Message-Id: <1272389874-2969-1-git-send-email-steve@digidescorp.com>
Date:	Tue, 27 Apr 2010 12:37:54 -0500
From:	"Steven J. Magnani" <steve@...idescorp.com>
To:	microblaze-uclinux@...e.uq.edu.au
Cc:	monstr@...str.eu, linux-kernel@...r.kernel.org,
	"Steven J. Magnani" <steve@...idescorp.com>
Subject: [PATCH V2] microblaze: add stack unwinder

Implement intelligent backtracing by searching for stack frame creation,
and emitting only return addresses. Use print_hex_dump() to display the
entire binary kernel stack.

Limitation: MMU kernels are not currently able to trace beyond a system trap
(interrupt, syscall, etc.). It is the intent of this patch to provide
infrastructure that can be extended to add this capability later.

Changes from V1:
* Removed checks in find_frame_creation() that prevented location of the frame
  creation instruction in heavily optimized code
* Various formatting/commenting/file location tweaks per review comments
* Dropped Kconfig option to enable STACKTRACE as something logically separate

Signed-off-by: Steven J. Magnani <steve@...idescorp.com>
---
diff -uprN a/arch/microblaze/include/asm/exceptions.h b/arch/microblaze/include/asm/exceptions.h
--- a/arch/microblaze/include/asm/exceptions.h	2010-04-27 10:14:33.000000000 -0500
+++ b/arch/microblaze/include/asm/exceptions.h	2010-04-27 10:30:59.000000000 -0500
@@ -14,6 +14,11 @@
 #define _ASM_MICROBLAZE_EXCEPTIONS_H
 
 #ifdef __KERNEL__
+
+#ifndef CONFIG_MMU
+#define EX_HANDLER_STACK_SIZ	(4*19)
+#endif
+
 #ifndef __ASSEMBLY__
 
 /* Macros to enable and disable HW exceptions in the MSR */
diff -uprN a/arch/microblaze/include/asm/system.h b/arch/microblaze/include/asm/system.h
--- a/arch/microblaze/include/asm/system.h	2010-04-27 10:14:33.000000000 -0500
+++ b/arch/microblaze/include/asm/system.h	2010-04-27 10:31:07.000000000 -0500
@@ -44,7 +44,6 @@ extern struct task_struct *_switch_to(st
 #define smp_rmb()		rmb()
 #define smp_wmb()		wmb()
 
-void show_trace(struct task_struct *task, unsigned long *stack);
 void __bad_xchg(volatile void *ptr, int size);
 
 static inline unsigned long __xchg(unsigned long x, volatile void *ptr,
diff -uprN a/arch/microblaze/include/asm/unwind.h b/arch/microblaze/include/asm/unwind.h
--- a/arch/microblaze/include/asm/unwind.h	1969-12-31 18:00:00.000000000 -0600
+++ b/arch/microblaze/include/asm/unwind.h	2010-04-26 22:09:39.000000000 -0500
@@ -0,0 +1,29 @@
+/*
+ * Backtrace support for Microblaze
+ *
+ * Copyright (C) 2010  Digital Design Corporation
+ *
+ * This file is subject to the terms and conditions of the GNU General Public
+ * License.  See the file "COPYING" in the main directory of this archive
+ * for more details.
+ */
+
+#ifndef __MICROBLAZE_UNWIND_H
+#define __MICROBLAZE_UNWIND_H
+
+struct stack_trace;
+
+struct trap_handler_info {
+	unsigned long start_addr;
+	unsigned long end_addr;
+	const char    *trap_name;
+};
+extern struct trap_handler_info microblaze_trap_handlers;
+
+extern const char _hw_exception_handler;
+extern const char ex_handler_unhandled;
+
+void microblaze_unwind(struct task_struct *task, struct stack_trace *trace);
+
+#endif	/* __MICROBLAZE_UNWIND_H */
+
diff -uprN a/arch/microblaze/kernel/entry-nommu.S b/arch/microblaze/kernel/entry-nommu.S
--- a/arch/microblaze/kernel/entry-nommu.S	2010-04-27 10:14:33.000000000 -0500
+++ b/arch/microblaze/kernel/entry-nommu.S	2010-04-27 10:29:25.000000000 -0500
@@ -586,3 +586,31 @@ sys_rt_sigsuspend_wrapper:
 #include "syscall_table.S"
 
