On Mon, 2010-02-22 at 20:05 +0100, Frederic Weisbecker wrote:

Btw, you described that mcount is used in some gcc versions and
__gnu_mcount_nc in newers.
Shouldn't we have a gcc version dependency here? (not sure we can
do this from Kconfig though).

Maybe we can add something to recordmcount.pl?

quoted

 config DEBUG_USER
 	bool "Verbose user fault messages"
 	help

diff --git a/arch/arm/kernel/armksyms.c b/arch/arm/kernel/armksyms.c
index 8214bfe..e5e1e53 100644
--- a/arch/arm/kernel/armksyms.c
+++ b/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

diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
index d412d7c..c98e3a3 100644
--- a/arch/arm/kernel/entry-common.S
+++ b/arch/arm/kernel/entry-common.S

@@ -169,6 +169,12 @@ 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

@@ -187,6 +193,7 @@ trace:
 	mov	pc, r2
 	ldr	lr, [fp, #-4]			@ restore lr
 	ldmia	sp!, {r0-r3, pc}
+#endif
 
 #endif /* CONFIG_DYNAMIC_FTRACE */
 

diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
index 6c22d8a..7468ffe 100644
--- a/kernel/trace/Kconfig
+++ b/kernel/trace/Kconfig

@@ -126,7 +126,7 @@ if FTRACE
 config FUNCTION_TRACER
 	bool "Kernel Function Tracer"
 	depends on HAVE_FUNCTION_TRACER
-	select FRAME_POINTER
+	select FRAME_POINTER if (!ARM_UNWIND)

So, if I understand well. If people have ARM_UNWIND but
FUNCTION_TRACER, it might crash at link time in case
they don't have a recent enough gcc version to
support the new -pg style?

That doesn't look good.

Ideally, we need HAVE_FUNCTION_TRACER to be enabled in arm
only if (old gcc && frame pointers) || new gcc

And then, we need config OLD_MCOUNT:
	 old gcc && FUNCTION_TRACER

and config NEW_MCOUNT:
	new gcc && FUNCTION_TRACER

so that we can selectively build mcount or __gnu_mcount_nc.

Hmm, I fear we can't check gcc version from Kconfig, as I'm
grepping on Kconfig files...

I would not check gcc version through KCONFIG, but you can have the gcc
version checked and define another macro in a header somewhere, like
asm/ftrace.h. Then we could do something different while it is
compiling.

Disclaimer:
I have to go back and read the entire thread, to know what exactly is
going on ;-)

-- Steve