implicit declaration of function 這種警告問題的原因及解決方案

最近在改進SD卡的SEP4020驅動時，在改掉所有的warning時老報一個implicit declaration of function 的警告錯誤，上網查了下原因，原來有兩種情況會產生這種情況

1  沒有把函數所在的c檔案產生.o目標檔案
2  在函數所在的c檔案中定義了，但是沒有在與之相關聯的.h檔案中聲明

[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
於是我在sep_mci.c的頭部將 <linux/clk.h>包含進去就行了

這樣就沒有任何warning產生了，呵呵