1 files changed, 80 insertions, 44 deletions

diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
index 2c1db77d7848..f05a35a59694 100644
--- a/arch/arm/kernel/entry-common.S
+++ b/arch/arm/kernel/entry-common.S
@@ -92,75 +92,111 @@ 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}
-        mov r0, lr
-        sub r0, r0, #MCOUNT_INSN_SIZE
+        stmdb   sp!, {r0-r3, lr}
+        mov     r0, lr
+        sub     r0, r0, #MCOUNT_INSN_SIZE
 
         .globl mcount_call
 mcount_call:
-        bl ftrace_stub
-        ldr lr, [fp, #-4]                       @ restore lr
-        ldmia sp!, {r0-r3, pc}
+        bl      ftrace_stub
+        ldr     lr, [fp, #-4]                   @ restore lr
+        ldmia   sp!, {r0-r3, pc}
 
 ENTRY(ftrace_caller)
-        stmdb sp!, {r0-r3, lr}
-        ldr r1, [fp, #-4]
-        mov r0, lr
-        sub r0, r0, #MCOUNT_INSN_SIZE
+        stmdb   sp!, {r0-r3, lr}
+        ldr     r1, [fp, #-4]
+        mov     r0, lr
+        sub     r0, r0, #MCOUNT_INSN_SIZE
 
         .globl ftrace_call
 ftrace_call:
-        bl ftrace_stub
-        ldr lr, [fp, #-4]                       @ restore lr
-        ldmia sp!, {r0-r3, pc}
+        bl      ftrace_stub
+        ldr     lr, [fp, #-4]                   @ restore lr
+        ldmia   sp!, {r0-r3, pc}
 
 #else
 
 ENTRY(__gnu_mcount_nc)
-        stmdb sp!, {r0-r3, lr}
-        ldr r0, =ftrace_trace_function
-        ldr r2, [r0]
-        adr r0, ftrace_stub
-        cmp r0, r2
-        bne gnu_trace
-        ldmia sp!, {r0-r3, ip, lr}
-        mov pc, ip
+        stmdb   sp!, {r0-r3, lr}
+        ldr     r0, =ftrace_trace_function
+        ldr     r2, [r0]
+        adr     r0, ftrace_stub
+        cmp     r0, r2
+        bne     gnu_trace
+        ldmia   sp!, {r0-r3, ip, lr}
+        mov     pc, ip
 
 gnu_trace:
-        ldr r1, [sp, #20]                       @ lr of instrumented routine
-        mov r0, lr
-        sub r0, r0, #MCOUNT_INSN_SIZE
-        mov lr, pc
-        mov pc, r2
-        ldmia sp!, {r0-r3, ip, lr}
-        mov pc, ip
+        ldr     r1, [sp, #20]                   @ lr of instrumented routine
+        mov     r0, lr
+        sub     r0, r0, #MCOUNT_INSN_SIZE
+        mov     lr, pc
+        mov     pc, r2
+        ldmia   sp!, {r0-r3, ip, lr}
+        mov     pc, ip
 
 ENTRY(mcount)
-        stmdb sp!, {r0-r3, lr}
-        ldr r0, =ftrace_trace_function
-        ldr r2, [r0]
-        adr r0, ftrace_stub
-        cmp r0, r2
-        bne trace
-        ldr lr, [fp, #-4]                       @ restore lr
-        ldmia sp!, {r0-r3, pc}
+        stmdb   sp!, {r0-r3, lr}
+        ldr     r0, =ftrace_trace_function
+        ldr     r2, [r0]
+        adr     r0, ftrace_stub
+        cmp     r0, r2
+        bne     trace
+        ldr     lr, [fp, #-4]                   @ restore lr
+        ldmia   sp!, {r0-r3, pc}
 
 trace:
-        ldr r1, [fp, #-4]                       @ lr of instrumented routine
-        mov r0, lr
-        sub r0, r0, #MCOUNT_INSN_SIZE
-        mov lr, pc
-        mov pc, r2
-        ldr lr, [fp, #-4]                       @ restore lr
-        ldmia sp!, {r0-r3, pc}
+        ldr     r1, [fp, #-4]                   @ lr of instrumented routine
+        mov     r0, lr
+        sub     r0, r0, #MCOUNT_INSN_SIZE
+        mov     lr, pc
+        mov     pc, r2
+        ldr     lr, [fp, #-4]                   @ restore lr
+        ldmia   sp!, {r0-r3, pc}
 
 #endif /* CONFIG_DYNAMIC_FTRACE */
 
         .globl ftrace_stub
 ftrace_stub:
-        mov pc, lr
+        mov     pc, lr
 
 #endif /* CONFIG_FUNCTION_TRACER */