ARM: 6314/1: ftrace: allow build without frame pointers on ARM

With a new enough GCC, ARM function tracing can be supported without the
need for frame pointers.  This is essential for Thumb-2 support, since
frame pointers aren't available then.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Rabin Vincent <rabin@rab.in>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/Kconfig.debug

@@ -27,6 +27,11 @@ config ARM_UNWIND
 	  the performance is not affected. Currently, this feature
 	  only works with EABI compilers. If unsure say Y.
 
+config OLD_MCOUNT
+	bool
+	depends on FUNCTION_TRACER && FRAME_POINTER
+	default y
+
 config DEBUG_USER
 	bool "Verbose user fault messages"
 	help

arch/arm/kernel/armksyms.c

@@ -165,6 +165,8 @@ EXPORT_SYMBOL(_find_next_bit_be);
 #endif
 
 #ifdef CONFIG_FUNCTION_TRACER
+#ifdef CONFIG_OLD_MCOUNT
 EXPORT_SYMBOL(mcount);
+#endif
 EXPORT_SYMBOL(__gnu_mcount_nc);
 #endif

arch/arm/kernel/entry-common.S

@@ -128,6 +128,13 @@ ENDPROC(ret_from_fork)
  * allows it to be clobbered in subroutines and doesn't use it to hold
  * parameters.)
  */
+
+#ifndef CONFIG_OLD_MCOUNT
+#if (__GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 4))
+#error Ftrace requires CONFIG_FRAME_POINTER=y with GCC older than 4.4.0.
+#endif
+#endif
+
 #ifdef CONFIG_DYNAMIC_FTRACE
 ENTRY(mcount)
 	stmdb	sp!, {r0-r3, lr}
@@ -173,6 +180,12 @@ gnu_trace:
 	ldmia	sp!, {r0-r3, ip, lr}
 	mov	pc, ip
 
+#ifdef CONFIG_OLD_MCOUNT
+/*
+ * This is under an ifdef in order to force link-time errors for people trying
+ * to build with !FRAME_POINTER with a GCC which doesn't use the new-style
+ * mcount.
+ */
 ENTRY(mcount)
 	stmdb	sp!, {r0-r3, lr}
 	ldr	r0, =ftrace_trace_function
@@ -191,6 +204,7 @@ trace:
 	mov	pc, r2
 	ldr	lr, [fp, #-4]			@ restore lr
 	ldmia	sp!, {r0-r3, pc}
+#endif
 
 #endif /* CONFIG_DYNAMIC_FTRACE */
 

kernel/trace/Kconfig

@@ -121,7 +121,7 @@ if FTRACE
 config FUNCTION_TRACER
 	bool "Kernel Function Tracer"
 	depends on HAVE_FUNCTION_TRACER
-	select FRAME_POINTER
+	select FRAME_POINTER if (!ARM_UNWIND)
 	select KALLSYMS
 	select GENERIC_TRACER
 	select CONTEXT_SWITCH_TRACER