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Subject[PATCH 02/10] ARM: ftrace: document mcount formats
Date
Add a comment describing the mcount variants and how the callsites look
like.

Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Rabin Vincent <rabin@rab.in>
---
v2: Reword/expand comment.

arch/arm/kernel/entry-common.S | 36 ++++++++++++++++++++++++++++++++++++
1 files changed, 36 insertions(+), 0 deletions(-)

diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
index 0b042bd..f05a35a 100644
--- a/arch/arm/kernel/entry-common.S
+++ b/arch/arm/kernel/entry-common.S
@@ -92,6 +92,42 @@ ENDPROC(ret_from_fork)
#define CALL(x) .long x

#ifdef CONFIG_FUNCTION_TRACER
+/*
+ * When compiling with -pg, gcc inserts a call to the mcount routine at the
+ * start of every function. In mcount, apart from the function's address (in
+ * lr), we need to get hold of the function's caller's address.
+ *
+ * Older GCCs (pre-4.4) inserted a call to a routine called mcount like this:
+ *
+ * bl mcount
+ *
+ * These versions have the limitation that in order for the mcount routine to
+ * be able to determine the function's caller's address, an APCS-style frame
+ * pointer (which is set up with something like the code below) is required.
+ *
+ * mov ip, sp
+ * push {fp, ip, lr, pc}
+ * sub fp, ip, #4
+ *
+ * With EABI, these frame pointers are not available unless -mapcs-frame is
+ * specified, and if building as Thumb-2, not even then.
+ *
+ * Newer GCCs (4.4+) solve this problem by introducing a new version of mcount,
+ * with call sites like:
+ *
+ * push {lr}
+ * bl __gnu_mcount_nc
+ *
+ * With these compilers, frame pointers are not necessary.
+ *
+ * mcount can be thought of as a function called in the middle of a subroutine
+ * call. As such, it needs to be transparent for both the caller and the
+ * callee: the original lr needs to be restored when leaving mcount, and no
+ * registers should be clobbered. (In the __gnu_mcount_nc implementation, we
+ * clobber the ip register. This is OK because the ARM calling convention
+ * allows it to be clobbered in subroutines and doesn't use it to hold
+ * parameters.)
+ */
#ifdef CONFIG_DYNAMIC_FTRACE
ENTRY(mcount)
stmdb sp!, {r0-r3, lr}
--
1.7.0


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