 syscall_table_size=(.-sys_call_table)
+
+type_SYSCALL:
+	.ascii "SYSCALL\0"
+type_IRQ:
+	.ascii "IRQ\0"
+type_IRQ_PREEMPT:
+	.ascii "IRQ (PREEMPTED)\0"
+type_SYSCALL_PREEMPT:
+	.ascii " SYSCALL (PREEMPTED)\0"
+
+	/*
+	 * Trap decoding for stack unwinder
+	 * Tuples are (start addr, end addr, string)
+	 * If return address lies on [start addr, end addr],
+	 * unwinder displays 'string'
+	 */
+
+	.align 4
+.global microblaze_trap_handlers
+microblaze_trap_handlers:
+	/* Exact matches come first */
+	.word ret_to_user  ; .word ret_to_user    ; .word type_SYSCALL
+	.word ret_from_intr; .word ret_from_intr  ; .word type_IRQ
+	/* Fuzzy matches go here */
+	.word ret_from_intr; .word no_intr_resched; .word type_IRQ_PREEMPT
+	.word work_pending ; .word no_work_pending; .word type_SYSCALL_PREEMPT
+	/* End of table */
+	.word 0             ; .word 0               ; .word 0
diff -uprN a/arch/microblaze/kernel/entry.S b/arch/microblaze/kernel/entry.S
--- a/arch/microblaze/kernel/entry.S	2010-04-27 10:14:33.000000000 -0500
+++ b/arch/microblaze/kernel/entry.S	2010-04-27 10:28:19.000000000 -0500
@@ -1127,3 +1127,30 @@ ENTRY(_break)
 
 syscall_table_size=(.-sys_call_table)
 
+type_SYSCALL:
+	.ascii "SYSCALL\0"
+type_IRQ:
+	.ascii "IRQ\0"
+type_IRQ_PREEMPT:
+	.ascii "IRQ (PREEMPTED)\0"
+type_SYSCALL_PREEMPT:
+	.ascii " SYSCALL (PREEMPTED)\0"
+
+	/*
+	 * Trap decoding for stack unwinder
+	 * Tuples are (start addr, end addr, string)
+	 * If return address lies on [start addr, end addr],
+	 * unwinder displays 'string'
+	 */
+
+	.align 4
+.global microblaze_trap_handlers
+microblaze_trap_handlers:
+	/* Exact matches come first */
+	.word ret_from_trap; .word ret_from_trap   ; .word type_SYSCALL
+	.word ret_from_irq ; .word ret_from_irq    ; .word type_IRQ
+	/* Fuzzy matches go here */
+	.word ret_from_irq ; .word no_intr_resched ; .word type_IRQ_PREEMPT
+	.word ret_from_trap; .word TRAP_return     ; .word type_SYSCALL_PREEMPT
+	/* End of table */
+	.word 0               ; .word 0               ; .word 0
diff -uprN a/arch/microblaze/kernel/hw_exception_handler.S b/arch/microblaze/kernel/hw_exception_handler.S
--- a/arch/microblaze/kernel/hw_exception_handler.S	2010-04-27 10:14:33.000000000 -0500
+++ b/arch/microblaze/kernel/hw_exception_handler.S	2010-04-27 10:25:15.000000000 -0500
@@ -78,9 +78,6 @@
 #include <asm/asm-offsets.h>
 
