ARM: fix function graph tracer and unwinder dependencies

Naresh Kamboju recently reported that the function-graph tracer crashes
on ARM. The function-graph tracer assumes that the kernel is built with
frame pointers.

We explicitly disabled the function-graph tracer when building Thumb2,
since the Thumb2 ABI doesn't have frame pointers.

We recently changed the way the unwinder method was selected, which
seems to have made it more likely that we can end up with the function-
graph tracer enabled but without the kernel built with frame pointers.

Fix up the function graph tracer dependencies so the option is not
available when we have no possibility of having frame pointers, and
adjust the dependencies on the unwinder option to hide the non-frame
pointer unwinder options if the function-graph tracer is enabled.
Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
Tested-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>

arch/arm/Kconfig

@@ -73,7 +73,7 @@ config ARM
 	select HAVE_EFFICIENT_UNALIGNED_ACCESS if (CPU_V6 || CPU_V6K || CPU_V7) && MMU
 	select HAVE_EXIT_THREAD
 	select HAVE_FTRACE_MCOUNT_RECORD if !XIP_KERNEL
-	select HAVE_FUNCTION_GRAPH_TRACER if !THUMB2_KERNEL
+	select HAVE_FUNCTION_GRAPH_TRACER if !THUMB2_KERNEL && !CC_IS_CLANG
 	select HAVE_FUNCTION_TRACER if !XIP_KERNEL
 	select HAVE_GCC_PLUGINS
 	select HAVE_HW_BREAKPOINT if PERF_EVENTS && (CPU_V6 || CPU_V6K || CPU_V7)

arch/arm/Kconfig.debug

@@ -47,8 +47,8 @@ config DEBUG_WX
 
 choice
 	prompt "Choose kernel unwinder"
-	default UNWINDER_ARM if AEABI && !FUNCTION_GRAPH_TRACER
-	default UNWINDER_FRAME_POINTER if !AEABI || FUNCTION_GRAPH_TRACER
+	default UNWINDER_ARM if AEABI
+	default UNWINDER_FRAME_POINTER if !AEABI
 	help
 	  This determines which method will be used for unwinding kernel stack
 	  traces for panics, oopses, bugs, warnings, perf, /proc/<pid>/stack,
@@ -65,7 +65,7 @@ config UNWINDER_FRAME_POINTER
 
 config UNWINDER_ARM
 	bool "ARM EABI stack unwinder"
-	depends on AEABI
+	depends on AEABI && !FUNCTION_GRAPH_TRACER
 	select ARM_UNWIND
 	help
 	  This option enables stack unwinding support in the kernel