ARM: 6314/1: ftrace: allow build without frame pointers on ARM

With a new enough GCC, ARM function tracing can be supported without the
need for frame pointers.  This is essential for Thumb-2 support, since
frame pointers aren't available then.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Rabin Vincent <rabin@rab.in>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

3 files changed, 21 insertions, 0 deletions

diff --git a/arch/arm/Kconfig.debug b/arch/arm/Kconfig.debug
index 91344af75f39..4dbce538fec4 100644
--- a/arch/arm/Kconfig.debug
+++ b/arch/arm/Kconfig.debug
@@ -27,6 +27,11 @@ config ARM_UNWIND
           the performance is not affected. Currently, this feature
           only works with EABI compilers. If unsure say Y.
 
+config OLD_MCOUNT
+        bool
+        depends on FUNCTION_TRACER && FRAME_POINTER
+        default y
+
 config DEBUG_USER
         bool "Verbose user fault messages"
         help
diff --git a/arch/arm/kernel/armksyms.c b/arch/arm/kernel/armksyms.c
index 8214bfebfaca..e5e1e5387678 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 f05a35a59694..6805a7216bf8 100644
--- a/arch/arm/kernel/entry-common.S
+++ b/arch/arm/kernel/entry-common.S
@@ -128,6 +128,13 @@ ENDPROC(ret_from_fork)
  * allows it to be clobbered in subroutines and doesn't use it to hold
  * parameters.)
  */
+
+#ifndef CONFIG_OLD_MCOUNT
+#if (__GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 4))
+#error Ftrace requires CONFIG_FRAME_POINTER=y with GCC older than 4.4.0.
+#endif
+#endif
+
 #ifdef CONFIG_DYNAMIC_FTRACE
 ENTRY(mcount)
         stmdb   sp!, {r0-r3, lr}
@@ -173,6 +180,12 @@ 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
@@ -191,6 +204,7 @@ trace:
         mov     pc, r2
         ldr     lr, [fp, #-4]                   @ restore lr
         ldmia   sp!, {r0-r3, pc}
+#endif
 
 #endif /* CONFIG_DYNAMIC_FTRACE */