 /* Helpful Macros */
-#ifndef CONFIG_MMU
-#define EX_HANDLER_STACK_SIZ	(4*19)
-#endif
 #define NUM_TO_REG(num)		r ## num
 
 #ifdef CONFIG_MMU
@@ -988,6 +985,7 @@ ex_unaligned_fixup:
 .end _unaligned_data_exception
 #endif /* CONFIG_MMU */
 
+.global ex_handler_unhandled
 ex_handler_unhandled:
 /* FIXME add handle function for unhandled exception - dump register */
 	bri 0
diff -uprN a/arch/microblaze/kernel/Makefile b/arch/microblaze/kernel/Makefile
--- a/arch/microblaze/kernel/Makefile	2010-04-27 10:14:33.000000000 -0500
+++ b/arch/microblaze/kernel/Makefile	2010-04-27 10:29:59.000000000 -0500
@@ -17,7 +17,7 @@ extra-y := head.o vmlinux.lds
 obj-y += dma.o exceptions.o \
 	hw_exception_handler.o init_task.o intc.o irq.o of_device.o \
 	of_platform.o process.o prom.o prom_parse.o ptrace.o \
-	setup.o signal.o sys_microblaze.o timer.o traps.o reset.o
+	reset.o setup.o signal.o sys_microblaze.o timer.o traps.o unwind.o
 
 obj-y += cpu/
 
diff -uprN a/arch/microblaze/kernel/stacktrace.c b/arch/microblaze/kernel/stacktrace.c
--- a/arch/microblaze/kernel/stacktrace.c	2010-04-27 10:14:33.000000000 -0500
+++ b/arch/microblaze/kernel/stacktrace.c	2010-04-27 10:22:04.000000000 -0500
@@ -14,52 +14,18 @@
 #include <linux/thread_info.h>
 #include <linux/ptrace.h>
 #include <linux/module.h>
+#include <asm/unwind.h>
 
-/* FIXME initial support */
 void save_stack_trace(struct stack_trace *trace)
 {
-	unsigned long *sp;
-	unsigned long addr;
-	asm("addik %0, r1, 0" : "=r" (sp));
-
-	while (!kstack_end(sp)) {
-		addr = *sp++;
-		if (__kernel_text_address(addr)) {
-			if (trace->skip > 0)
-				trace->skip--;
-			else
-				trace->entries[trace->nr_entries++] = addr;
-
-			if (trace->nr_entries >= trace->max_entries)
-				break;
-		}
-	}
+	/* Exclude our helper functions from the trace*/
+	trace->skip += 2;
+	microblaze_unwind(NULL, trace);
 }
 EXPORT_SYMBOL_GPL(save_stack_trace);
 
 void save_stack_trace_tsk(struct task_struct *tsk, struct stack_trace *trace)
 {
-	unsigned int *sp;
-	unsigned long addr;
-
-	struct thread_info *ti = task_thread_info(tsk);
-
-	if (tsk == current)
-		asm("addik %0, r1, 0" : "=r" (sp));
-	else
-		sp = (unsigned int *)ti->cpu_context.r1;
-
-	while (!kstack_end(sp)) {
-		addr = *sp++;
-		if (__kernel_text_address(addr)) {
-			if (trace->skip > 0)
-				trace->skip--;
-			else
-				trace->entries[trace->nr_entries++] = addr;
-
-			if (trace->nr_entries >= trace->max_entries)
-				break;
-		}
-	}
+	microblaze_unwind(tsk, trace);
 }
 EXPORT_SYMBOL_GPL(save_stack_trace_tsk);
diff -uprN a/arch/microblaze/kernel/traps.c b/arch/microblaze/kernel/traps.c
--- a/arch/microblaze/kernel/traps.c	2010-04-27 10:14:33.000000000 -0500
+++ b/arch/microblaze/kernel/traps.c	2010-04-27 10:21:22.000000000 -0500
@@ -16,13 +16,14 @@
 
