That thread "MIPS: Add option to pass return address location to
_mcount" from "David Daney <ddaney@caviumnetworks.com>" have added a new
option -mmcount-ra-address to gcc(4.5) for MIPS to transfer the location
of the return address to _mcount.

Benefit from this new feature, function graph tracer on MIPS will be
easier and safer to hijack the return address of the kernel function,
which will save some overhead and make the whole thing more reliable.

In this patch, at first, try to enable the option -mmcount-ra-address in
arch/mips/Makefile with cc-option, if gcc support it, it will be
enabled, otherwise, no side effect.

and then, we need to support this new option of gcc 4.5 and also support
the old gcc versions.

with _mcount in the old gcc versions, it's not easy to get the location
of return address(tracing: add function graph tracer support for MIPS),
   so, we do it in a C function: ftrace_get_parent_addr(ftrace.c), but
   with -mmcount-ra-address, only several instructions need to get what
   we want, so, I put into asm(mcount.S). and also, as the $12(t0) is
   used by -mmcount-ra-address for transferring the localtion of return
   address to _mcount, we need to save it into the stack and restore it
   when enabled dynamic function tracer, 'Cause we have called
   "ftrace_call" before "ftrace_graph_caller", which may destroy
   $12(t0).

(Thanks to David for providing that -mcount-ra-address and giving the
 idea of KBUILD_MCOUNT_RA_ADDRESS, both of them have made the whole
 thing more beautiful!)

Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Nicholas Mc Guire <der.herr@hofr.at>
Cc: zhangfx@lemote.com
Cc: Wu Zhangjin <wuzhangjin@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: linux-kernel@vger.kernel.org
Cc: linux-mips@linux-mips.org
Patchwork: http://patchwork.linux-mips.org/patch/681/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>