内核运行之前访问IO

Access IO before Kernel start

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：

#define           ((volatile unsigned long *)0x40100000)
#define           ((volatile unsigned long *)0x40200000)
#define           ((volatile unsigned long *)0x40700000)

#define             


static  void (char )
{
        while (!([5] & 0x20));
        [0] = ;
}

/*
 * This does not append a newline
 */
static void (const char *)
{
        while (*) {
                (*);
                if (* == '\n')
                        ('\r');
                ++;
        }
}

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))