 #include <asm/exceptions.h>
 #include <asm/system.h>
+#include <asm/unwind.h>
 
 void trap_init(void)
 {
 	__enable_hw_exceptions();
 }
 
-static unsigned long kstack_depth_to_print = 24;
+static unsigned long kstack_depth_to_print;	/* 0 == entire stack */
 
 static int __init kstack_setup(char *s)
 {
@@ -30,31 +31,47 @@ static int __init kstack_setup(char *s)
 }
 __setup("kstack=", kstack_setup);
 
-void show_trace(struct task_struct *task, unsigned long *stack)
+void show_stack(struct task_struct *task, unsigned long *sp)
 {
-	unsigned long addr;
+	unsigned long words_to_show;
+	u32 fp = (u32) sp;
 
-	if (!stack)
-		stack = (unsigned long *)&stack;
+	if (fp == 0) {
+		if (task) {
+			fp = ((struct thread_info *)
+				(task->stack))->cpu_context.r1;
+		} else {
+			/* Pick up caller of dump_stack() */
+			fp = (u32)&sp - 8;
+		}
+	}
 
-	printk(KERN_NOTICE "Call Trace: ");
-#ifdef CONFIG_KALLSYMS
-	printk(KERN_NOTICE "\n");
-#endif
-	while (!kstack_end(stack)) {
-		addr = *stack++;
-		/*
-		 * If the address is either in the text segment of the
-		 * kernel, or in the region which contains vmalloc'ed
-		 * memory, it *may* be the address of a calling
-		 * routine; if so, print it so that someone tracing
-		 * down the cause of the crash will be able to figure
-		 * out the call path that was taken.
-		 */
-		if (kernel_text_address(addr))
-			print_ip_sym(addr);
+	words_to_show = (THREAD_SIZE - (fp & (THREAD_SIZE - 1))) >> 2;
+	if (kstack_depth_to_print && (words_to_show > kstack_depth_to_print))
+		words_to_show = kstack_depth_to_print;
+
+	pr_info("Kernel Stack:\n");
+
+	/*
+	 * Make the first line an 'odd' size if necessary to get
+	 * remaining lines to start at an address multiple of 0x10
+	 */
+	if (fp & 0xF) {
+		unsigned long line1_words = (0x10 - (fp & 0xF)) >> 2;
+		if (line1_words < words_to_show) {
+			print_hex_dump(KERN_INFO, "", DUMP_PREFIX_ADDRESS, 32,
+				       4, (void *)fp, line1_words << 2, 0);
+			fp += line1_words << 2;
+			words_to_show -= line1_words;
+		}
 	}
-	printk(KERN_NOTICE "\n");
+	print_hex_dump(KERN_INFO, "", DUMP_PREFIX_ADDRESS, 32, 4, (void *)fp,
+		       words_to_show << 2, 0);
+	printk(KERN_INFO "\n\n");
+
+	pr_info("Call Trace:\n");
+	microblaze_unwind(task, NULL);
+	pr_info("\n");
 
 	if (!task)
 		task = current;
@@ -62,34 +79,6 @@ void show_trace(struct task_struct *task
 	debug_show_held_locks(task);
 }
 
