On Tue, Feb 23, 2010 at 08:18:33AM -0500, Steven Rostedt wrote:

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

quoted

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?

May be yeah, with an explicit message that describes the issue.


 

quoted

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.

Yeah. I understand now why we can't do that on Kconfig time, HOSTCC
can be different than CC.

The most important thing is to have a message that describes the issue
if the compiler-config combination is impossible.