powerpc/ftrace: Disable ftrace on ppc32 if using clang

Ftrace on ppc32 expects a three instruction sequence at the beginning of
each function when specifying -pg:
	mflr	r0
	stw	r0,4(r1)
	bl	_mcount

This is the case with all supported versions of gcc. Clang however emits
a branch to _mcount after the function prologue, similar to the pre
-mprofile-kernel ABI on ppc64. This is not supported.

Disable ftrace on ppc32 if using clang for now. This can be re-enabled
later if clang picks up support for -fpatchable-function-entry on ppc32.

Signed-off-by: Naveen N Rao <[email protected]>
Acked-by: Nick Desaulniers <[email protected]>
Link: llvm/llvm-project#63220
Signed-off-by: Michael Ellerman <[email protected]>
Link: https://msgid.link/[email protected]

Author: rnav

arch/powerpc/Kconfig

@@ -236,7 +236,7 @@ config PPC
 	select HAVE_FUNCTION_DESCRIPTORS	if PPC64_ELF_ABI_V1
 	select HAVE_FUNCTION_ERROR_INJECTION
 	select HAVE_FUNCTION_GRAPH_TRACER
-	select HAVE_FUNCTION_TRACER
+	select HAVE_FUNCTION_TRACER		if PPC64 || (PPC32 && CC_IS_GCC)
 	select HAVE_GCC_PLUGINS			if GCC_VERSION >= 50200   # plugin support on gcc <= 5.1 is buggy on PPC
 	select HAVE_GENERIC_VDSO
 	select HAVE_HARDLOCKUP_DETECTOR_ARCH	if PPC_BOOK3S_64 && SMP