ARM: 6288/1: ftrace: document mcount formats

#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}