Linux/DRA72: mmap read fail

/**********************************************************************
**
** �ļ���:     	api_mmap.c
** ��Ȩ����:   (c) 2016-2017 ���������ţ��������޹�˾
** �ļ�����:  mmap api
**
***********************************************************************
**             �޸���ʷ��¼
**=====================================================================
**|    ����  	   |  ����    |  �޸ļ�¼
**=====================================================================
**| 2017/06/15 | ��ѧ�� |  ������һ�汾
***********************************************************************/
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>
#include <sys/ioctl.h>  
#include <unistd.h>
#include <errno.h>
#include <sys/mman.h>

#define BASE_ADDR 				0x48051000
#define GPIO_CTRL				0x130
#define GPIO_OE				    0x134			//0 = Output enabled ; 1 = Output disabled
#define GPIO_DATAIN				0x138
#define GPIO_DATAOUT			0x13C
#define GPIO_SETDATAOUT			0x194

#define GPIO7_3			        1<<3
#define GPIO7_4			        1<<4


void _writel(void *addr, unsigned int val)
{
	*((volatile unsigned int *)addr) = val;
}

unsigned int _readl(void *addr)
{	
	return *((volatile unsigned int *)addr);
}

int main()
{	
	int fd_mem;
	unsigned char *map_addr = NULL;
	
	if((fd_mem = open("/dev/mem", O_RDWR | O_NDELAY))<0)
	{
		perror("open mem");
		return -1;
	}
	map_addr = mmap(NULL, 512, PROT_READ | PROT_WRITE, MAP_SHARED, fd_mem, BASE_ADDR);
	if(map_addr == NULL)
	{
		perror("mmap");
		return -1;
	}
	printf("<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<%p\n",map_addr);
	printf("1GPIO_OE:0x%x",_readl(map_addr+GPIO_OE));
	//_writel(map_addr+GPIO_OE, _readl(map_addr+GPIO_OE) & ~(GPIO7_3 | GPIO7_4));
	//printf("2GPIO_OE:0x%x",_readl(map_addr+GPIO_OE));
	

	return 0;
}