DevMem note DevMem note 0

1.devmem ADDRESS [WIDTH [VALUE]]

Read/write from physical address
 
        ADDRESS Address to act upon
        WIDTH   Width (8/16/...)
        VALUE   Data to be written

2. devmem -> mmap

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;
}

3. mmap

#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);

详细参数如下:

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:

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;
}

vm_area_struct结构体