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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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