如何使用devmem及mmap直接访问物理内存

devmem 是一个在 Linux 系统下用于直接访问物理内存的命令行工具，它能让用户在用户空间对指定的物理内存地址进行读写操作。以下是关于它的使用方法介绍：

安装 devmem

在大多数 Linux 发行版中，devmem 通常包含在 util-linux 软件包内。你可以通过以下命令来安装：

sudo apt-get update
sudo apt-get install util-linux

基本使用方法

读取内存
要读取指定物理内存地址处的数据，可使用以下命令格式：

sudo devmem2  0x2050000000 
sudo devmem2  0x2050000000  b  0x55
Usage:  devmem2 { address } [ type [ data ] ]
        address : memory address to act upon
        type    : access operation type : [b]yte, [h]alfword, [w]ord, [l]ong
        data    : data to be written

问题1：
报错Operation not permitted
修改内核编译config选项：
CONFIG_STRICT_DEVMEM disable

问题2：
报错Invalid argument
内核编译config选项：
CONFIG_X86_PAT disable ;
CONFIG_EXPERT enable

devmem工具实现代码

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <signal.h>
#include <fcntl.h>
#include <ctype.h>
#include <termios.h>
#include <sys/types.h>
#include <sys/mman.h>
  
#define FATAL do { fprintf(stderr, "Error at line %d, file %s (%d) [%s]\n", \
  __LINE__, __FILE__, errno, strerror(errno)); exit(1); } while(0)
 
#define MAP_SIZE 4096UL
#define MAP_MASK (MAP_SIZE - 1)

int main(int argc, char **argv) {
    int fd;
    void *map_base, *virt_addr; 
	unsigned long read_result, writeval;
	off_t target;
	int access_type = 'w';
	
	if(argc < 2) {
		fprintf(stderr, "\nUsage:\t%s { address } [ type [ data ] ]\n"
			"\taddress : memory address to act upon\n"
			"\ttype    : access operation type : [b]yte, [h]alfword, [w]ord, [l]ong\n"
			"\tdata    : data to be written\n\n",
			argv[0]);
		exit(1);
	}
	target = strtoul(argv[1], 0, 0);

	if(argc > 2)
		access_type = tolower(argv[2][0]);


    if((fd = open("/dev/mem", O_RDWR | O_SYNC)) == -1) FATAL;
    printf("/dev/mem opened.\n"); 
    fflush(stdout);
    
    /* Map one page */
    map_base = mmap(0, MAP_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, target & ~MAP_MASK);
    if(map_base == (void *) -1) FATAL;
    printf("Memory mapped at address %p.\n", map_base); 
    fflush(stdout);
    
    virt_addr = map_base + (target & MAP_MASK);
    switch(access_type) {
		case 'b':
			read_result = *((unsigned char *) virt_addr);
			break;
		case 'h':
			read_result = *((unsigned short *) virt_addr);
			break;
		case 'w':
			read_result = *((unsigned int/*long*/ *) virt_addr);
			break;
		case 'l':
			read_result = *((unsigned long *) virt_addr);
			break;
		default:
			fprintf(stderr, "Illegal data type '%c'.\n", access_type);
			exit(2);
	}
    printf("Value at address 0x%X (%p): 0x%lX\n", target, virt_addr, read_result); 
    fflush(stdout);

	if(argc > 3) {
		writeval = strtoul(argv[3], 0, 0);
		switch(access_type) {
			case 'b':
				*((unsigned char *) virt_addr) = writeval;
				read_result = *((unsigned char *) virt_addr);
				break;
			case 'h':
				*((unsigned short *) virt_addr) = writeval;
				read_result = *((unsigned short *) virt_addr);
				break;
			case 'w':
				//printf("%p\n", virt_addr);
				//printf("0x%X\n", writeval);

				*((unsigned int/*long*/ *) virt_addr) = writeval;
				//*((unsigned char *) (virt_addr+0)) = writeval;
				//*((unsigned char *) (virt_addr+1)) = (writeval >> 8);
				//*((unsigned char *) (virt_addr+2)) = (writeval >> 16);
				//*((unsigned char *) (virt_addr+3)) = (writeval >> 24);

				read_result = *((unsigned int/*long*/ *) virt_addr);
				break;
			case 'l':
				*((unsigned long *) virt_addr) = writeval;
				read_result = *((unsigned long *) virt_addr);
				break;
		}
		printf("Written 0x%lX; readback 0x%lX\n", writeval, read_result); 
		fflush(stdout);
	}
	
	if(munmap(map_base, MAP_SIZE) == -1) FATAL;
    close(fd);
    return 0;
}

Linux应用层操作寄存器

除了直接使用devmem，我们也可以在Linux应用层自己实现一个devmem。

devmem的实现原理，就是打开/dev/mem，然后通过mmap映射物理地址，从而实现读写寄存器。因此，我们只要实现这些操作，就可以自己实现类似devmem的功能。

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <unistd.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/mman.h>

#define MAP_SIZE 0x80000
#define base 0x40000000

int main(int argc, char **argv)
{
 int fd = open("/dev/mem",O_RDWR|O_NDELAY);
    
    if (fd < 0)
    {
        printf("open /dev/mem error!\n");
        return -1;
 }
    
    void *map_base = mmap(NULL,MAP_SIZE,PROT_READ|PROT_WRITE,MAP_SHARED,fd,base);
    if (map_base == MAP_FAILED)
    return -1;
    
    printf("%x \n",*(volatile unsigned int*)(map_base));

    close(fd);
    munmap(map_base,MAP_SIZE);
    
 return 0;
}