如何初始化gpio口为低电平

我用的的mini6410.最近写一个电机驱动，发现当设置为输出模式时，在加载驱动模块后(加载前引脚为低电平），总是不能把gpio口的m组的引脚电平设为低电平，下面是初始化函数
static int __init dev_init(void)
{
    int ret;
    {
        unsigned tmp;
        tmp = readl(S3C64XX_GPMCON);/*set GPM port output mode*/
        tmp &= ~(0xffffU);
        tmp |= (0x1111U);
        writel(tmp, S3C64XX_GPMCON);
        
        tmp = readl(S3C64XX_GPMDAT);
        tmp &= ~(0xfU))
        tmp |= (0x0U);
        writel(tmp, S3C64XX_GPMDAT);
        
        /*tmp = readl(S3C64XX_GPMPUD);
        tmp &= ~(0xff);
        tmp |= (0x00);
        writel(tmp,S3C64XX_GPMPUD);*/
    }

    ret = misc_register(&misc);

    printk (DEVICE_NAME"\tinitialized\n");

    return ret;
}