LXR linux/arch/arm/kernel/ftrace.c

   1/*
   2 * Dynamic function tracing support.
   3 *
   4 * Copyright (C) 2008 Abhishek Sagar <sagar.abhishek@gmail.com>
   5 *
   6 * For licencing details, see COPYING.
   7 *
   8 * Defines low-level handling of mcount calls when the kernel
   9 * is compiled with the -pg flag. When using dynamic ftrace, the
  10 * mcount call-sites get patched lazily with NOP till they are
  11 * enabled. All code mutation routines here take effect atomically.
  12 */
  13
  14#include <linux/ftrace.h>
  15
  16#include <asm/cacheflush.h>
  17#include <asm/ftrace.h>
  18
  19#define PC_OFFSET      8
  20#define BL_OPCODE      0xeb000000
  21#define BL_OFFSET_MASK 0x00ffffff
  22
  23static unsigned long bl_insn;
  24static const unsigned long NOP = 0xe1a00000; /* mov r0, r0 */
  25
  26unsigned char *ftrace_nop_replace(void)
  27{
  28        return (char *)&NOP;
  29}
  30
  31/* construct a branch (BL) instruction to addr */
  32unsigned char *ftrace_call_replace(unsigned long pc, unsigned long addr)
  33{
  34        long offset;
  35
  36        offset = (long)addr - (long)(pc + PC_OFFSET);
  37        if (unlikely(offset < -33554432 || offset > 33554428)) {
  38                /* Can't generate branches that far (from ARM ARM). Ftrace
  39                 * doesn't generate branches outside of kernel text.
  40                 */
  41                WARN_ON_ONCE(1);
  42                return NULL;
  43        }
  44        offset = (offset >> 2) & BL_OFFSET_MASK;
  45        bl_insn = BL_OPCODE | offset;
  46        return (unsigned char *)&bl_insn;
  47}
  48
  49int ftrace_modify_code(unsigned long pc, unsigned char *old_code,
  50                       unsigned char *new_code)
  51{
  52        unsigned long err = 0, replaced = 0, old, new;
  53
  54        old = *(unsigned long *)old_code;
  55        new = *(unsigned long *)new_code;
  56
  57        __asm__ __volatile__ (
  58                "1:  ldr    %1, [%2]  \n"
  59                "    cmp    %1, %4    \n"
  60                "2:  streq  %3, [%2]  \n"
  61                "    cmpne  %1, %3    \n"
  62                "    movne  %0, #2    \n"
  63                "3:\n"
  64
  65                ".section .fixup, \"ax\"\n"
  66                "4:  mov  %0, #1  \n"
  67                "    b    3b      \n"
  68                ".previous\n"
  69
  70                ".section __ex_table, \"a\"\n"
  71                "    .long 1b, 4b \n"
  72                "    .long 2b, 4b \n"
  73                ".previous\n"
  74
  75                : "=r"(err), "=r"(replaced)
  76                : "r"(pc), "r"(new), "r"(old), "0"(err), "1"(replaced)
  77                : "memory");
  78
  79        if (!err && (replaced == old))
  80                flush_icache_range(pc, pc + MCOUNT_INSN_SIZE);
  81
  82        return err;
  83}
  84
  85int ftrace_update_ftrace_func(ftrace_func_t func)
  86{
  87        int ret;
  88        unsigned long pc, old;
  89        unsigned char *new;
  90
  91        pc = (unsigned long)&ftrace_call;
  92        memcpy(&old, &ftrace_call, MCOUNT_INSN_SIZE);
  93        new = ftrace_call_replace(pc, (unsigned long)func);
  94        ret = ftrace_modify_code(pc, (unsigned char *)&old, new);
  95        return ret;
  96}
  97
  98/* run from ftrace_init with irqs disabled */
  99int __init ftrace_dyn_arch_init(void *data)
 100{
 101        ftrace_mcount_set(data);
 102        return 0;
 103}
 104