ARM: 6312/1: ftrace: allow building without frame pointers

With current gcc, compiling with both -pg and -fomit-frame-pointer is
not allowed.  However, -pg can be used to build without actually
specifying -fno-omit-frame-pointer, upon which the default behaviour
for the target will be used.

On ARM, it is not possible to build a Thumb-2 kernel with
-fno-omit-frame-pointer (FRAME_POINTERS depends on !THUMB2_KERNEL). In
order to support ftrace for Thumb-2, we need to be able to allow a
combination of FUNCTION_TRACER and !FRAME_POINTER.  We do this by
omitting -fomit-frame-pointer if ftrace is enabled.

Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Rabin Vincent <rabin@rab.in>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

Makefile

@@ -554,8 +554,15 @@ endif
 ifdef CONFIG_FRAME_POINTER
 KBUILD_CFLAGS	+= -fno-omit-frame-pointer -fno-optimize-sibling-calls
 else
+# Some targets (ARM with Thumb2, for example), can't be built with frame
+# pointers.  For those, we don't have FUNCTION_TRACER automatically
+# select FRAME_POINTER.  However, FUNCTION_TRACER adds -pg, and this is
+# incompatible with -fomit-frame-pointer with current GCC, so we don't use
+# -fomit-frame-pointer with FUNCTION_TRACER.
+ifndef CONFIG_FUNCTION_TRACER
 KBUILD_CFLAGS	+= -fomit-frame-pointer
 endif
+endif
 
 ifdef CONFIG_DEBUG_INFO
 KBUILD_CFLAGS	+= -g