fb_mmap函数疑问

Alan0521

最近看LCD驱动，fb_mmap函数有些地方有些疑问，望大家提点，谢谢！
主要是代码中红字部分，不清楚什么意思。

static int
fb_mmap(struct file *file, struct vm_area_struct * vma)
__acquires(&info->lock)
__releases(&info->lock)
{
int fbidx = iminor(file->f_path.dentry->d_inode);
struct fb_info *info = registered_fb[fbidx];
struct fb_ops *fb = info->fbops;
unsigned long off;
unsigned long start;
u32 len;

if (vma->vm_pgoff > (~0UL >> PAGE_SHIFT))
   return -EINVAL;
off = vma->vm_pgoff << PAGE_SHIFT;
if (!fb)
   return -ENODEV;
//如果在fb_ops里实现了fb_mmap，调用用户自己实现的fb_mmap即可，如前所述，这里没有实现

if (fb->fb_mmap) {
   int res;
   mutex_lock(&info->lock);
   res = fb->fb_mmap(info, vma);
   mutex_unlock(&info->lock);
   return res;
}

mutex_lock(&info->lock);

/* frame buffer memory */
start = info->fix.smem_start;
//起始地址是页对齐的
len = PAGE_ALIGN((start & ~PAGE_MASK) + info->fix.smem_len);
//mmap中的off含义根据具体的驱动不同而不同
//显卡设备里往往是用作把显卡寄存器也直接映射给用户空间，加速操作

if (off >= len) {
   /* memory mapped io */
   off -= len;
   if (info->var.accel_flags) {
    mutex_unlock(&info->lock);
    return -EINVAL;
   }
   start = info->fix.mmio_start;
   len = PAGE_ALIGN((start & ~PAGE_MASK) + info->fix.mmio_len);
}

mutex_unlock(&info->lock);
start &= PAGE_MASK;
if ((vma->vm_end - vma->vm_start + off) > len)
   return -EINVAL;
off += start;
//指定该段内存属性
vma->vm_pgoff = off >> PAGE_SHIFT;
/* This is an IO map - tell maydump to skip this VMA */
vma->vm_flags |= VM_IO | VM_RESERVED;
fb_pgprotect(file, vma, off);
//正式映射物理内存到用户空间虚拟地址
if (io_remap_pfn_range(vma, vma->vm_start, off >> PAGE_SHIFT,
        vma->vm_end - vma->vm_start, vma->vm_page_prot))
   return -EAGAIN;
return 0;
}