struct pt_regs *regs,
 }
 
 /* Valid only for Kernel mode traps. */
-static inline unsigned long kernel_stack_pointer(struct pt_regs *regs)
+static inline unsigned long kernel_stack_pointer(const struct pt_regs *regs)
 {
 	return regs->sp;
 }
 
 /* Helpers for working with the frame pointer */
-static inline unsigned long frame_pointer(struct pt_regs *regs)
+static inline unsigned long frame_pointer(const struct pt_regs *regs)
 {
 	return regs->s0;
 }
@@ -101,7 +101,7 @@ static inline void frame_pointer_set(struct pt_regs *regs,
 	regs->s0 = val;
 }
 
-static inline unsigned long regs_return_value(struct pt_regs *regs)
+static inline unsigned long regs_return_value(const struct pt_regs *regs)
 {
 	return regs->a0;
 }
diff --git a/arch/riscv/include/asm/unwind.h b/arch/riscv/include/asm/unwind.h
index 9e5c8b5420..00f7845147 100644
--- a/arch/riscv/include/asm/unwind.h
+++ b/arch/riscv/include/asm/unwind.h
@@ -4,6 +4,12 @@
 
 struct pt_regs;
 
-void unwind_backtrace(struct pt_regs *regs);
+#if defined CONFIG_RISCV_UNWIND && !defined __PBL__
+void unwind_backtrace(const struct pt_regs *regs);
+#else
+static inline void unwind_backtrace(const struct pt_regs *regs)
+{
+}
+#endif
 
 #endif
diff --git a/arch/riscv/lib/Makefile b/arch/riscv/lib/Makefile
index a399de7399..f3db5320f7 100644
--- a/arch/riscv/lib/Makefile
+++ b/arch/riscv/lib/Makefile
@@ -9,3 +9,4 @@ obj-$(CONFIG_RISCV_OPTIMZED_STRING_FUNCTIONS) += memcpy.o memset.o memmove.o
 obj-$(CONFIG_RISCV_SBI) += sbi.o
 obj-$(CONFIG_CMD_RISCV_CPUINFO) += cpuinfo.o
 obj-$(CONFIG_BOOTM) += bootm.o
+obj-$(CONFIG_RISCV_UNWIND) += stacktrace.o
diff --git a/arch/riscv/lib/stacktrace.c b/arch/riscv/lib/stacktrace.c
new file mode 100644
index 0000000000..663938019e
--- /dev/null
+++ b/arch/riscv/lib/stacktrace.c
@@ -0,0 +1,79 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2008 ARM Limited
+ * Copyright (C) 2014 Regents of the University of California
+ * Copyright (C) 2021 Ahmad Fatoum, Pengutronix
+ *
+ * Framepointer assisted stack unwinder
+ */
+
+#include <linux/kernel.h>
+#include <printk.h>
+#include <asm/unwind.h>
+#include <asm/ptrace.h>
+#include <asm-generic/memory_layout.h>
+#include <asm/sections.h>
+
+struct stackframe {
+	unsigned long fp;
+	unsigned long ra;
+};
+
+static void dump_backtrace_entry(unsigned long where, unsigned long from)
+{
+#ifdef CONFIG_KALLSYMS
+	printf("[<%08lx>] (%pS) from [<%08lx>] (%pS)\n", where, (void *)where, from, (void *)from);
+#else
+	printf("Function entered at [<%08lx>] from [<%08lx>]\n", where, from);
+#endif
+}
+
+static int unwind_frame(struct stackframe *frame, unsigned long *sp)
+{
+	unsigned long low, high;
+	unsigned long fp = frame->fp;
+
+	low = *sp;
+	high = ALIGN(low, STACK_SIZE);
+
+	if (fp < low || fp > high - sizeof(struct stackframe) || fp & 0x7)
+		return -1;
+
+	*frame = *((struct stackframe *)fp - 1);
+	*sp = fp;
+
+	return 0;
+}
+
+void unwind_backtrace(const struct pt_regs *regs)
+{
+        struct stackframe frame = {};
+	register unsigned long current_sp asm ("sp");
+	unsigned long sp = 0;
+
+	if (regs) {
+		frame.fp = frame_pointer(regs);
+		frame.ra = regs->ra;
+	} else {
+		sp = current_sp;
+		frame.fp = (unsigned long)__builtin_frame_address(0);
+		frame.ra = (unsigned long)unwind_backtrace;
+	}
+
+	printf("Call trace:\n");
+	for (;;) {
+		unsigned long where = frame.ra;
+		int ret;
+
+		ret = unwind_frame(&frame, &sp);
+		if (ret < 0)
+			break;
+
+		dump_backtrace_entry(where, frame.ra);
+	}
+}
+
+void dump_stack(void)
+{
+	unwind_backtrace(NULL);
+}