implicit declaration of function 'int func(...)'

最近在改进SD卡的SEP4020驱动时，在改掉所有的warning时老报一个implicit declaration of function 的警告错误，上网查了下原因，原来有两种情况会产生这种情况
1  没有把函数所在的c文件生成.o目标文件
2  在函数所在的c文件中定义了，但是没有在与之相关联的.h文件中声明
[root@mail linux]# make
  CHK     include/linux/version.h
make[1]: “include/asm-arm/mach-types.h”是最新的。
  CHK     include/linux/compile.h
  CHK     usr/initramfs_list
  CC      drivers/mmc/sep_mci.o
drivers/mmc/sep_mci.c: In function `sep4020sdi_probe':
drivers/mmc/sep_mci.c:745: warning: implicit declaration of function `clk_get'
drivers/mmc/sep_mci.c:745: warning: assignment makes pointer from integer without a cast
drivers/mmc/sep_mci.c:754: warning: implicit declaration of function `clk_enable'
drivers/mmc/sep_mci.c:800: warning: implicit declaration of function `clk_put'
drivers/mmc/sep_mci.c: In function `sep4020sdi_remove':
drivers/mmc/sep_mci.c:830: warning: implicit declaration of function `clk_disable'
drivers/mmc/sep_mci.c: In function `sep4020sdi_irq':
drivers/mmc/sep_mci.c:197: warning: 'sdi_cclear' might be used uninitialized in this function
drivers/mmc/sep_mci.c: In function `sep4020sdi_probe':
drivers/mmc/sep_mci.c:673: warning: 'host' might be used uninitialized in this function
drivers/mmc/sep_mci.c: At top level:
drivers/mmc/sep_mci.c:337: warning: 'sep4020sdi_irq_cd' defined but not used
  LD      drivers/mmc/built-in.o
  LD      drivers/built-in.o
  GEN     .version
  CHK     include/linux/compile.h
  UPD     include/linux/compile.h
  CC      init/version.o
  LD      init/built-in.o
  LD      .tmp_vmlinux1
  KSYM    .tmp_kallsyms1.S
  AS      .tmp_kallsyms1.o
  LD      .tmp_vmlinux2
  KSYM    .tmp_kallsyms2.S
  AS      .tmp_kallsyms2.o
  LD      vmlinux
  SYSMAP  System.map
  SYSMAP  .tmp_System.map
  OBJCOPY arch/arm/boot/Image
  Kernel: arch/arm/boot/Image is ready
  GZIP    arch/arm/boot/compressed/piggy.gz
  AS      arch/arm/boot/compressed/piggy.o
  LD      arch/arm/boot/compressed/vmlinux
  OBJCOPY arch/arm/boot/zImage
  Kernel: arch/arm/boot/zImage is ready
  Building modules, stage 2.
  MODPOST
于是我在sep_mci.c的头部将 <linux/clk.h>包含进去就行了
这样就没有任何warning产生了，呵呵