-void show_stack(struct task_struct *task, unsigned long *sp)
-{
-	unsigned long *stack;
-	int i;
-
-	if (sp == NULL) {
-		if (task)
-			sp = (unsigned long *) ((struct thread_info *)
-						(task->stack))->cpu_context.r1;
-		else
-			sp = (unsigned long *)&sp;
-	}
-
-	stack = sp;
-
-	printk(KERN_INFO "\nStack:\n  ");
-
-	for (i = 0; i < kstack_depth_to_print; i++) {
-		if (kstack_end(sp))
-			break;
-		if (i && ((i % 8) == 0))
-			printk("\n  ");
-		printk("%08lx ", *sp++);
-	}
-	printk("\n");
-	show_trace(task, stack);
-}
-
 void dump_stack(void)
 {
 	show_stack(NULL, NULL);
diff -uprN a/arch/microblaze/kernel/unwind.c b/arch/microblaze/kernel/unwind.c
--- a/arch/microblaze/kernel/unwind.c	1969-12-31 18:00:00.000000000 -0600
+++ b/arch/microblaze/kernel/unwind.c	2010-04-27 09:43:03.000000000 -0500
@@ -0,0 +1,318 @@
+/*
+ * Backtrace support for Microblaze
+ *
+ * Copyright (C) 2010  Digital Design Corporation
+ *
+ * Based on arch/sh/kernel/cpu/sh5/unwind.c code which is:
+ * Copyright (C) 2004  Paul Mundt
+ * Copyright (C) 2004  Richard Curnow
+ *
+ * This file is subject to the terms and conditions of the GNU General Public
+ * License.  See the file "COPYING" in the main directory of this archive
+ * for more details.
+ */
+
+/* #define DEBUG 1 */
+#include <linux/kallsyms.h>
+#include <linux/kernel.h>
+#include <linux/sched.h>
+#include <linux/stacktrace.h>
+#include <linux/types.h>
+#include <linux/errno.h>
+#include <linux/module.h>
+#include <linux/io.h>
+#include <asm/sections.h>
+#include <asm/exceptions.h>
+#include <asm/unwind.h>
+
+struct stack_trace;
+
+/*
+ * On Microblaze, finding the previous stack frame is a little tricky.
+ * At this writing (3/2010), Microblaze does not support CONFIG_FRAME_POINTERS,
+ * and even if it did, gcc (4.1.2) does not store the frame pointer at
+ * a consistent offset within each frame. To determine frame size, it is
+ * necessary to search for the assembly instruction that creates or reclaims
+ * the frame and extract the size from it.
+ *
+ * Microblaze stores the stack pointer in r1, and creates a frame via
+ *
+ *     addik r1, r1, -FRAME_SIZE
+ *
+ * The frame is reclaimed via
+ *
+ *     addik r1, r1, FRAME_SIZE
+ *
+ * Frame creation occurs at or near the top of a function.
+ * Depending on the compiler, reclaim may occur at the end, or before
+ * a mid-function return.
+ *
+ * A stack frame is usually not created in a leaf function.
+ *
+ */
+
+/**
+ * get_frame_size - Extract the stack adjustment from an
+ *                  "addik r1, r1, adjust" instruction
+ * @instr : Microblaze instruction
+ *
+ * Return - Number of stack bytes the instruction reserves or reclaims
+ */
+inline long get_frame_size(unsigned long instr)
+{
+	return abs((s16)(instr & 0xFFFF));
+}
+
+/**
+ * find_frame_creation - Search backward to find the instruction that creates
+ *                       the stack frame (hopefully, for the same function the
+ *                       initial PC is in).
+ * @pc : Program counter at which to begin the search
+ *
+ * Return - PC at which stack frame creation occurs
+ *          NULL if this cannot be found, i.e. a leaf function
+ */
+static unsigned long *find_frame_creation(unsigned long *pc)
+{
+	int i;
+
+	/* NOTE: Distance to search is arbitrary
+	 *	 250 works well for most things,
+	 *	 750 picks up things like tcp_recvmsg(),
+	 *	1000 needed for fat_fill_super()
