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。 
      mapped_size = page_size = getpagesize(); //4K bytes
      //if cross page, mapped_size *=2 
      map_base = mmap(NULL,
            mapped_size, // 
            argv[3] ? (PROT_READ | PROT_WRITE) : PROT_READ,
            MAP_SHARED,
            fd,
            target & ~(off_t)(page_size - 1)//offset from 0x0000, page start address
          );
      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. Mus be multiple of PAGE SIZE
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. mmap to /dev/mem
Questions:
offset 如何变成是物理地址? offset 跟vma->pgoff 的关系?
vma->vm_start = addr ? 若 addr=NULL 如何给址?

系统调用的入口是entry_SYSCALL_64_fastpath，然后根据系统调用号在sys_call_table中找到对应的函数。
mmap()和munmap()对应的系统调用分别是SyS_mmap()和SyS_munmap()

arch/arc/kernel/sys.c

#define __SYSCALL(nr, call) [nr] = (call),
void *sys_call_table[NR_syscalls] = {
	[0 ... NR_syscalls-1] = sys_ni_syscall,
#include <asm/unistd.h>
};

include/uapiasm-generic/unistd.h

5./dev/mem driver in inux-kernel/drivers/char/mem.c

static int mmap_mem(struct file *file, struct vm_area_struct *vma)
{
    size_t size = vma->vm_end - vma->vm_start;
    if (!valid_mmap_phys_addr_range(vma->vm_pgoff, size))//没检查
          return -EINVAL;
    if (!private_mapping_ok(vma))//vm_flags must have VM_MAYSHARE
        return -ENOSYS;
 
    if (!range_is_allowed(vma->vm_pgoff, size))//check each page is allowed by calling devmem_is_allowed. ARCH_HAS_VALID_PHYS_ADDR_RANGE 
        return -EPERM;
 
    if (!phys_mem_access_prot_allowed(file, vma->vm_pgoff, size,
                        &vma->vm_page_prot))//没检查
        return -EINVAL;
 
    vma->vm_page_prot = phys_mem_access_prot(file, vma->vm_pgoff,
                         size,
                         vma->vm_page_prot);// 跟 cache 有关 先不管
 
    vma->vm_ops = &mmap_mem_ops;
 
    /* Remap-pfn-range will mark the range VM_IO */
    if (remap_pfn_range(vma,
                vma->vm_start, //Virt 开头位址 
                vma->vm_pgoff,
                size,
                vma->vm_page_prot)) {
        return -EAGAIN;
    }
    return 0;
}

remap_pfn_range im ./mm/memory.c

remap_pfn_range(vma, //user vma to map to
                addr, //target user address
                pfn, //physic address of kernel memory. Page #?
                size, //size of map area
                prot //page protection flags for this mapping
               )
{
    vma->vm_pgoff = pfn;
    vma->flags |= VM_IO|VM_PFNMAP|VM_DONTEXPAD |VM_DONTDUMP;
    mm = vma->mm;
    do {
        //for each page, update mm
        next = pgd_addr_end(addr, end)
        remap_pud_range(mm, pgd, addr, next, pfn+page#, prot)
        pgd++, addr = next,
    }while (addr != end)

Question：

如何确认offset 合理区域? length 可以有多大?

Reference:
https://blog.csdn.net/junllee/article/details/82146351