sh: Generalize CALLER_ADDRx support.

This splits out the unwinder implementation and adds a new
return_address() abstraction modelled after the ARM code. The DWARF
unwinder is tied in to this, returning NULL otherwise in the case of
being unable to support arbitrary depths.

This enables us to get correct behaviour with the unwinder enabled,
as well as disabling the arbitrary depth support when frame pointers are
enabled, as arbitrary depths with __builtin_return_address() are not
supported regardless.

With this abstraction it's also possible to layer on a simplified
implementation with frame pointers in the event that the unwinder isn't
enabled, although this is left as a future exercise.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>

arch/sh/kernel/Makefile

@@ -11,6 +11,7 @@ endif
 
 obj-y	:= debugtraps.o dumpstack.o idle.o io.o io_generic.o irq.o	\
 	   machvec.o nmi_debug.o process_$(BITS).o ptrace_$(BITS).o	\
+	   return_address.o						\
 	   setup.o signal_$(BITS).o sys_sh.o sys_sh$(BITS).o		\
 	   syscalls_$(BITS).o time.o topology.o traps.o			\
 	   traps_$(BITS).o unwinder.o

arch/sh/kernel/return_address.c

@@ -0,0 +1,54 @@
+/*
+ * arch/sh/kernel/return_address.c
+ *
+ * Copyright (C) 2009  Matt Fleming
+ * Copyright (C) 2009  Paul Mundt
+ *
+ * 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.
+ */
+#include <linux/kernel.h>
+#include <asm/dwarf.h>
+
+#ifdef CONFIG_DWARF_UNWINDER
+
+void *return_address(unsigned int depth)
+{
+	struct dwarf_frame *frame;
+	unsigned long ra;
+	int i;
+
+	for (i = 0, frame = NULL, ra = 0; i <= depth; i++) {
+		struct dwarf_frame *tmp;
+
+		tmp = dwarf_unwind_stack(ra, frame);
+
+		if (frame)
+			dwarf_free_frame(frame);
+
+		frame = tmp;
+
+		if (!frame || !frame->return_addr)
+			break;
+
+		ra = frame->return_addr;
+	}
+
+	/* Failed to unwind the stack to the specified depth. */
+	WARN_ON(i != depth + 1);
+
+	if (frame)
+		dwarf_free_frame(frame);
+
+	return (void *)ra;
+}
+
+#else
+
+void *return_address(unsigned int depth)
+{
+	return NULL;
+}
+
+#endif