+	 */
+	for (i = 0; i < 1000; i++, pc--) {
+		unsigned long instr;
+		s16 frame_size;
+
+		if (!kernel_text_address((unsigned long) pc))
+			return NULL;
+
+		instr = *pc;
+
+		/* addik r1, r1, foo ? */
+		if ((instr & 0xFFFF0000) != 0x30210000)
+			continue;	/* No */
+
+		frame_size = get_frame_size(instr);
+		if ((frame_size < 8) || (frame_size & 3)) {
+			pr_debug("    Invalid frame size %d at 0x%p\n",
+				 frame_size, pc);
+			return NULL;
+		}
+
+		pr_debug("    Found frame creation at 0x%p, size %d\n", pc,
+			 frame_size);
+		return pc;
+	}
+
+	return NULL;
+}
+
+/**
+ * lookup_prev_stack_frame - Find the stack frame of the previous function.
+ * @fp          : Frame (stack) pointer for current function
+ * @pc          : Program counter within current function
+ * @leaf_return : r15 value within current function. If the current function
+ *		  is a leaf, this is the caller's return address.
+ * @pprev_fp    : On exit, set to frame (stack) pointer for previous function
+ * @pprev_pc    : On exit, set to current function caller's return address
+ *
+ * Return - 0 on success, -EINVAL if the previous frame cannot be found
+ */
+static int lookup_prev_stack_frame(unsigned long fp, unsigned long pc,
+				   unsigned long leaf_return,
+				   unsigned long *pprev_fp,
+				   unsigned long *pprev_pc)
+{
+	unsigned long *prologue = NULL;
+
+	/* _switch_to is a special leaf function */
+	if (pc != (unsigned long) &_switch_to)
+		prologue = find_frame_creation((unsigned long *)pc);
+
+	if (prologue) {
+		long frame_size = get_frame_size(*prologue);
+
+		*pprev_fp = fp + frame_size;
+		*pprev_pc = *(unsigned long *)fp;
+	} else {
+		if (!leaf_return)
+			return -EINVAL;
+		*pprev_pc = leaf_return;
+		*pprev_fp = fp;
+	}
+
+	/* NOTE: don't check kernel_text_address here, to allow display
+	 *	 of userland return address
+	 */
+	return (!*pprev_pc || (*pprev_pc & 3)) ? -EINVAL : 0;
+}
+
+static void microblaze_unwind_inner(struct task_struct *task,
+				    unsigned long pc, unsigned long fp,
+				    unsigned long leaf_return,
+				    struct stack_trace *trace);
+
+/**
+ * unwind_trap - Unwind through a system trap, that stored previous state
+ *		 on the stack.
+ */
+#ifdef CONFIG_MMU
+static inline void unwind_trap(struct task_struct *task, unsigned long pc,
+				unsigned long fp, struct stack_trace *trace)
+{
+	/* To be implemented */
+}
+#else
+static inline void unwind_trap(struct task_struct *task, unsigned long pc,
+				unsigned long fp, struct stack_trace *trace)
+{
+	const struct pt_regs *regs = (const struct pt_regs *) fp;
+	microblaze_unwind_inner(task, regs->pc, regs->r1, regs->r15, trace);
+}
+#endif
+
+/**
+ * microblaze_unwind_inner - Unwind the stack from the specified point
+ * @task  : Task whose stack we are to unwind (may be NULL)
+ * @pc    : Program counter from which we start unwinding
+ * @fp    : Frame (stack) pointer from which we start unwinding
+ * @leaf_return : Value of r15 at pc. If the function is a leaf, this is
+ *				  the caller's return address.
+ * @trace : Where to store stack backtrace (PC values).
+ *	    NULL == print backtrace to kernel log
+ */
+void microblaze_unwind_inner(struct task_struct *task,
+			     unsigned long pc, unsigned long fp,
+			     unsigned long leaf_return,
+			     struct stack_trace *trace)
+{
+	int ofs = 0;
+
