查看DevMem note的源代码 DevMem note 0

1.devmem ADDRESS [WIDTH [VALUE]] 
<source lang="awk">
Read/write from physical address

        ADDRESS Address to act upon
        WIDTH   Width (8/16/...)
        VALUE   Data to be written

</source>

2. devmem -> mmap
<source lang="c">
int devmem_main(int argc UNUSED_PARAM, char **argv)
{
fd = xopen("/dev/mem", argv[3] ? (O_RDWR | O_SYNC) : (O_RDONLY | O_SYNC)); //根据第三个参数确定是以只读形式打开，还是以读写形式打开。 
map_base = mmap(NULL,
            mapped_size,
            argv[3] ? (PROT_READ | PROT_WRITE) : PROT_READ,
            MAP_SHARED,
            fd,
            target & ~(off_t)(page_size - 1));
virt_addr = (char*)map_base + offset_in_page;
}
</source>
3. mmap
<source lang="c">
#include <sys/mman.h>

void *mmap(void *addr, size_t length, int prot, int flags, int fd, off_t offset);
int munmap(void *addr, size_t length);
</source>

详细参数如下:
{| class="wikitable" style="width:100%"
! 参数      !! 详细说明              
|- 
|| addr     || 需要映射的虚拟内存地址；如果为NULL，系统会自动选定。映射成功后返回该地址 
|-  
|| length     || 需要映射多大的数据量
|-  
|| prot||描述映射区域内存保护方式，包括：PROT_EXEC、PROT_READ、PROT_WRITE、PROT_NONE.
|-  
||flags||描述映射区域的特性，比如是否对其他进程共享，是否建立匿名映射，是否创建私有的cow.
|-  
||fd||要映射到内存中的文件描述符
|-  
||offset||文件映射的偏移量
|}

4. /dev/mem mmap operator:
<source lang="c">
static int mmap_mem(struct file *file, struct vm_area_struct *vma)
{
    size_t size = vma->vm_end - vma->vm_start;
    phys_addr_t offset = (phys_addr_t)vma->vm_pgoff << PAGE_SHIFT;

    /* It's illegal to wrap around the end of the physical address space. */
    if (offset + (phys_addr_t)size - 1 < offset)
        return -EINVAL;

    if (!valid_mmap_phys_addr_range(vma->vm_pgoff, size))return -EINVAL;

    if (!private_mapping_ok(vma))
        return -ENOSYS;

    if (!range_is_allowed(vma->vm_pgoff, size))
        return -EPERM;

    if (!phys_mem_access_prot_allowed(file, vma->vm_pgoff, size,
                        &vma->vm_page_prot))
        return -EINVAL;

    vma->vm_page_prot = phys_mem_access_prot(file, vma->vm_pgoff,
                         size,
                         vma->vm_page_prot);

    vma->vm_ops = &mmap_mem_ops;

    /* Remap-pfn-range will mark the range VM_IO */
    if (remap_pfn_range(vma,---------------------------------------------将内核中vma->vm_pgoff对应的size个页面，映射到vma区域，返回的虚拟空间起始地址是vma->vm_start。
                vma->vm_start,
                vma->vm_pgoff,
                size,
                vma->vm_page_prot)) {
        return -EAGAIN;
    }
    return 0;
}
</source>

vm_area_struct结构体