linux-kernel - [PATCH 02/10] ARM: ftrace: document mcount formats

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Message-Id: <1266090518-31120-3-git-send-email-rabin@rab.in>
Date:	Sun, 14 Feb 2010 01:18:30 +0530
From:	Rabin Vincent <rabin@....in>
To:	linux-kernel@...r.kernel.org
Cc:	linux-arm-kernel@...ts.infradead.org, Rabin Vincent <rabin@....in>,
	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Abhishek Sagar <sagar.abhishek@...il.com>,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
Subject: [PATCH 02/10] ARM: ftrace: document mcount formats

Add a comment describing the mcount variants and how the callsites look
like.

Signed-off-by: Rabin Vincent <rabin@....in>
---
 arch/arm/kernel/entry-common.S |   32 ++++++++++++++++++++++++++++++++
 1 files changed, 32 insertions(+), 0 deletions(-)

diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
index 0b042bd..d412d7c 100644
--- a/arch/arm/kernel/entry-common.S
+++ b/arch/arm/kernel/entry-common.S
@@ -92,6 +92,38 @@ 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.
+ *
+ * With both the mcount types, we need to restore the original lr before
+ * returning.  In the __gnu_mcount_nc, version we're allowed to clobber ip.
+ * No other registers should be clobbered.
+ */
 #ifdef CONFIG_DYNAMIC_FTRACE
 ENTRY(mcount)
 	stmdb	sp!, {r0-r3, lr}
-- 
1.6.6

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