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ftrace.c

/*
 * Dynamic function tracing support.
 *
 * Copyright (C) 2008 Abhishek Sagar <sagar.abhishek@gmail.com>
 *
 * For licencing details, see COPYING.
 *
 * Defines low-level handling of mcount calls when the kernel
 * is compiled with the -pg flag. When using dynamic ftrace, the
 * mcount call-sites get patched lazily with NOP till they are
 * enabled. All code mutation routines here take effect atomically.
 */

#include <linux/ftrace.h>

#include <asm/cacheflush.h>
#include <asm/ftrace.h>

#define PC_OFFSET      8
#define BL_OPCODE      0xeb000000
#define BL_OFFSET_MASK 0x00ffffff

static unsigned long bl_insn;
static const unsigned long NOP = 0xe1a00000; /* mov r0, r0 */

unsigned char *ftrace_nop_replace(void)
{
      return (char *)&NOP;
}

/* construct a branch (BL) instruction to addr */
unsigned char *ftrace_call_replace(unsigned long pc, unsigned long addr)
{
      long offset;

      offset = (long)addr - (long)(pc + PC_OFFSET);
      if (unlikely(offset < -33554432 || offset > 33554428)) {
            /* Can't generate branches that far (from ARM ARM). Ftrace
             * doesn't generate branches outside of kernel text.
             */
            WARN_ON_ONCE(1);
            return NULL;
      }
      offset = (offset >> 2) & BL_OFFSET_MASK;
      bl_insn = BL_OPCODE | offset;
      return (unsigned char *)&bl_insn;
}

int ftrace_modify_code(unsigned long pc, unsigned char *old_code,
                   unsigned char *new_code)
{
      unsigned long err = 0, replaced = 0, old, new;

      old = *(unsigned long *)old_code;
      new = *(unsigned long *)new_code;

      __asm__ __volatile__ (
            "1:  ldr    %1, [%2]  \n"
            "    cmp    %1, %4    \n"
            "2:  streq  %3, [%2]  \n"
            "    cmpne  %1, %3    \n"
            "    movne  %0, #2    \n"
            "3:\n"

            ".section .fixup, \"ax\"\n"
            "4:  mov  %0, #1  \n"
            "    b    3b      \n"
            ".previous\n"

            ".section __ex_table, \"a\"\n"
            "    .long 1b, 4b \n"
            "    .long 2b, 4b \n"
            ".previous\n"

            : "=r"(err), "=r"(replaced)
            : "r"(pc), "r"(new), "r"(old), "0"(err), "1"(replaced)
            : "memory");

      if (!err && (replaced == old))
            flush_icache_range(pc, pc + MCOUNT_INSN_SIZE);

      return err;
}

int ftrace_update_ftrace_func(ftrace_func_t func)
{
      int ret;
      unsigned long pc, old;
      unsigned char *new;

      pc = (unsigned long)&ftrace_call;
      memcpy(&old, &ftrace_call, MCOUNT_INSN_SIZE);
      new = ftrace_call_replace(pc, (unsigned long)func);
      ret = ftrace_modify_code(pc, (unsigned char *)&old, new);
      return ret;
}

/* run from kstop_machine */
int __init ftrace_dyn_arch_init(void *data)
{
      ftrace_mcount_set(data);
      return 0;
}

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