如果要在内核运行之前访问CPU的某些IO端口，直接使用指针方式定义寄存器进行操作即可。例如，在解压内核的时候喂狗，通过操作IO进行，可以这样操作：

在arch/arm/boot/compressed/misc.c文件：

 

    307     arch_decomp_setup();

    308

    309     makecrc();

    310     *((volatile unsigned long *)0x40E00054) &= (~(3<<28)); //ABING GPAF0_L

    311     *((volatile unsigned long *)0x40E0001C) = (1<<14); //ABING GPDR0

    312     *((volatile unsigned long *)0x40E00024) = (1<<14); //ABING GPCR0

    313     putstr("Uncompressing Linux...");

 

  314     *((volatile unsigned long *)0x40E00018) = (1<<14); //ABING GPSR0

    315     gunzip();

 

  316     *((volatile unsigned long *)0x40E00024) = (1<<14); //ABING GPCR0

    317     putstr(" done, booting the kernel.\n");

    318     return output_ptr;

 

 

 

 

参考include/asm-arm/arch-pxa/uncompress.h文件访问串口的代码：

12#define           ((volatile unsigned long *)0x40100000)  13#define           ((volatile unsigned long *)0x40200000)  14#define           ((volatile unsigned long *)0x40700000)  15  16#define               17  18  19static  void (char )  20{  21        while (!([5] & 0x20));  22        [0] = ;  23}  24  25/*  26 * This does not append a newline  27 */  28static void (const char *)  29{  30        while (*) {  31                (*);  32                if (* == '\n')  33                        ('\r');  34                ++;  35        }  36}

可以这样定义：

#define __raw_readl(a)    (*(volatile unsigned int *)(a))#define __raw_writel(v,a) (*(volatile unsigned int *)(a) = (v))#define __raw_readw(a)    (*(volatile unsigned short *)(a))#define __raw_writew(v,a) (*(volatile unsigned short *)(a) = (v))