+	pr_debug("    Unwinding with PC=%p, FP=%p\n", (void *)pc, (void *)fp);
+	if (!pc || !fp || (pc & 3) || (fp & 3)) {
+		pr_debug("    Invalid state for unwind, aborting\n");
+		return;
+	}
+	for (; pc != 0;) {
+		unsigned long next_fp, next_pc = 0;
+		unsigned long return_to = pc +  2 * sizeof(unsigned long);
+		const struct trap_handler_info *handler =
+			&microblaze_trap_handlers;
+
+		/* Is previous function the HW exception handler? */
+		if ((return_to >= (unsigned long)&_hw_exception_handler)
+		    &&(return_to < (unsigned long)&ex_handler_unhandled)) {
+			/*
+			 * HW exception handler doesn't save all registers,
+			 * so we open-code a special case of unwind_trap()
+			 */
+#ifndef CONFIG_MMU
+			const struct pt_regs *regs =
+				(const struct pt_regs *) fp;
+#endif
+			pr_info("HW EXCEPTION\n");
+#ifndef CONFIG_MMU
+			microblaze_unwind_inner(task, regs->r17 - 4,
+						fp + EX_HANDLER_STACK_SIZ,
+						regs->r15, trace);
+#endif
+			return;
+		}
+
+		/* Is previous function a trap handler? */
+		for (; handler->start_addr; ++handler) {
+			if ((return_to >= handler->start_addr)
+			    && (return_to <= handler->end_addr)) {
+				if (!trace)
+					pr_info("%s\n", handler->trap_name);
+				unwind_trap(task, pc, fp, trace);
+				return;
+			}
+		}
+		pc -= ofs;
+
+		if (trace) {
+#ifdef CONFIG_STACKTRACE
+			if (trace->skip > 0)
+				trace->skip--;
+			else
+				trace->entries[trace->nr_entries++] = pc;
+
+			if (trace->nr_entries >= trace->max_entries)
+				break;
+#endif
+		} else {
+			/* Have we reached userland? */
+			if (unlikely(pc == task_pt_regs(task)->pc)) {
+				pr_info("[<%p>] PID %lu [%s]\n",
+					(void *) pc,
+					(unsigned long) task->pid,
+					task->comm);
+				break;
+			} else
+				print_ip_sym(pc);
+		}
+
+		/* Stop when we reach anything not part of the kernel */
+		if (!kernel_text_address(pc))
+			break;
+
+		if (lookup_prev_stack_frame(fp, pc, leaf_return, &next_fp,
+					    &next_pc) == 0) {
+			ofs = sizeof(unsigned long);
+			pc = next_pc & ~3;
+			fp = next_fp;
+			leaf_return = 0;
+		} else {
+			pr_debug("    Failed to find previous stack frame\n");
+			break;
+		}
+
+		pr_debug("    Next PC=%p, next FP=%p\n",
+			 (void *)next_pc, (void *)next_fp);
+	}
+}
+
+/**
+ * microblaze_unwind - Stack unwinder for Microblaze (external entry point)
+ * @task  : Task whose stack we are to unwind (NULL == current)
+ * @trace : Where to store stack backtrace (PC values).
+ *	    NULL == print backtrace to kernel log
+ */
+void microblaze_unwind(struct task_struct *task, struct stack_trace *trace)
+{
+	if (task) {
+		if (task == current) {
+			const struct pt_regs *regs = task_pt_regs(task);
+			microblaze_unwind_inner(task, regs->pc, regs->r1,
+						regs->r15, trace);
+		} else {
+			struct thread_info *thread_info =
+				(struct thread_info *)(task->stack);
+			const struct cpu_context *cpu_context =
+				&thread_info->cpu_context;
+
+			microblaze_unwind_inner(task,
+						(unsigned long) &_switch_to,
+						cpu_context->r1,
+						cpu_context->r15, trace);
+		}
+	} else {
+		unsigned long pc, fp;
+
+		__asm__ __volatile__ ("or %0, r1, r0" : "=r" (fp));
+
+		__asm__ __volatile__ (
+			"brlid %0, 0f;"
+			"nop;"
+			"0:"
+			: "=r" (pc)
+		);
+
+		/* Since we are not a leaf function, use leaf_return = 0 */
+		microblaze_unwind_inner(current, pc, fp, 0, trace);
+	}
+}
+

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